Calcium (Ca)
Calcium is element 20, an alkaline earth metal in group 2, period 4. A neutral atom has 20 protons, 20 neutrons and 20 electrons, 2 of them valence electrons.
Alkaline earth metal- Atomic number
- 20
- Atomic mass
- 40.08 u
- Valence electrons
- 2
- Protons · neutrons
- 20 · 20
- Electrons per shell
- 2, 8, 8, 2
- At room temperature
- solid
Bohr model of calcium
The Bohr model draws calcium as a nucleus of 20 protons and 20 neutrons, with its 20 electrons on 4 circular shells: 2 in shell 1, 8 in shell 2, 8 in shell 3, 2 in shell 4. Press Build the atom to watch the shells fill from the inside out. The outer ring (the N shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of calcium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Short form (noble gas notation): [Ar] 4s². The [Ar] stands for the electrons of argon, the noble gas before it.
How many valence electrons does calcium have?
Calcium has 2 valence electrons.
Calcium is in group 2, so it has 2 valence electrons. It tends to lose them and form a +2 ion.
Protons, neutrons and electrons
| Protons | 20 | equal to the atomic number |
|---|---|---|
| Electrons | 20 | equal to the protons in a neutral atom |
| Neutrons | 20 | mass number 40 − 20 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Calcium properties
| Symbol | Ca |
|---|---|
| Category | Alkaline earth metal |
| Group · period · block | 2 · 4 · s |
| Atomic mass | 40.08 u |
| State at 25 °C | solid |
| Melting point | 1,115 K (842 °C, 1,550 °F) |
| Boiling point | 1,757 K (1,480 °C, 2,700 °F) |
| Density | 1.54 g/cm³ |
| Electronegativity (Pauling) | 1 |
| Atomic radius | 231 pm |
| First ionization energy | 6.113 eV |
| Electron affinity | — |
| Oxidation states | +2 |
| Common ion charge | +2 |
| Discovered | Known since ancient times |
Where calcium sits on the periodic table
Group 2: Beryllium, Magnesium, Strontium, Barium, Radium.
Questions about calcium
How many valence electrons does calcium have?
Calcium has 2 valence electrons.
How many protons, neutrons and electrons does calcium have?
A neutral calcium atom has 20 protons and 20 electrons. Its most common form has about 20 neutrons (mass number 40 − 20 protons).
What is the electron configuration of calcium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s², written in short form as [Ar] 4s².
What does the Bohr model of calcium look like?
A nucleus of 20 protons and 20 neutrons, circled by 4 electron shells holding 2, 8, 8, 2 electrons.
What is the atomic number of calcium?
20. The atomic number is the number of protons in the nucleus, and it is what makes an atom calcium.