The s, p, and d Blocks

The periodic table is divided into blocks based on the subshell being filled by the outermost (highest-energy) electrons. This classification explains many patterns in properties and behaviour. Understanding the blocks helps you predict an element's chemistry from its position in the table.

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Block Definitions

The block of an element is determined by the subshell that contains its highest-energy electron:

Block	Subshell Being Filled	Groups	Elements
s-block	s subshell	1 and 2 (plus He)	Li, Be, Na, Mg, K, Ca, etc.
p-block	p subshell	13–18	B, C, N, O, F, Ne, Al, Si, etc.
d-block	d subshell	3–12	Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc.
f-block	f subshell	Lanthanides, Actinides	La, Ce, Pr... / Ac, Th, Pa...

Note: Helium (1s²) is placed in Group 18 with the noble gases due to its chemical properties, but it is technically an s-block element by electron configuration.

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The s-Block: Groups 1 and 2

The s-block elements have their outermost electron(s) in an s subshell.