There is an
approximate correspondence between this nomenclature of blocks, based on
electronic configuration, and sets of elements based on chemical properties. The s-block and p-block together are usually considered
main-group elements, the d-block corresponds to the
transition metals, and the f-block corresponds to the inner transition metals and encompasses nearly all of the
lanthanides (like
lanthanum,
praseodymium and
dysprosium) and the
actinides (like
actinium,
uranium and
einsteinium). The
group 12 elements
zinc,
cadmium, and
mercury are sometimes regarded as main group, rather than transition group, because they are chemically and physically more similar to the p-block elements than the other d-block elements. The
group 3 elements are occasionally considered main group elements due to their similarities to the s-block elements. However, they remain d-block elements even when considered to be main group. Groups (columns) in the f-block (between groups 2 and 3) are not numbered.
Helium is an s-block element, with its outer (and only) electrons in the 1s
atomic orbital, although its chemical properties are more similar to the p-block
noble gases in group 18 due to its full shell.
s-block The s-block, with the s standing for "sharp" and
azimuthal quantum number 0, is on the left side of the conventional periodic table and is composed of elements from the first two columns plus one element in the rightmost column, the nonmetals
hydrogen and
helium and the
alkali metals (in group 1) and
alkaline earth metals (group 2). Their general valence configuration is
ns1–2.
Helium is an s-element, but nearly always finds its place to the far right in
group 18, above the p-element
neon. Each
row of the table has two s-elements. The metals of the s-block (from
the second period onwards) are mostly soft and have generally low melting and boiling points. Most impart colour to a flame. Chemically, all s-elements except
helium are highly reactive. Metals of the s-block are highly electropositive and often form essentially ionic compounds with nonmetals, especially with the highly electronegative halogen nonmetals.
p-block The p-block, with the p standing for "principal" and
azimuthal quantum number 1, is on the right side of the standard periodic table and encompasses elements in groups 13 to 18. Their general electronic configuration is
ns2
np1–6.
Helium, though being the first element in group 18, is not included in the p-block. Each row of the table has a place for six p-elements except for
the first row (which has none). This block is the only one having all three types of elements:
metals,
nonmetals, and
metalloids. The p-block elements can be described on a group-by-group basis as: group 13, the
triels; 14, the
tetrels; 15, the
pnictogens; 16, the
chalcogens; 17, the
halogens; and 18, the
helium group, composed of the
noble gases (excluding helium) and
oganesson. Alternatively, the p-block can be described as containing
post-transition metals;
metalloids;
reactive nonmetals including the
halogens; and
noble gases (excluding helium). The p-block elements are unified by the fact that their valence (outermost) electrons are in the p orbital. The p orbital consists of six lobed shapes coming from a central point at evenly spaced angles. The p orbital can hold a maximum of six electrons, hence there are six columns in the p-block. Elements in column 13, the first column of the p-block, have one p-orbital electron. Elements in column 14, the second column of the p-block, have two p-orbital electrons. The trend continues this way until column 18, which has six p-orbital electrons. The block is a stronghold of the
octet rule in its first row, but elements in subsequent rows often display
hypervalence. The p-block elements show variable oxidation states usually differing by multiples of two. The reactivity of elements in a group generally decreases downwards. (Helium breaks this trend in group 18 by being more reactive than neon, but since helium is actually an s-block element, the p-block portion of the trend remains intact.) The bonding between metals and nonmetals depends on the electronegativity difference. Ionicity is possible when the electronegativity difference is high enough (e.g.
Li3N,
NaCl,
PbO). Metals in relatively high oxidation states tend to form covalent structures (e.g.
WF6,
OsO4,
TiCl4,
AlCl3), as do the more noble metals even in low oxidation states (e.g.
AuCl,
HgCl2). There are also some metal oxides displaying
electrical (metallic) conductivity, like
RuO2,
ReO3, and
IrO2. The metalloids tend to form either covalent compounds or alloys with metals, though even then ionicity is possible with the most electropositive metals (e.g.
Mg2Si).
d-block The d-block, with the d standing for "diffuse" and azimuthal quantum number 2, is in the middle of the periodic table and encompasses elements from groups 3 to 12; it starts in the
4th period. Periods from the fourth onwards have a space for ten d-block elements. Most or all of these elements are also known as
transition metals because they occupy a transitional zone in properties, between the strongly electropositive metals of groups 1 and 2, and the weakly electropositive metals of groups 13 to 16. Group 3 or group 12, while still counted as d-block metals, are sometimes not counted as transition metals because they do not show the chemical properties characteristic of transition metals as much, for example, multiple
oxidation states and coloured compounds. The d-block elements are all metals and most have one or more chemically active d-orbital electrons. Because there is a relatively small difference in the energy of the different d-orbital electrons, the number of electrons participating in chemical bonding can vary. The d-block elements have a tendency to exhibit two or more oxidation states, differing by multiples of one. The most
common oxidation states are +2 and +3.
Chromium,
iron,
molybdenum,
ruthenium,
tungsten, and
osmium can have formal oxidation numbers as low as −4;
iridium holds the singular distinction of being capable of achieving an oxidation state of
+9, though only under far-from-standard conditions. The d-orbitals (four shaped as
four-leaf clovers, and the fifth as a
dumbbell with a ring around it) can contain up to five pairs of electrons. Some sources list
group 11 and
group 12 elements with full d-orbitals separately as ds-block elements.
f-block The f-block, with the f standing for "fundamental" and azimuthal quantum number 3, appears as a footnote in a standard 18-column table but is located at the center-left of a 32-column full-width table, between groups 2 and 3. Periods from the sixth onward have a place for fourteen f-block elements. These elements are generally not considered part of any
group. They are sometimes called inner transition metals because they provide a transition between the s-block and d-block in the
6th and
7th row (period), in the same way that the d-block
transition metals provide a transitional bridge between the s-block and p-block in the 4th and 5th rows. The f-block elements come in two series:
lanthanum through
ytterbium in period 6, and
actinium through
nobelium in period 7. All are metals. The f-orbital electrons are less active in the chemistry of the period 6 f-block elements, although they do make some contribution; these are rather similar to each other. They are more active in the early period 7 f-block elements, where the energies of the 5f, 7s, and 6d shells are quite similar; consequently these elements tend to show as much chemical variability as their transition metals analogues. The later period 7 f-block elements from about
curium onwards behave more like their period 6 counterparts. The f-block elements are unified by mostly having one or more electrons in an inner f-orbital. Of the f-orbitals, six have six lobes each, and the seventh looks like a dumbbell with a donut with two rings. They can contain up to seven pairs of electrons; hence, the block occupies fourteen columns in the periodic table. They are not assigned group numbers, since vertical periodic trends cannot be discerned in a "group" of two elements. The two 14-member rows of the f-block elements are sometimes confused with the
lanthanides and the
actinides, which are names for sets of elements based on chemical properties more so than electron configurations. Those sets have 15 elements rather than 14, extending into the first members of the d-block in their periods,
lutetium and
lawrencium respectively. In many periodic tables, the f-block is shifted one element to the right, so that lanthanum and actinium become d-block elements, and Ce–Lu and Th–Lr form the f-block tearing the d-block into two very uneven portions. This is a holdover from early erroneous measurements of electron configurations, in which the 4f shell was thought to complete its filling only at lutetium. In fact ytterbium completes the 4f shell, and on this basis
Lev Landau and
Evgeny Lifshitz considered in 1948 that lutetium cannot correctly be considered an f-block element. Since then, physical, chemical, and electronic evidence has overwhelmingly supported that the f-block contains the elements La–Yb and Ac–No, and 2021.
g-block A g-block, with azimuthal quantum number 4, is predicted to begin in the vicinity of
element 121. Though g-orbitals are not expected to start filling in the ground state until around element
124–
126 (see
extended periodic table), they are likely already low enough in energy to start participating chemically in element 121, similar to the situation of the 4f and 5f orbitals. If the trend of the previous rows continued, then the g-block would have eighteen elements. However, calculations predict a very strong blurring of periodicity in the eighth period, to the point that individual blocks become hard to delineate. It is likely that the eighth period will not quite follow the trend of previous rows. ==See also==