Magnesium at a glance
- Atomic number
- 12
- Symbol
- Mg
- Atomic mass
- 24.305 u
- Category
- Alkaline earth
- Group / period
- 2 / 3
- Electron configuration
- [Ne] 3s 2
- State at 25 °C
- Solid
- Melting point
- 649.9 °C
- Boiling point
- 1090 °C
- Density
- 1.738 g/cm³
- Electronegativity
- 1.31
- 1st ionization energy
- 738 kJ/mol
- Oxidation states
- +2
- Discovered
- 1755
Atomic structure
A neutral magnesium atom has 12 protons and 12 electrons, and its most common isotope carries about 12 neutrons. One atom weighs about 4.04 × 10⁻²⁶ kg, so a mole of magnesium (6.022 × 10²³ atoms) weighs 24.305 g. That nucleus holds 1 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. The electrons fill 3 shells (K 2, L 8, M 2), which gives the configuration [Ne] 3s 2. With 2 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 3 stable isotopes: Mg-24, Mg-25 and Mg-26. No other element has a stable isotope with mass number 24, so Mg-24 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) magnesium is a solid. It melts at 649.9 °C and boils at 1090 °C, so it stays liquid across a 440-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 1.738 g/cm³, 1.74 times as dense as water. Its atoms have a radius of about 150 pm (Sodium next door is 180 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.31 on the Pauling scale. Pulling off its outermost electron takes 738 kJ/mol, less than Beryllium above it (900 kJ/mol). A bond between magnesium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. Its common oxidation states are +2, so in compounds you will usually meet it as Mg²⁺. Group 2 also holds Beryllium, Calcium, Strontium, Barium and Radium, which share its outer-shell pattern. Its Lewis symbol is Mg 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 2.33 % of Earth's crust, the 7th most abundant element there. In the human body it accounts for about 0.1 % of body mass. It was first identified in 1755, making it the 18th 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 518.4 nm, 517.3 nm and 516.7 nm.