Calcium

Calcium (Ca) is an alkaline earth metal with atomic number 20 and an atomic mass of 40.078. Its electron configuration is [Ar] 4s2, which gives it 2 valence electrons.

Calcium key data

Symbol	Ca
Atomic number	20
Atomic mass (standard atomic weight)	40.078
Molar mass	40.078 g/mol
Protons	20
Neutrons (most abundant isotope, calcium-40)	20
Electrons (neutral atom)	20
Valence electrons	2
Category	Alkaline earth metal
Period	4
Group	2
Block	s-block
Noble gas notation	[Ar] 4s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 4s2
Electrons per shell	2, 8, 8, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of calcium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown. The discovery year is the year calcium was first isolated, from Wikipedia’s timeline of chemical element discoveries, in place of PubChem’s “Ancient”.

Melting point	1115 K (841.85 °C), 55th of 103 by melting point
Boiling point	1757 K (1483.9 °C), 61st of 93 by boiling point
Density	1.54 g/cm³, 81st of 96 by density
State at room temperature	Solid
Electronegativity	1 (Pauling), 86th of 95 by electronegativity
Van der Waals radius	231 pm, 26th of 99 by atomic radius
First ionisation energy	6.113 eV (589.8 kJ/mol), 72nd of 102 by first ionisation energy
Common oxidation states	+2
Discovered	1808

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Calcium electron configuration

The full electron configuration of calcium is 1s2 2s2 2p6 3s2 3p6 4s2. With the noble-gas core collapsed it is [Ar] 4s2, and the outermost subshell is 4s2.

Bohr model of calcium (Ca): a nucleus and four shells holding 2, 8, 8 and 2 electrons from the inside out. — Bohr model of calcium (Ca). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of calcium (Ca), 1s2 2s2 2p6 3s2 3p6 4s2: one box per orbital, filled by Hund’s rule, showing every orbital drawn holds a pair of electrons. — Orbital diagram of calcium (Ca). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one calcium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 40.078 / (6.02214076 × 10²³)
= 6.6551 × 10⁻²³ g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 40.078
= 1.5026 × 10²² atoms

Calcium on the periodic table

Calcium is in period 4, group 2, in the s-block.

The other elements of group 2 are beryllium, magnesium, strontium, barium and radium.