A block of the periodic table is a set of chemical elements having their differentiating electrons predominately in the same atomic orbital type. A differentiating electron is the electron that differentiates an element from the previous one. For example, sodium [Ne] 3s1, when compared to neon [He] 2s22p6, has a difference in one s-electron. The term appears to have been first used by Charles Janet.^[1] 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 values of electrons' azimuthal quantum number: sharp (0), principal (1), diffuse (2), and fundamental (3).

There is an approximate correspondence between this nomenclature of blocks, based on electronic configuration, and groupings 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, some scientists regard the group 12 elements Zn, Cd and Hg 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 3 and 4) are not numbered.

Helium is from the s-block, with its outer (and only) electrons in the 1s atomic orbital, although its chemical properties are more similar to the p-block noble gases due to its full shell.

The s-block is on the left side of the conventional periodic table and it is composed of elements from the first two columns, the alkali metals (group 1) and alkaline earth metals (group 2), plus the nonmetals hydrogen and helium. Their general valence configuration is ns1–2. Helium is an s-element, but nearly always finds its place far right into 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 row onwards) are 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 generally form ionic compounds with nonmetals.

The p-block is on the right side of the periodic table and includes elements from the six columns beginning with column 13 and ending with column 18. Their general valence configuration is ns2 np1–6. Helium, though being in the top of group 18, is not included in the p-block. Except for the first row (which has none), each row of the table has place for six p-elements.

This block contains a variety of elements and is the only block that contains all three types of elements: metals, nonmetals, and metalloids. Generally, the p-block elements are best described in terms of element type or group.

The p-block elements are unified by the fact that their valence electrons (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.

They are mostly covalent in nature and the block is a stronghold of the octet rule, especially columns 14–17. They show variable oxidation states. The reactivity of elements in a group generally decreases downwards.

The d-block is on the middle of the periodic table and includes elements from columns 3 through 12. It appears beginning with the 4th row. These elements are also known as the transition metals because they show a transitivity in their properties i.e. they show a trend in their properties in simple incomplete d orbitals. Transition basically means d orbital lies between s and p orbitals and shows a transition from properties of s to p.

The d-block elements are all metals which exhibit two or more ways of forming chemical bonds. 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. This results in the same element exhibiting two or more oxidation states, which determines the type and number of its nearest neighbors in chemical compounds.

The d-block elements are unified by mostly having one or more chemically active d-orbital electrons. The d-orbitals can contain up to five pairs of electrons; they have a crucial importance for chemistry of elements in columns 3–11. Due to capacity of the d-subshell the block includes 10 columns in the periodic table.

The f-block is in the center-left of a 32-column periodic table but is footnoted in 18-column tables. These elements are not generally considered part of any group. They are often 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 known f-block elements come in two series, the lanthanides of period 6 and the radioactive actinides of period 7. All are metals. Because the f-orbital electrons are less active in determining the chemistry of these elements, their chemical properties are mostly determined by outer s-orbital electrons. Consequently, there is much less chemical variability within the f-block than within the s-, p-, or d-blocks.

The f-block elements are unified by mostly having one or more of their outermost electrons in an f-orbital. The f-orbitals can contain up to seven pairs of electrons; hence, the block includes fourteen columns in the periodic table.