Radium at a glance
- Atomic number
- 88
- Symbol
- Ra
- Atomic mass
- 226 u
- Category
- Alkaline earth
- Group / period
- 2 / 7
- Electron configuration
- [Rn] 7s 2
- State at 25 °C
- Solid
- Melting point
- 699.9 °C
- Boiling point
- 1737 °C
- Density
- 5.5 g/cm³
- Electronegativity
- 0.9
- 1st ionization energy
- 509 kJ/mol
- Oxidation states
- +2
- Discovered
- 1898
Atomic structure
A neutral radium atom has 88 protons and 88 electrons, and its most common isotope carries about 138 neutrons. One atom weighs about 3.75 × 10⁻²⁵ kg, so a mole of radium (6.022 × 10²³ atoms) weighs 226 g. That nucleus holds 1.57 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 18, P 8, Q 2), which gives the configuration [Rn] 7s 2. With 2 electrons in its outer shell, it tends to lose electrons and form positive ions. All of its isotopes are radioactive, so any radium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) radium is a solid. It melts at 699.9 °C and boils at 1737 °C, so it stays liquid across a 1037-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 5.5 g/cm³, 5.5 times as dense as water. Its atoms have a radius of about 215 pm (Actinium next door is 195 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.9 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 509 kJ/mol, more than Barium above it (503 kJ/mol). A bond between radium and chlorine would be mostly ionic, with an electronegativity difference of 2.3. It releases 10 kJ/mol when it gains an extra electron. Its common oxidation states are +2, so in compounds you will usually meet it as Ra²⁺. Group 2 also holds Beryllium, Magnesium, Calcium, Strontium and Barium, which share its outer-shell pattern. Its Lewis symbol is Ra surrounded by 2 dots. Handle with care: as a pure element it is radioactive.
Where it comes from
It makes up 9e-7 mg/kg of Earth's crust, the 85th most abundant element there. It was first identified in 1898, making it the 81st element to be discovered (counting the 11 known since antiquity together).