Classification of elements in the periodic tableAQA A-Level Chemistry: Revision notes

The Periodic Table lists the elements in order of increasing proton number (atomic number). The proton number is the number of protons in the nucleus, and in a neutral atom it also equals the number of electrons.

Because the electrons of an atom fill sub-shells in a fixed order, the proton number fixes the electron configuration. This is why elements with similar configurations of outer electrons fall in the same region of the table.

An element is classified as s, p, d or f block according to its position in the Periodic Table. The block is the type of sub-shell that contains the highest-energy electron, which is the last electron added when the elements are built up in order of proton number.

• s block: Groups 1 and 2. The outer electrons are in an s sub-shell, e.g. sodium 1s²2s²2p⁶3s¹.
• p block: the six columns on the right (Groups 3 to 0, or 13 to 18). The last electron enters a p sub-shell, e.g. chlorine 1s²2s²2p⁶3s²3p⁵.
• d block: the block of 10 columns between Groups 2 and 3 in Period 4 and below, which includes the transition metals. The last electron enters a d sub-shell, e.g. iron [Ar]3d⁶4s².
• f block: the two rows set apart at the bottom (lanthanides and actinides). The last electron enters an f sub-shell.

Each orbital holds up to two electrons, and each type of sub-shell has a fixed number of orbitals.

• s: 1 orbital, maximum 2 electrons, 2 elements per row
• p: 3 orbitals, maximum 6 electrons, 6 elements per row
• d: 5 orbitals, maximum 10 electrons, 10 elements per row
• f: 7 orbitals, maximum 14 electrons, 14 elements per row

The block widths in the table match the number of electrons each sub-shell can hold.

To find the block of an element from its proton number:

1. Find the element in the Periodic Table, or write its electron configuration by filling sub-shells in order of energy (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, ...).
2. Identify the sub-shell that holds the highest-energy electron.

Worked examples

• Proton number 12 (Mg): 1s²2s²2p⁶3s², last electron in 3s, so s block.
• Proton number 17 (Cl): 1s²2s²2p⁶3s²3p⁵, last electron in 3p, so p block.
• Proton number 26 (Fe): 1s²2s²2p⁶3s²3p⁶3d⁶4s², last sub-shell filled is 3d, so d block.

For Period 4, the 4s sub-shell fills at K and Ca (proton numbers 19 and 20), 3d at Sc to Zn (21 to 30) and 4p at Ga to Kr (31 to 36).