Strontium (Sr)
Strontium (Sr) is an alkaline earth metal with atomic number 38 and a standard atomic weight of 87.62. At room temperature it is a solid that melts at 777 °C and boils at 1,377 °C.
Strontium key facts
Strontium at a glance
Identity
| Symbol | Sr |
|---|---|
| Atomic number (Z) | 38 |
| Atomic weight | 87.62 ± 0.01 (CIAAW 2024) |
| Category | Alkaline earth metal |
| Group | 2 — alkaline earth metals |
| Period | 5 |
| Block | s-block |
| CAS number | 7440-24-6 |
Atomic properties
| Electron configuration | [Kr] 5s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 8, 2 |
| Oxidation states | +2 Less common: +1 |
| Electronegativity (Pauling) | 0.95 |
| First ionisation energy | 5.695 eV (549.5 kJ/mol) 2nd: 11.03 eV · 3rd: 42.883 eV |
| Electron affinity | 0.048 eV |
| Atomic radius (empirical) | 200 pm |
| Covalent radius | 185 pm |
| Van der Waals radius | 249 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 777 °C · 1,430.6 °F · 1,050.15 K |
| Boiling point | 1,377 °C · 2,510.6 °F · 1,650.15 K |
| Density | 2.64 g/cm³ |
| Specific heat capacity | 0.306 J/(g·K) |
| Crystal structure | Face-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 88Sr 82.58%, 86Sr 9.86%, 87Sr 7%, 84Sr 0.56% |
| Stable isotopes | 4 |
| Known isotopes | 35 |
| Abundance in Earth's crust | 370 mg/kg |
| Abundance in seawater | 7.9 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Strontium electron configuration
Strontium has 38 electrons. Written in short form, its ground-state configuration is [Kr] 5s2 — the electrons of the noble gas krypton, plus 5s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2.
As a member of group 2 it has 2 valence electrons in its outer shell (shell 5), which largely decide its chemistry.
Discovery and name
1790 — Adair Crawford and William Cruickshank (isolated by Humphry Davy, 1808) · Scotland.
Strontian, a village in Scotland where the mineral was found.
Known for
Gives fireworks and flares their red colour. Strontium-90 is a hazardous fission product; strontium ranelate has been used to treat osteoporosis.
Isotopes of strontium
Naturally occurring strontium is a mix of 4 isotopes. Each has 38 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 88Sr | 38 | 50 | 82.58% | stable |
| 86Sr | 38 | 48 | 9.86% | stable |
| 87Sr | 38 | 49 | 7% | stable |
| 84Sr | 38 | 46 | 0.56% | stable |
35 isotopes of strontium are known in our data set (half-lives as published in nuclear data tables).
Strontium compared with group 2
The other elements in group 2 (alkaline earth metals), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Beryllium | 4 | 9.0122 | 1,287 °C | 2,468 °C | 1.85 g/cm³ | 1.57 |
| Magnesium | 12 | 24.305 | 650 °C | 1,090 °C | 1.74 g/cm³ | 1.31 |
| Calcium | 20 | 40.078 | 842 °C | 1,484 °C | 1.54 g/cm³ | 1 |
| Strontium | 38 | 87.62 | 777 °C | 1,377 °C | 2.64 g/cm³ | 0.95 |
| Barium | 56 | 137.33 | 727 °C | 1,845 °C | 3.51 g/cm³ | 0.89 |
| Radium | 88 | [226] | 696 °C | 1,737 °C | 5 g/cm³ | 0.9 |