Throughout the 19th century, chemistry was divided between those who followed the atomic theory of
John Dalton and the
energeticists, such as
Wilhelm Ostwald and
Ernst Mach. Although such proponents of the atomic theory as
Amedeo Avogadro and
Ludwig Boltzmann made great advances in explaining the behavior of
gases, this dispute was not finally settled until
Jean Perrin's experimental investigation of
Einstein's atomic explanation of
Brownian motion in the first decade of the 20th century. Discovered in 1801, this concept is also known as Dalton's law of partial pressures. Dalton also proposed a modern
atomic theory in 1803 which stated that all matter was composed of small indivisible particles termed atoms, atoms of a given element possess unique characteristics and weight, and three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). In 1808, Dalton first published
New System of Chemical Philosophy (1808–1827), in which he outlined the first modern scientific description of the atomic theory. This work identified chemical elements as a specific type of atom, therefore rejecting
Newton's theory of chemical affinities. Instead, Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight of hydrogen to be identically one. Following
Jeremias Benjamin Richter (known for introducing the term
stoichiometry), he proposed that chemical elements combine in integral ratios. This is known as the
law of multiple proportions or Dalton's law, and Dalton included a clear description of the law in his
New System of Chemical Philosophy. The law of multiple proportions is one of the basic laws of stoichiometry used to establish the atomic theory. Despite the importance of the work as the first view of atoms as physically real entities and the introduction of a system of chemical symbols,
New System of Chemical Philosophy devoted almost as much space to the caloric theory as to atomism. French chemist
Joseph Proust proposed the
law of definite proportions, which states that elements always combine in small, whole number ratios to form compounds, based on several experiments conducted between 1797 and 1804. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry. The law of definite proportions and constant composition do not prove that atoms exist, but they are difficult to explain without assuming that chemical compounds are formed when atoms combine in constant proportions.
Jöns Jacob Berzelius , the chemist who worked out the modern technique of
chemical formula notation and is considered one of the fathers of modern chemistry A Swedish chemist and disciple of Dalton,
Jöns Jacob Berzelius embarked on a systematic program to try to make accurate and precise quantitative measurements and to ensure the purity of chemicals. Along with Lavoisier, Boyle, and Dalton, Berzelius is known as the father of modern chemistry. In 1828 he compiled a table of relative atomic weights, where
oxygen was used as a standard, with its weight set at 100, and which included all of the elements known at the time. This work provided evidence in favor of Dalton's atomic theory – that inorganic chemical compounds are composed of atoms combined in
whole number amounts. He determined the exact elementary constituents of a large number of compounds; the results strongly supported Proust's Law of Definite Proportions. In discovering that atomic weights are not integer multiples of the weight of hydrogen, Berzelius also disproved
Prout's hypothesis that elements are built up from atoms of hydrogen. Motivated by his extensive atomic weight determinations and in a desire to aid his experiments, he introduced the classical system of
chemical symbols and notation with his 1808 publication
Lärbok i Kemien, in which elements are abbreviated to one or two letters to make a distinct symbol from their Latin name. This system of chemical notation—in which the elements were given simple written labels, such as O for oxygen, or Fe for iron, with proportions denoted by numbers—is the same basic system used today. The only difference is that instead of the subscript number used today (e.g., H2O), Berzelius used a superscript (H2O). Berzelius is credited with identifying the chemical elements
silicon,
selenium,
thorium, and
cerium. Students working in Berzelius's laboratory also discovered
lithium and
vanadium. Berzelius developed the
radical theory of chemical combination, which holds that reactions occur as stable groups of atoms called
radicals are exchanged between molecules. He believed that salts are compounds formed of
acids and
bases, and discovered that the anions in acids were attracted to a positive electrode (the
anode), whereas the cations in a base were attracted to a negative electrode (the
cathode). Berzelius did not believe in the
Vitalism Theory, but instead in a regulative force which produced organization of tissues in an organism. Berzelius is also credited with originating the chemical terms "
catalysis", "
polymer", "
isomer", and "
allotrope", although his original definitions differ dramatically from modern usage. For example, he coined the term "polymer" in 1833 to describe organic compounds which shared identical empirical formulas but which differed in overall molecular weight, the larger of the compounds being described as "polymers" of the smallest. By this long-superseded, pre-structural definition,
glucose (C6H12O6) was viewed as a polymer of
formaldehyde (CH2O).
New elements and gas laws , the discover of several
alkali and
alkaline earth metals, as well as contributions to the discoveries of the elemental nature of
chlorine and
iodine English chemist
Humphry Davy was a pioneer in the field of
electrolysis, using Alessandro Volta's voltaic pile to split up common compounds and thus isolate a series of new elements. He went on to electrolyse molten salts and discovered several new metals, especially
sodium and
potassium, highly reactive elements known as the
alkali metals. Potassium, the first metal that was isolated by electrolysis, was discovered in 1807 by Davy, who derived it from
caustic potash (KOH). Before the 19th century, no distinction was made between potassium and sodium. Sodium was first isolated by Davy in the same year by passing an electric current through molten
sodium hydroxide (NaOH). When Davy heard that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. Davy was successful, and discovered
calcium in 1808 by electrolyzing a mixture of
lime and
mercuric oxide. He worked with electrolysis throughout his life and, in 1808, he isolated
magnesium,
strontium and
barium. Davy also experimented with gases by inhaling them. This experimental procedure nearly proved fatal on several occasions, but led to the discovery of the unusual effects of
nitrous oxide, which came to be known as laughing gas.
Chlorine was discovered in 1774 by Swedish chemist
Carl Wilhelm Scheele, who called it
"dephlogisticated marine acid" (see
phlogiston theory) and mistakenly thought it contained
oxygen. Scheele observed several properties of chlorine gas, such as its bleaching effect on litmus, its deadly effect on insects, its yellow-green colour, and the similarity of its smell to that of
aqua regia. However, Scheele was unable to publish his findings at the time. In 1810, chlorine was given its current name by Humphry Davy (derived from the Greek word for green), who insisted that chlorine was in fact an
element. He also showed that
oxygen could not be obtained from the substance known as
oxymuriatic acid (HCl solution). This discovery overturned
Lavoisier's definition of acids as compounds of oxygen. Davy was a popular lecturer and able experimenter. , who stated that the ratio between the volumes of the reactant gases and the products can be expressed in simple whole numbers French chemist
Joseph Louis Gay-Lussac shared the interest of Lavoisier and others in the quantitative study of the properties of gases. From his first major program of research in 1801–1802, he concluded that equal volumes of all gases expand equally with the same increase in temperature: this conclusion is usually called "
Charles's law", as Gay-Lussac gave credit to
Jacques Charles, who had arrived at nearly the same conclusion in the 1780s but had not published it. The law was independently discovered by British natural philosopher John Dalton by 1801, although Dalton's description was less thorough than Gay-Lussac's. In 1804 Gay-Lussac made several daring ascents of over 7,000 meters above sea level in hydrogen-filled balloons—a feat not equaled for another 50 years—that allowed him to investigate other aspects of gases. Not only did he gather magnetic measurements at various altitudes, but he also took pressure, temperature, and humidity measurements and samples of air, which he later analyzed chemically. In 1808 Gay-Lussac announced what was probably his single greatest achievement: from his own and others' experiments he deduced that gases at constant temperature and pressure combine in simple numerical proportions by volume, and the resulting product or products—if gases—also bear a simple proportion by volume to the volumes of the reactants. In other words, gases under equal conditions of temperature and pressure react with one another in volume ratios of small whole numbers. This conclusion subsequently became known as "
Gay-Lussac's law" or the "
Law of Combining Volumes". With his fellow professor at the
École Polytechnique,
Louis Jacques Thénard, Gay-Lussac also participated in early electrochemical research, investigating the elements discovered by its means. Among other achievements, they decomposed
boric acid by using fused potassium, thus discovering the element
boron. The two also took part in contemporary debates that modified Lavoisier's definition of acids and furthered his program of analyzing organic compounds for their oxygen and hydrogen content. The element
iodine was discovered by French chemist
Bernard Courtois in 1811. Courtois gave samples to his friends,
Charles Bernard Desormes (1777–1862) and
Nicolas Clément (1779–1841), to continue research. He also gave some of the substance to Gay-Lussac and to physicist
André-Marie Ampère. On December 6, 1813, Gay-Lussac announced that the new substance was either an element or a compound of oxygen. It was Gay-Lussac who suggested the name
"iode", from the Greek word ιώδες (iodes) for violet (because of the color of iodine vapor). Davy sent a letter dated December 10 to the
Royal Society of London stating that he had identified a new element. Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists acknowledged Courtois as the first to isolate the element. In 1815, Humphry Davy invented the
Davy lamp, which allowed miners within
coal mines to work safely in the presence of flammable gases. There had been many mining explosions caused by
firedamp or
methane often ignited by open flames of the lamps then used by miners. Davy conceived of using an iron gauze to enclose a lamp's flame, and so prevent the methane burning inside the lamp from passing out to the general atmosphere. Although the idea of the
safety lamp had already been demonstrated by
William Reid Clanny and by the then unknown (but later very famous) engineer
George Stephenson, Davy's use of wire gauze to prevent the spread of flame was used by many other inventors in their later designs. There was some discussion as to whether Davy had discovered the principles behind his lamp without the help of the work of
Smithson Tennant, but it was generally agreed that the work of both men had been independent. Davy refused to patent the lamp, and its invention led to him being awarded the
Rumford medal in 1816. , who postulated that, under controlled conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules. This is known as
Avogadro's law. After Dalton published his atomic theory in 1808, certain of his central ideas were soon adopted by most chemists. However, uncertainty persisted for half a century about how atomic theory was to be configured and applied to concrete situations; chemists in different countries developed several different incompatible atomistic systems. A paper that suggested a way out of this difficult situation was published as early as 1811 by the Italian physicist
Amedeo Avogadro (1776–1856), who hypothesized that equal volumes of gases at the same
temperature and
pressure contain equal numbers of molecules, from which it followed that relative
molecular weights of any two gases are the same as the ratio of the densities of the two gases under the same conditions of temperature and pressure. Avogadro also reasoned that simple gases were not formed of solitary atoms but were instead compound molecules of two or more atoms. Thus Avogadro was able to overcome the difficulty that Dalton and others had encountered when Gay-Lussac reported that above 100 °C the volume of water vapor was twice the volume of the oxygen used to form it. According to Avogadro, the molecule of oxygen had split into two atoms in the course of forming water vapor. Avogadro's hypothesis was neglected for half a century after it was first published. Many reasons for this neglect have been cited, including some theoretical problems, such as Jöns Jacob Berzelius's "dualism", which asserted that compounds are held together by the attraction of positive and negative electrical charges, making it inconceivable that a molecule composed of two electrically similar atoms—as in oxygen—could exist. An additional barrier to acceptance was the fact that many chemists were reluctant to adopt physical methods (such as vapour-density determinations) to solve their problems. By mid-century, however, some leading figures had begun to view the chaotic multiplicity of competing systems of atomic weights and molecular formulas as intolerable. Moreover, purely chemical evidence began to mount that suggested Avogadro's approach might be right after all. During the 1850s, younger chemists, such as
Alexander Williamson in England,
Charles Gerhardt and
Charles-Adolphe Wurtz in France, and
August Kekulé in Germany, began to advocate reforming theoretical chemistry to make it consistent with Avogadrian theory.
Wöhler, von Liebig, organic chemistry and the vitalism debate , which
Friedrich Wöhler used for seminal contributions in
organic chemistry, for which
Justus von Liebig also made major contributions In 1825,
Friedrich Wöhler and
Justus von Liebig performed the first confirmed discovery and explanation of
isomers, earlier named by Berzelius. Working with
cyanic acid and
fulminic acid, they correctly deduced that isomerism was caused by differing arrangements of atoms within a molecular structure. In 1827,
William Prout classified biomolecules into their modern groupings:
carbohydrates,
proteins and
lipids. After the nature of combustion was settled, a dispute about
vitalism and the essential distinction between organic and inorganic substances began. The vitalism question was revolutionized in 1828 when Friedrich Wöhler synthesized
urea, thereby establishing that organic compounds could be produced from inorganic starting materials and disproving the theory of vitalism. This opened a new research field in chemistry, and by the end of the 19th century, scientists were able to synthesize hundreds of organic compounds. The most important among them are
mauve,
magenta, and other synthetic
dyes, as well as the widely used drug
aspirin. The discovery of the artificial synthesis of urea contributed greatly to the theory of
isomerism, as the empirical chemical formulas for urea and
ammonium cyanate are identical (see
Wöhler synthesis). In 1832, Friedrich Wöhler and Justus von Liebig discovered and explained
functional groups and
radicals in relation to organic chemistry, as well as first synthesizing
benzaldehyde. Liebig, a German chemist, made major contributions to
agricultural and
biological chemistry, and worked on the organization of
organic chemistry, being considered one of its principal founders. Liebig is also considered the "father of the
fertilizer industry" for his discovery of
nitrogen as an essential plant
nutrient, and his formulation of the
Law of the Minimum which described the effect of individual nutrients on crops.
Vladimir Markovnikov Vladimir Markovnikov, born in 1838, was a Russian scientist who did most of his work at Kazan University in Russia. At Kazan, he studied under
Butlerov in a laboratory better known as "the cradle of Russian organic chemistry", after which he also studied chemistry in Germany for two years. Products in chemistry that follow this rule are considered Markovnikov products and those that did not are considered anti-Markovnikov products.
Mid-1800s In 1840,
Germain Hess proposed
Hess's law, an early statement of the
law of conservation of energy, which establishes that
energy changes in a chemical process depend only on the states of the starting and product materials and not on the specific pathway taken between the two states. In 1847,
Hermann Kolbe obtained
acetic acid from completely inorganic sources, further disproving vitalism. In 1848,
William Thomson, 1st Baron Kelvin (commonly known as Lord Kelvin) established the concept of
absolute zero, the temperature at which all molecular motion ceases. In 1849,
Louis Pasteur discovered that the
racemic form of
tartaric acid is a mixture of the levorotatory and dextrotatory forms, thus clarifying the nature of
optical rotation and advancing the field of
stereochemistry. In 1852,
August Beer proposed
Beer's law, which explains the relationship between the composition of a mixture and the amount of light it will absorb. Based partly on earlier work by
Pierre Bouguer and
Johann Heinrich Lambert, it established the
analytical technique known as
spectrophotometry. In 1855,
Benjamin Silliman, Jr. pioneered methods of
petroleum cracking, which made the entire modern
petrochemical industry possible. in 1861 Avogadro's hypothesis began to gain broad appeal among chemists only after his compatriot and fellow scientist
Stanislao Cannizzaro demonstrated its value in 1858, two years after Avogadro's death. Cannizzaro's chemical interests had originally centered on natural products and on reactions of
aromatic compounds; in 1853 he discovered that when
benzaldehyde is treated with concentrated base, both
benzoic acid and
benzyl alcohol are produced—a phenomenon known today as the
Cannizzaro reaction. In his 1858 pamphlet, Cannizzaro showed that a complete return to the ideas of Avogadro could be used to construct a consistent and robust theoretical structure that fit nearly all of the available empirical evidence. For instance, he pointed to evidence that suggested that not all elementary gases consist of two atoms per molecule—some were
monatomic, most were
diatomic, and a few were even more complex. Another point of contention had been the formulas for compounds of the
alkali metals (such as
sodium) and the
alkaline earth metals (such as
calcium), which, in view of their striking chemical analogies, most chemists had wanted to assign to the same formula type. Cannizzaro argued that placing these metals in different categories had the beneficial result of eliminating certain anomalies when using their physical properties to deduce atomic weights. Unfortunately, Cannizzaro's pamphlet was published initially only in Italian and had little immediate impact. The real breakthrough came with an
international chemical congress held in the German town of
Karlsruhe in September 1860, at which most of the leading European chemists were present. The Karlsruhe Congress had been arranged by Kekulé, Wurtz, and a few others who shared Cannizzaro's sense of the direction chemistry should go. Speaking in French (as everyone there did), Cannizzaro's eloquence and logic made an indelible impression on the assembled body. Moreover, his friend Angelo Pavesi distributed Cannizzaro's pamphlet to attendees at the end of the meeting; more than one chemist later wrote of the decisive impression the reading of this document provided. For instance,
Lothar Meyer later wrote that on reading Cannizzaro's paper, "The scales seemed to fall from my eyes." Cannizzaro thus played a crucial role in winning the battle for reform. The system advocated by him, and soon thereafter adopted by most leading chemists, is substantially identical to what is still used today.
Perkin, Crookes, and Nobel In 1856, Sir
William Henry Perkin, age 18, given a challenge by his professor,
August Wilhelm von Hofmann, sought to synthesize
quinine, the anti-
malaria drug, from
coal tar. In one attempt, Perkin
oxidized aniline using
potassium dichromate, whose
toluidine impurities reacted with the aniline and yielded a black solid—suggesting a "failed" organic synthesis. Cleaning the flask with alcohol, Perkin noticed purple portions of the solution: a byproduct of the attempt was the first synthetic dye, known as
mauveine or Perkin's mauve. Perkin's discovery is the foundation of the dye synthesis industry, one of the earliest successful chemical industries. German chemist
August Kekulé von Stradonitz's most important single contribution was his structural theory of organic composition, outlined in two articles published in 1857 and 1858 and treated in great detail in the pages of his extraordinarily popular ("Textbook of Organic Chemistry"), the first installment of which appeared in 1859 and gradually extended to four volumes. Kekulé argued that tetravalent
carbon atoms – that is, carbon forming exactly four
chemical bonds – could link together to form what he called a "carbon chain" or a "carbon skeleton," to which other atoms with other valences (such as hydrogen, oxygen, nitrogen, and chlorine) could join. He was convinced that it was possible for the chemist to specify this detailed molecular architecture for at least the simpler organic compounds known in his day. Kekulé was not the only chemist to make such claims in this era. The Scottish chemist
Archibald Scott Couper published a substantially similar theory nearly simultaneously, and the Russian chemist
Aleksandr Butlerov did much to clarify and expand structure theory. However, it was predominantly Kekulé's ideas that prevailed in the chemical community. (two views): light and dark. Electrons travel in straight lines from the
cathode (left), as evidenced by the shadow cast from the
Maltese cross on the fluorescence of the righthand end. The anode is at the bottom wire. British chemist and physicist
William Crookes is noted for his
cathode ray studies, fundamental in the development of
atomic physics. His researches on electrical discharges through a rarefied gas led him to observe the dark space around the cathode, now called the Crookes dark space. He demonstrated that cathode rays travel in straight lines and produce phosphorescence and heat when they strike certain materials. A pioneer of vacuum tubes, Crookes invented the
Crookes tube – an early experimental discharge tube, with partial vacuum with which he studied the behavior of cathode rays. With the introduction of
spectrum analysis by
Robert Bunsen and
Gustav Kirchhoff (1859–1860), Crookes applied the new technique to the study of
selenium compounds. Bunsen and Kirchhoff had previously used spectroscopy as a means of chemical analysis to discover
caesium and
rubidium. In 1861, Crookes used this process to discover
thallium in some seleniferous deposits. He continued work on that new element, isolated it, studied its properties, and in 1873 determined its atomic weight. During his studies of thallium, Crookes discovered the principle of the
Crookes radiometer, a device that converts light radiation into rotary motion. The principle of this radiometer has found numerous applications in the development of sensitive measuring instruments. In 1862,
Alexander Parkes exhibited
Parkesine, one of the earliest
synthetic polymers, at the International Exhibition in London. This discovery formed the foundation of the modern
plastics industry. In 1864,
Cato Maximilian Guldberg and
Peter Waage, building on Claude Louis Berthollet's ideas, proposed the
law of mass action. In 1865,
Johann Josef Loschmidt determined the number of molecules in a
mole, later named
Avogadro's number. In 1865, August Kekulé, based partially on the work of Loschmidt and others, established the structure of benzene as a six carbon ring with alternating single and
double bonds. Kekulé's novel proposal for benzene's cyclic structure was much contested but was never replaced by a superior theory. This theory provided the scientific basis for the dramatic expansion of the German chemical industry in the last third of the 19th century. Kekulé is also famous for having clarified the nature of aromatic compounds, which are compounds based on the benzene molecule. In 1865,
Adolf von Baeyer began work on
indigo dye, a milestone in modern industrial organic chemistry which revolutionized the dye industry. Swedish chemist and inventor
Alfred Nobel found that when
nitroglycerin was incorporated in an absorbent inert substance like
kieselguhr (
diatomaceous earth) it became safer and more convenient to handle, and this mixture he patented in 1867 as
dynamite. Nobel later on combined nitroglycerin with various nitrocellulose compounds, similar to
collodion, but settled on a more efficient recipe combining another nitrate explosive, and obtained a transparent, jelly-like substance, which was a more powerful explosive than dynamite.
Gelignite, or blasting gelatin, as it was named, was patented in 1876; and was followed by a host of similar combinations, modified by the addition of
potassium nitrate and various other substances.
Mendeleev's periodic table , responsible for organizing the known chemical elements in a
periodic table An important breakthrough in making sense of the list of known chemical elements (as well as in understanding the internal structure of atoms) was
Dmitri Mendeleev's development of the first modern
periodic table, or the periodic classification of the elements. Mendeleev, a Russian chemist, felt that there was some type of order to the elements and he spent more than thirteen years of his life collecting data and assembling the concept, initially with the idea of resolving some of the disorder in the field for his students. Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. Mendeleev's law allowed him to build up a systematic periodic table of all the 66 elements then known based on atomic mass, which he published in
Principles of Chemistry in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. Mendeleev had such faith in the validity of the periodic law that he proposed changes to the generally accepted values for the atomic weight of a few elements and, in his version of the periodic table of 1871, predicted the locations within the table of unknown elements together with their properties. He even predicted the likely properties of three yet-to-be-discovered elements, which he called
ekaboron (Eb), ekaaluminium (Ea), and ekasilicon (Es), which proved to be good predictors of the properties of
scandium,
gallium, and
germanium, respectively, which each fill the spot in the periodic table assigned by Mendeleev. At first the periodic system did not raise interest among chemists. However, with the discovery of the predicted elements, notably gallium in 1875, scandium in 1879, and germanium in 1886, it began to win wide acceptance. The subsequent proof of many of his predictions within his lifetime brought fame to Mendeleev as the founder of the periodic law. This organization surpassed earlier attempts at classification by
Alexandre-Émile Béguyer de Chancourtois, who published the telluric helix, an early, three-dimensional version of the periodic table of the elements in 1862,
John Newlands, who proposed the law of octaves (a precursor to the periodic law) in 1864, and
Lothar Meyer, who developed an early version of the periodic table with 28 elements organized by
valence in 1864. Mendeleev's table did not include any of the
noble gases, however, which had not yet been discovered. Gradually the periodic law and table became the framework for a great part of chemical theory. By the time Mendeleev died in 1907, he enjoyed international recognition and had received distinctions and awards from many countries. In 1873,
Jacobus Henricus van 't Hoff and
Joseph Achille Le Bel, working independently, developed a model of
chemical bonding that explained the chirality experiments of Pasteur and provided a physical cause for
optical activity in chiral compounds. van 't Hoff's publication, called , etc. (Proposal for the development of 3-dimensional chemical structural formulae) and consisting of twelve pages of text and one page of diagrams, gave the impetus to the development of
stereochemistry. The concept of the "asymmetrical carbon atom", dealt with in this publication, supplied an explanation of the occurrence of numerous isomers, inexplicable by means of the then current structural formulae. At the same time he pointed out the existence of relationship between optical activity and the presence of an asymmetrical carbon atom.
Josiah Willard Gibbs formulated a concept of
thermodynamic equilibrium of a system in terms of energy and entropy. He also did extensive work on chemical equilibrium, and equilibria between phases. American mathematical physicist
J. Willard Gibbs's work on the applications of
thermodynamics was instrumental in transforming
physical chemistry into a rigorous deductive science. During the years from 1876 to 1878, Gibbs worked on the principles of thermodynamics, applying them to the complex processes involved in chemical reactions. He discovered the concept of
chemical potential, or the "fuel" that makes chemical reactions work. In 1876 he published his most famous contribution, "
On the Equilibrium of Heterogeneous Substances", a compilation of his work on thermodynamics and physical chemistry which laid out the concept of
free energy to explain the physical basis of chemical equilibria. In these essays were the beginnings of Gibbs' theories of phases of matter: he considered each state of matter a phase, and each substance a component. Gibbs took all of the variables involved in a chemical reaction – temperature, pressure, energy, volume, and entropy – and included them in one simple equation known as
Gibbs' phase rule. Within this paper was perhaps his most outstanding contribution, the introduction of the concept of free energy, now universally called
Gibbs free energy in his honor. The Gibbs free energy relates the tendency of a physical or chemical system to simultaneously lower its energy and increase its disorder, or
entropy, in a spontaneous natural process. Gibbs's approach allows a researcher to calculate the change in free energy in the process, such as in a chemical reaction, and how fast it will happen. Since virtually all chemical processes and many physical ones involve such changes, his work has significantly impacted both the theoretical and experiential aspects of these sciences. In 1877,
Ludwig Boltzmann established statistical derivations of many important physical and chemical concepts, including
entropy, and distributions of molecular velocities in the gas phase. Together with Boltzmann and
James Clerk Maxwell, Gibbs created a new branch of theoretical physics called
statistical mechanics (a term that he coined), explaining the laws of thermodynamics as consequences of the statistical properties of large ensembles of particles. Gibbs also worked on the application of Maxwell's equations to problems in physical optics. Gibbs's derivation of the phenomenological laws of thermodynamics from the statistical properties of systems with many particles was presented in his highly influential textbook
Elementary Principles in Statistical Mechanics, published in 1902, a year before his death. In that work, Gibbs reviewed the relationship between the laws of thermodynamics and the statistical theory of molecular motions. The overshooting of the original function by partial sums of
Fourier series at points of discontinuity is known as the
Gibbs phenomenon.
Late 19th century Carl von Linde and the modern chemical process , father of the industrial gas industry and modern refrigiation German engineer
Carl von Linde's invention of a continuous process of liquefying gases in large quantities formed a basis for the modern technology of
refrigeration and provided both impetus and means for conducting scientific research at low temperatures and very high vacuums. He developed a
dimethyl ether refrigerator (1874) and an ammonia refrigerator (1876). Though other refrigeration units had been developed earlier, Linde's were the first to be designed with the aim of precise calculations of efficiency. In 1895 he set up a large-scale plant for the production of liquid air. Six years later he developed a method for separating pure liquid oxygen from liquid air that resulted in widespread industrial conversion to processes utilizing oxygen (e.g., in
steel manufacture). He founded the Linde plc, the world's largest
industrial gas company by market share and revenue. In 1883,
Svante Arrhenius developed an
ion theory to explain conductivity in
electrolytes. In 1884,
Jacobus Henricus van 't Hoff published (Studies in Dynamic Chemistry), a seminal study on
chemical kinetics. In this work, van 't Hoff entered for the first time the field of physical chemistry. Of great importance was his development of the general thermodynamic relationship between the heat of conversion and the displacement of the equilibrium as a result of temperature variation. At constant volume, the equilibrium in a system will tend to shift in such a direction as to oppose the temperature change which is imposed upon the system. Thus, lowering the temperature results in heat development while increasing the temperature results in heat absorption. This principle of mobile equilibrium was subsequently (1885) put in a general form by
Henry Louis Le Chatelier, who extended the principle to include compensation, by change of volume, for imposed pressure changes. The van 't Hoff-Le Chatelier principle, or simply
Le Chatelier's principle, explains the response of
dynamic chemical equilibria to external stresses. In 1884,
Hermann Emil Fischer proposed the structure of
purine, a key structure in many biomolecules, which he later synthesized in 1898. He also began work on the chemistry of
glucose and related
sugars. In 1885,
Eugen Goldstein named the
cathode ray, later discovered to be composed of electrons, and the
canal ray, later discovered to be positive hydrogen ions that had been stripped of their electrons in a
cathode-ray tube; these would later be named
protons. The year 1885 also saw the publishing of J. H. van 't Hoff's (Chemical equilibria in gaseous systems or strongly diluted solutions), which dealt with this theory of dilute solutions. Here he demonstrated that the "
osmotic pressure" in solutions which are sufficiently dilute is proportionate to the
concentration and the absolute temperature so that this pressure can be represented by a formula that only deviates from the formula for gas pressure by a coefficient
i. He also determined the value of
i by various methods, for example by means of the
vapor pressure and
François-Marie Raoult's results on the lowering of the freezing point. Thus van 't Hoff was able to prove that thermodynamic laws are not only valid for gases, but also for dilute solutions. His pressure laws, given general validity by the electrolytic dissociation theory of Arrhenius (1884–1887) – the first foreigner who came to work with him in Amsterdam (1888) – are considered the most comprehensive and important in the realm of natural sciences. In 1893,
Alfred Werner discovered the octahedral structure of cobalt complexes, thus establishing the field of
coordination chemistry.
Ramsay's discovery of the noble gases The most celebrated discoveries of Scottish chemist
William Ramsay were made in inorganic chemistry. Ramsay was intrigued by the British physicist
John Strutt, 3rd Baron Rayleigh's 1892 discovery that the atomic weight of
nitrogen found in chemical compounds was lower than that of nitrogen found in the atmosphere. He ascribed this discrepancy to a light gas included in chemical compounds of nitrogen, while Ramsay suspected a hitherto undiscovered heavy gas in atmospheric nitrogen. Using two different methods to remove all known gases from air, Ramsay and Lord Rayleigh were able to announce in 1894 that they had found a monatomic, chemically inert gaseous element that constituted nearly 1 percent of the atmosphere; they named it
argon. The following year, Ramsay liberated another inert gas from a mineral called
cleveite; this proved to be
helium, previously known only in the solar spectrum. In his book
The Gases of the Atmosphere (1896), Ramsay showed that the positions of helium and argon in the periodic table of elements indicated that at least three more noble gases might exist. In 1898 Ramsay and the British chemist
Morris W. Travers isolated these elements—called
neon,
krypton, and
xenon—from air and brought them to a liquid state at low temperature and high pressure. Sir William Ramsay worked with
Frederick Soddy to demonstrate, in 1903, that alpha particles (helium nuclei) were continually produced during the radioactive decay of a sample of radium. Ramsay was awarded the 1904
Nobel Prize for Chemistry in recognition of "services in the discovery of the inert gaseous elements in the air, and his determination of their place in the periodic system." In 1897,
J. J. Thomson discovered the
electron using the cathode-ray tube. In 1898,
Wilhelm Wien demonstrated that canal rays (streams of positive ions) can be deflected by magnetic fields and that the amount of deflection is proportional to the
mass-to-charge ratio. This discovery would lead to the
analytical technique known as
mass spectrometry in 1912.
Marie and Pierre Curie , a pioneer in the field of radioactivity and the first twice-honored Nobel laureate (and still the only one in two different sciences)
Marie Skłodowska-Curie was a Polish-born French physicist and chemist who is famous for her pioneering research on
radioactivity. She and her husband are considered to have laid the cornerstone of the nuclear age with their research on radioactivity. Marie was fascinated with the work of
Henri Becquerel, a French physicist who discovered in 1896 that uranium casts off rays similar to the
X-rays discovered by
Wilhelm Röntgen. Marie Curie began studying uranium in late 1897 and theorized, according to a 1904 article she wrote for
Century magazine, "that the emission of rays by the compounds of uranium is a property of the metal itself—that it is an atomic property of the element uranium independent of its chemical or physical state." Curie took Becquerel's work a few steps further, conducting her own experiments on uranium rays. She discovered that the rays remained constant, no matter the condition or form of the uranium. The rays, she theorized, came from the element's atomic structure. This revolutionary idea created the field of
atomic physics and the Curies coined the word
radioactivity to describe the phenomenon. , known for his work on radioactivity as well as on
ferromagnetism,
paramagnetism, and
diamagnetism; notably
Curie's law and
Curie point Pierre and Marie further explored radioactivity by working to separate the substances in uranium ores and then using the
electrometer to make radiation measurements to 'trace' the minute amount of unknown radioactive element among the fractions that resulted. Working with the mineral
pitchblende, the pair discovered a new radioactive element in 1898. They named the element
polonium, after Marie's native country of Poland. On December 21, 1898, the Curies detected the presence of another radioactive material in the pitchblende. They presented this finding to the
French Academy of Sciences on December 26, proposing that the new element be called
radium. The Curies then went to work isolating polonium and radium from naturally occurring compounds to prove that they were new elements. In 1902, the Curies announced that they had produced a decigram of pure radium, demonstrating its existence as a unique chemical element. While it took three years for them to isolate radium, they were never able to isolate polonium. Along with the discovery of two new elements and finding techniques for isolating radioactive isotopes, Curie oversaw the world's first studies into the treatment of
neoplasms, using radioactive isotopes. With Henri Becquerel and her husband, Pierre Curie, she was awarded the 1903
Nobel Prize for Physics. She was the sole winner of the 1911
Nobel Prize for Chemistry. She was the first woman to win a Nobel Prize, and she is the only woman to win the award in two different fields. While working with Marie to extract pure substances from ores, an undertaking that really required industrial resources but that they achieved in relatively primitive conditions, Pierre himself concentrated on the physical study (including luminous and chemical effects) of the new radiations. Through the action of magnetic fields on the rays given out by the radium, he proved the existence of particles that were electrically positive, negative, and neutral; these
Ernest Rutherford was afterward to call alpha, beta, and gamma rays. Pierre then studied these radiations by
calorimetry and also observed the physiological effects of radium, thus opening the way to radium therapy. Among Pierre Curie's discoveries were that ferromagnetic substances exhibited a critical temperature transition, above which the substances lost their ferromagnetic behavior – this is known as the "
Curie point." He was elected to the Academy of Sciences (1905), having in 1903 jointly with Marie received the Royal Society's prestigious Davy Medal and jointly with her and Becquerel the Nobel Prize for Physics. He was run over by a carriage in the
rue Dauphine in Paris in 1906 and died instantly. His complete works were published in 1908.
Ernest Rutherford , discoverer of the nucleus and considered the father of nuclear physics New Zealand-born chemist and physicist
Ernest Rutherford is considered to be "the father of
nuclear physics." Rutherford is best known for devising the names
alpha,
beta, and
gamma to classify various forms of radioactive "rays" which were poorly understood at his time (alpha and beta rays are particle beams, while gamma rays are a form of high-energy
electromagnetic radiation). Rutherford deflected alpha rays with both electric and magnetic fields in 1903. Working with
Frederick Soddy, Rutherford explained that
radioactivity is due to the
transmutation of elements, now known to involve
nuclear reactions. He also observed that the intensity of radioactivity of a radioactive element decreases over a unique and regular amount of time until a point of stability, and he named the halving time the "
half-life". In 1901 and 1902 he worked with Frederick Soddy to prove that atoms of one radioactive element would spontaneously turn into another, by expelling a piece of the atom at high velocity. In 1906 at the University of Manchester, Rutherford oversaw an experiment conducted by his students
Hans Geiger (known for the
Geiger counter) and
Ernest Marsden. In the
Geiger–Marsden experiment, a beam of alpha particles, generated by the radioactive decay of
radon, was directed normally onto a sheet of very thin gold foil in an evacuated chamber. Under the prevailing
plum pudding model, the alpha particles should all have passed through the foil and hit the detector screen, or have been deflected by, at most, a few degrees. However, the actual results surprised Rutherford. Although many of the alpha particles did pass through as expected, many others were deflected at small angles while others were reflected back to the alpha source. They observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. The gold foil experiment showed large deflections for a small fraction of incident particles. Rutherford realized that, because some of the alpha particles were deflected or reflected, the atom had a concentrated centre of positive charge and of relatively large mass – Rutherford later termed this positive center the "
atomic nucleus". The alpha particles had either hit the positive centre directly or passed by it close enough to be affected by its positive charge. Since many other particles passed through the gold foil, the positive centre would have to be a relatively small size compared to the rest of the atom – meaning that the atom is mostly open space. From his results, Rutherford developed a model of the atom that was similar to the
Solar System, known as the
Rutherford model. Like planets, electrons orbited a central, Sun-like nucleus. For his work with radiation and the atomic nucleus, Rutherford received the 1908 Nobel Prize in Chemistry. == 20th century ==