Calcium (Ca)
Calcium is the 20th element, a alkaline earth metal in the s-block. Its standard atomic weight is 40.078.
Calcium
40.078
What the table records
- Atomic number
- 20
- Standard atomic weight
- 40.078
- Group
- 2
- Period
- 4
- Block
- s-block
- Category
- Alkaline earth metal
- [Ar] 4s2
- Valence electrons
- 2
- Typical valence
- 2
- Van der Waals radius
- 2.4 Å
- Covalent radius
- 1.76 Å
Molecules containing calcium
The lightest named structures in this library that contain Ca.
How to read these numbers
The standard atomic weight is an average over the isotopes as they occur naturally, which is why it is rarely a whole number. It is the figure a molar mass calculation uses. Work in mass spectrometry wants the monoisotopic mass of a single isotope instead, which is a different number.
The two radii are not the same measurement. The covalent radius is half the distance between two bonded atoms of the element; the van der Waals radius is how close a non-bonded atom gets before repulsion takes over, and is always the larger. Space-filling models are drawn with the second, ball-and-stick models with the first — which is why the same molecule looks so different in the two.