Calcium at a glance
- Atomic number
- 20
- Symbol
- Ca
- Atomic mass
- 40.078 u
- Category
- Alkaline earth
- Group / period
- 2 / 4
- Electron configuration
- [Ar] 4s 2
- State at 25 °C
- Solid
- Melting point
- 841.9 °C
- Boiling point
- 1484 °C
- Density
- 1.54 g/cm³
- Electronegativity
- 1
- 1st ionization energy
- 590 kJ/mol
- Oxidation states
- +2
- Discovered
- 1808
Atomic structure
A neutral calcium atom has 20 protons and 20 electrons, and its most common isotope carries about 20 neutrons. One atom weighs about 6.66 × 10⁻²⁶ kg, so a mole of calcium (6.022 × 10²³ atoms) weighs 40.078 g. That nucleus holds 1 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. The electrons fill 4 shells (K 2, L 8, M 8, N 2), which gives the configuration [Ar] 4s 2. With 2 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 6 stable isotopes: Ca-40, Ca-42, Ca-43, Ca-44, Ca-46 and Ca-48. Its most common isotope, Ca-40, has the same mass number as Ar-40: isobars, nuclei of different elements with 40 nucleons each.
Physical properties
At room temperature (25 °C) calcium is a solid. It melts at 841.9 °C and boils at 1484 °C, so it stays liquid across a 642-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 1.54 g/cm³, 1.54 times as dense as water. Its atoms have a radius of about 180 pm (Potassium next door is 220 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1 on the Pauling scale. Pulling off its outermost electron takes 590 kJ/mol, less than Magnesium above it (738 kJ/mol). A bond between calcium and chlorine would be mostly ionic, with an electronegativity difference of 2.2. It releases 2 kJ/mol when it gains an extra electron. Its common oxidation states are +2, so in compounds you will usually meet it as Ca²⁺. Group 2 also holds Beryllium, Magnesium, Strontium, Barium and Radium, which share its outer-shell pattern. Its Lewis symbol is Ca surrounded by 2 dots. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 4.15 % of Earth's crust, the 5th most abundant element there. In the human body it accounts for about 1.5 % of body mass, making it one of the main building blocks of life. It was first identified in 1808, making it the 45th element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 422.7 nm, 616.2 nm and 643.9 nm. In a flame test its compounds colour the flame orange-red.