Chemical element · Atomic number 20
Calcium
Calcium in the periodic table: atomic number 20, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Alkaline earth metal
solid
40.08 u
Documented element samplePure calcium is kept under argon because the metal reacts quickly in moist air.
Image credit: Matthias Zepper
Auto-oriented, limited to 1600 × 1200 pixels and re-encoded as WebP; the subject was not altered.
Atomic classification
Shell occupancy
Calcium in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 8 electrons
- N · n=4
- 2 electrons
- Electron configuration
- [Ar]4s2
- Electrons per shell
- 2 · 8 · 8 · 2
- Group
- 2
- Period
- 4
- Block
- S
- Element category
- Alkaline earth metal
Physical and chemical properties
- Atomic mass
- 40.08 u
- Standard state
- solid
- Density
- 1.54 g/cm³
- Melting point
- 1,115 K
- Boiling point
- 1,757 K
- Electronegativity
- 1 (Pauling)
- First ionisation energy
- 6.113 eV
- Oxidation states
- +2
- Discovery
- known since antiquity
Safety and periodic classification
Safety
Safe handling cannot be inferred from Calcium's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Calcium is in period 4, group 2 and the S block. Its direct neighbours by atomic number are Potassium and Scandium. The recorded Pauling electronegativity is 1. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Calcium sits among the 118 elements
The table compares the reported values for Calcium with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Calcium | Position in the data set |
|---|---|---|
| Melting point | 1,115 K | Rank 55 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 1,757 K | Rank 61 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 1.54 g/cm³ | Rank 81 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 1 | Rank 86 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 6.113 eV | Rank 71 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Calcium in the periodic table
- Place in the table
- Group 2, period 4, S block. Classified as alkaline earth metal, solid at standard conditions.
- Direct neighbours
- Atomic number 20. Preceded by Potassium, followed by Scandium.
- Same group
- Same number of outer electrons, and therefore similar reactivity to Beryllium, Magnesium, Strontium, Barium, Radium.
- Discovery
- Recorded in the snapshot as: known since antiquity. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +2. They follow from the electron configuration [Ar]4s2 and indicate which compounds to expect.
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