Calcium (Ca)

Element 20 · alkaline earth metal · group 2, period 4

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

Calcium is element 20, a alkaline earth metal in group 2 and period 4. Its electron configuration is [Ar] 4s2. That leaves 2 valence electrons in the outer shell, with the shells filling 2, 8, 8, 2. Losing its 2 valence electrons is the shorter route to a full shell, so it carries a charge of +2.

20 protons
2 valence e−
+2
s-block

Properties

Atomic number	20
Symbol	Ca
Standard atomic weight	40.078 u
Group	2
Period	4
Block	s-block
Category	alkaline earth metal
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Noble-gas notation	[Ar] 4s²
Electrons per shell	2, 8, 8, 2
Valence electrons	2
Common ionic charge	+2
Electronegativity (Pauling)	1
Protons / electrons	20 / 20

Electron configuration of calcium

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

Everything up to Ar is the same as in that noble gas, so it is usually abbreviated to [Ar] 4s². 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
4s	2	2

Calcium ion charge

Calcium commonly forms ions with a charge of +2.

It has 2 valence electrons, so it is 6 short of a full octet and 2 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 calcium have?

Calcium has 2 valence electrons. For groups 1 and 2 the count is the group number, and calcium is in group 2.

What is the electron configuration of calcium?

1s2 2s2 2p6 3s2 3p6 4s2, or [Ar] 4s2 written from the previous noble gas. Orbitals fill lowest energy first.

What is the charge of calcium?

Calcium commonly forms ions with a charge of +2. With 2 valence electrons it is 6 short of an octet and 2 above an empty shell, so it takes whichever route is shorter.

How many protons, electrons and neutrons does calcium have?

A neutral calcium atom has 20 protons and 20 electrons. Its standard atomic weight is 40.078, so the most common isotope has about 20 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 20 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.