---
title: "Calcium (Ca) — element 20"
description: "Calcium is element 20, symbol Ca, atomic weight 40.078. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/calcium/
site: "At One Place"
---

# Calcium (Ca)

**Calcium — element 20: Ca**

40.078 u · Alkaline earth metal · period 4, group 2

About 99% of the calcium in a human body is in bone and teeth, but the other 1% does most of the interesting work: a calcium ion crossing a membrane is the trigger for every muscle contraction, including each heartbeat. Limestone, chalk, marble and seashells are all calcium carbonate, laid down mostly by organisms, and heating it to about 900 °C to drive off carbon dioxide gives quicklime — a reaction humans have been running for at least 9,000 years. The metal itself is silvery and soft and almost nobody has seen it, because it tarnishes within seconds in air.

## Properties

- **Atomic number:** 20
- **Symbol:** Ca
- **Standard atomic weight:** 40.078 u
- **Category:** Alkaline earth metal
- **Group:** 2
- **Period:** 4
- **Block:** s-block
- **Electron configuration:** [Ar] 4s²
- **Electrons per shell:** 2, 8, 8, 2
- **Melting point:** 1,115 K — 841.9 °C — 1547.3 °F
- **Boiling point:** 1,757 K — 1483.8 °C — 2702.9 °F
- **Density:** 1.54 g/cm³
- **Electronegativity:** 1 (Pauling)
- **Oxidation states:** +2
- **Discovered:** 1808 by Humphry Davy

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
- **Noble gas shorthand:** [Ar] 4s²
- **Shells:** K: 2 · L: 8 · M: 8 · N: 2

## What it is used for

- Cement and concrete, via limestone and quicklime
- Steelmaking, where lime removes phosphorus and sulfur
- Plaster, mortar and agricultural lime
- Calcium carbonate as an antacid and a dietary supplement
- A reducing agent for extracting uranium, thorium and zirconium

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 18 | Argon | Ar | 39.95 |
| 19 | Potassium | K | 39.098 |
| 20 | Calcium ← | Ca | 40.078 |
| 21 | Scandium | Sc | 44.956 |
| 22 | Titanium | Ti | 47.867 |

## Group 2

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Calcium sits in group 2 with beryllium, magnesium, strontium, barium, radium.

## Common questions

### What is the atomic number of calcium?

20. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for calcium?

Ca.

### What is the electron configuration of calcium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s², or [Ar] 4s² in noble-gas shorthand.

### What is the melting point of calcium?

1,115 K — 841.9 °C — 1547.3 °F

## Related pages

- [Argon (Ar)](https://atoneplace.net/element/argon/)
- [Potassium (K)](https://atoneplace.net/element/potassium/)
- [Scandium (Sc)](https://atoneplace.net/element/scandium/)
- [Titanium (Ti)](https://atoneplace.net/element/titanium/)
- [Beryllium (Be)](https://atoneplace.net/element/beryllium/)
- [Magnesium (Mg)](https://atoneplace.net/element/magnesium/)
- [Strontium (Sr)](https://atoneplace.net/element/strontium/)
- [Barium (Ba)](https://atoneplace.net/element/barium/)
- [Radium (Ra)](https://atoneplace.net/element/radium/)

## Sources

- Official vendor product documentation — At One Place (/about/)