Periodic table blocks

Periodic table blocks are regions whose elements are grouped by the type of atomic orbital being filled by their differentiating electrons: s, p, d, or f.

Atomic orbital: An atomic orbital is a quantum state that describes an electron’s spatial distribution and energy in an atom. Each block is named for the orbital type that receives the differentiating electron.
s subshell: An s subshell contains one orbital and can hold up to two electrons. Its two-electron capacity gives the s block its characteristic width.
Alkali metal: An alkali metal is a Group 1 element, excluding hydrogen, with one valence electron in an s orbital. These elements occupy the leftmost part of the s block.
Helium: Helium is a noble gas whose two electrons occupy the 1s orbital. Its configuration places it in the s block by filling, although it is displayed with noble gases.
Electron configuration: An electron configuration specifies how electrons occupy an atom’s shells, subshells, and orbitals. Configurations reveal which subshell is being filled across each block.
p subshell: A p subshell contains three orbitals and can hold up to six electrons. Its six-electron capacity accounts for the p block's six columns.
Noble gas: A noble gas is a Group 18 element with a filled outer electron shell, except helium's filled 1s shell. Most noble gases end the p block; helium is the s-block exception.
Group (periodic table): A group is a vertical column of elements in the periodic table, often sharing chemical properties. Groups organize elements by recurring properties, a different principle from block subshell filling.
Aufbau principle: The Aufbau principle describes the usual ordering in which electrons occupy atomic orbitals from lower to higher energy. Its filling sequence predicts the broad block layout, despite notable exceptions.
d subshell: A d subshell contains five orbitals and can hold up to ten electrons. Its ten-electron capacity corresponds to the d block's usual width.

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