A block of the periodic table is a set of elements unified by the orbitals their valence electrons or vacancies lie in. The term appears to have been first used by Charles Janet. Each block is named after its characteristic orbital: s-block, p-block, d-block, and f-block.
The block names (s, p, d, and f) are derived from the spectroscopic notation for the value of an electron's azimuthal quantum number: sharp (0), principal (1), diffuse (2), or fundamental (3). Succeeding notations proceed in alphabetical order, as g, h, etc.
In Foundations of Chemistry, 2017
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 encompasses nearly all of the lanthanides (like lanthanum) and the actinides (like actinium). Not everyone agrees on the exact membership of each set of elements. For example, the group 12 elements zinc, cadmium, and mercury are often 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 sometimes considered main group elements due to their similarities to the s-block elements. 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.
Wilkins RG and Wilkins PC (2003)
The role of calcium and comparable cations in animal behaviour, RSC, Cambridge, p. 1
The s-block 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.
The p-block 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 icosagens; 14, the crystallogens; 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. This is not case in group 18, where reactivity increases in the following sequence: Ne < He < Ar < Kr < Xe < Rn < Og (although helium, which breaks the trend, is not a part of the p-block; therefore the p-block portion of group 18 conforms to the trend).
Oxygen and the halogens tend to form more ionic compounds with metals; the remaining reactive nonmetals tend to form more covalent compounds, although ionicity is possible when the electronegativity difference is high enough (e.g. Li3N). The metalloids tend to form either covalent compounds or alloys with metals.
Kneen WR, Rogers MJW, and Simpson P 1972
Chemistry: Facts, patterns, and principles, Addison-Wesley, London, pp. 487-489
The d-block is in the middle of the periodic table and encompasses elements from groups 3 to 12; it starts in the 4th period. 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, 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 oxidation numbers as low as -4; iridium holds the singular distinction of being capable of achieving an oxidation state of +9.
Dolg M 2015 (ed.)
Computational methods in lanthanide and actinide chemistry, John Wiley & Sons, Chichester, p. xvii
The f-block appears as a footnote in a standard 18-column table but is located at the center-left of a 32-column full width table. While these elements are generally not considered part of any group, some authors consider them to be part of group 3. 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, in periods 6 and 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 f-block elements 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. The lanthanides are the 15 elements running from lanthanum (La) to lutetium (Lu); the actinides are the 15 elements running from actinium (Ac) to lawrencium (Lr).
A g-block 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.
The tetrahedral periodic table of elements. Animation showing a transition from the conventional table into a tetrahedron.