Calcium
Calcium is a chemical element with symbol Ca and atomic number 20. Calcium is a soft gray alkaline earth metal, fifth-most-abundant element by mass in the Earth's crust. The ion Ca2+ is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.
At a glance
- Atomic number
- 20
- Atomic mass
- 40.0784 u
- Phase
- Solid
- Density
- 1.55 g/cm³
- Melting point
- 1,115 K
- Boiling point
- 1,757 K
- First ionization energy
- 589.8 kJ/mol
- Electronegativity
- 1
- Electron affinity
- 2.37 kJ/mol
- Molar heat capacity
- 25.929 J/(mol·K)
- Period
- 4
- Appearance
- Unavailable
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Calcium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
18 is the [Ar] core total across three shells.
Preparing the orbital view…
20 protons20 neutrons
Calcium-40: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, calcium is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
CaNeutral atom · no charge
A neutral calcium atom. Removing its first electron takes 590 kJ/mol.
Calcium’s outer shell holds 2 electrons. Electron 3 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 4.3× as much energy as the one before. That’s why calcium forms Ca²⁺ ions. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 2, then 8, then 8, then 2 bars. Read backwards, that’s 2, 8, 8, 2: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 2 electrons before the first big jump are calcium’s valence electrons, the ones it uses in chemical reactions. That’s why calcium sits in group 2: every element in the group has the same number of outer electrons, so they react in similar ways.
- The numbers explain the charge. Removing electron 3 as well, to make Ca³⁺, would cost 4,912 kJ/mol. Forming a compound gives back far less energy than that, so Ca³⁺ doesn’t form in ordinary chemistry.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 4 | 590 | – | Measured |
| 2 | 4 | 1,145 | 1.9 | Measured |
| 3 | 3 | 4,912 | 4.3 | Measured |
| 4 | 3 | 6,491 | 1.3 | Estimated |
| 5 | 3 | 8,138 | 1.3 | Estimated |
| 6 | 3 | 10,496 | 1.3 | Estimated |
| 7 | 3 | 12,274 | 1.2 | Estimated |
| 8 | 3 | 14,207 | 1.2 | Measured |
| 9 | 3 | 18,191 | 1.3 | Estimated |
| 10 | 3 | 20,385 | 1.1 | Estimated |
| 11 | 2 | 57,081 | 2.8 | Estimated |
| 12 | 2 | 63,507 | 1.1 | Estimated |
| 13 | 2 | 70,299 | 1.1 | Estimated |
| 14 | 2 | 78,848 | 1.1 | Estimated |
| 15 | 2 | 86,258 | 1.1 | Estimated |
| 16 | 2 | 93,948 | 1.1 | Estimated |
| 17 | 2 | 104,860 | 1.1 | Estimated |
| 18 | 2 | 111,704 | 1.1 | Calculated |
| 19 | 1 | 494,860 | 4.4 | Calculated |
| 20 | 1 | 527,762 | 1.1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
How does it compare?
Density
1.55 g/cm³16th lowest of 99
Melting point
1,115 K44th lowest of 96
Boiling point
1,757 K35th lowest of 97
Electronegativity
110th lowest of 96
First ionization energy
590 kJ/mol31st lowest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- CalciumRoyal Society of Chemistry
- CalciumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database