Caesium (Cs)

Element 55 · alkali metal · group 1, period 6

Electron shells, 2 + 8 + 18 + 18 + 8 + 1. The outer shell is highlighted - it is the only one that takes part in bonding.

Caesium is element 55, a alkali metal in group 1 and period 6. Its electron configuration is [Xe] 6s1. That leaves 1 valence electron in the outer shell, with the shells filling 2, 8, 18, 18, 8, 1. Losing its 1 valence electron is the shorter route to a full shell, so it carries a charge of +1.

55 protons
1 valence e−
+1
s-block

Properties

Atomic number	55
Symbol	Cs
Standard atomic weight	132.91 u
Group	1
Period	6
Block	s-block
Category	alkali metal
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹
Noble-gas notation	[Xe] 6s¹
Electrons per shell	2, 8, 18, 18, 8, 1
Valence electrons	1
Common ionic charge	+1
Electronegativity (Pauling)	0.79
Protons / electrons	55 / 55

Electron configuration of caesium

Orbitals fill from the lowest energy upward. Writing that out for caesium gives 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹, which is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 in plain text.

Everything up to Xe is the same as in that noble gas, so it is usually abbreviated to [Xe] 6s¹. The part outside the bracket is what does the chemistry.

Subshell	Electrons	Capacity
1s	2	2
2s	2	2
2p	6	6
3s	2	2
3p	6	6
3d	10	10
4s	2	2
4p	6	6
4d	10	10
5s	2	2
5p	6	6
6s	1	2

Caesium ion charge

Caesium commonly forms ions with a charge of +1.

It has 1 valence electron, so it is 7 short of a full octet and 1 above an empty outer shell. An atom takes whichever route is shorter, which is why the charge comes out as it does.

Common questions

How many valence electrons does caesium have?

Caesium has 1 valence electron. For groups 1 and 2 the count is the group number, and caesium is in group 1.

What is the electron configuration of caesium?

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1, or [Xe] 6s1 written from the previous noble gas. Orbitals fill lowest energy first.

What is the charge of caesium?

Caesium commonly forms ions with a charge of +1. With 1 valence electron it is 7 short of an octet and 1 above an empty shell, so it takes whichever route is shorter.

How many protons, electrons and neutrons does caesium have?

A neutral caesium atom has 55 protons and 55 electrons. Its standard atomic weight is 132.91, so the most common isotope has about 78 neutrons.

See the ionic charge of every element →

How this page was produced. The configuration was built by filling orbitals in energy order from atomic number 55 alone, not looked up. The shell counts, the valence count and the charge all follow from it. Nothing on this page is written per element.