Strontium at a glance
- Atomic number
- 38
- Symbol
- Sr
- Atomic mass
- 87.62 u
- Category
- Alkaline earth
- Group / period
- 2 / 5
- Electron configuration
- [Kr] 5s 2
- State at 25 °C
- Solid
- Melting point
- 776.9 °C
- Boiling point
- 1382 °C
- Density
- 2.64 g/cm³
- Electronegativity
- 0.95
- 1st ionization energy
- 550 kJ/mol
- Oxidation states
- +2
- Discovered
- 1790
Atomic structure
A neutral strontium atom has 38 protons and 38 electrons, and its most common isotope carries about 50 neutrons. One atom weighs about 1.45 × 10⁻²⁵ kg, so a mole of strontium (6.022 × 10²³ atoms) weighs 87.62 g. That nucleus holds 1.32 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 8, O 2), which gives the configuration [Kr] 5s 2. With 2 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 4 stable isotopes: Sr-84, Sr-86, Sr-87 and Sr-88. No other element has a stable isotope with mass number 88, so Sr-88 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) strontium is a solid. It melts at 776.9 °C and boils at 1382 °C, so it stays liquid across a 605-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 2.64 g/cm³, 2.64 times as dense as water. Its atoms have a radius of about 200 pm (Rubidium next door is 235 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.95 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 550 kJ/mol, less than Calcium above it (590 kJ/mol). A bond between strontium and chlorine would be mostly ionic, with an electronegativity difference of 2.2. It releases 5 kJ/mol when it gains an extra electron. Its common oxidation states are +2, so in compounds you will usually meet it as Sr²⁺. Group 2 also holds Beryllium, Magnesium, Calcium, Barium and Radium, which share its outer-shell pattern. Its Lewis symbol is Sr 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 370 mg/kg of Earth's crust, the 15th most abundant element there. In the human body it accounts for about 0.00046 % of body mass. It was first identified in 1790, making it the 29th 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 460.7 nm, 407.8 nm and 640.8 nm. In a flame test its compounds colour the flame scarlet red.