Electronic configuration of helium

Electrons in each shell

Helium has 2 electrons arranged as 2 across 1 shell, which is why it sits in period 1.

How to write it

Electrons fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… Each s orbital holds 2 electrons, p holds 6 and d holds 10. Placing Helium's 2 electrons in that order gives 1s².

Helium at a glance

Atomic number	2
Symbol	He
Atomic mass	4.0026 u
Group	18
Period	1
Block	s-block
Category	Noble gas
Valence electrons	2

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Other elements in group 18

Elements in the same group have the same number of outer electrons, which is why they behave alike.