Strontium (Sr): Bohr Model, Protons, Neutrons & Electrons
Strontium is element number 38, an alkaline earth metal in group 2, period 5. Its atom has 38 protons, 50 neutrons and 38 electrons arranged in 5 shells: 2, 8, 18, 8, 2.
Protons, neutrons and electrons in strontium
The atomic number of strontium is 38, so every strontium atom has 38 protons. A neutral atom has the same number of electrons, 38. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For strontium-88, that is 88 − 38 = 50 neutrons. Strontium has 4 stable isotopes, with 46, 48, 49, 50 neutrons.
How to draw the Bohr model of strontium
- Draw the nucleus and write 38 p⁺ and 50 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Draw the second shell and put 8 electrons on it.
- Draw the third shell and put 18 electrons on it.
- Draw the fourth shell and put 8 electrons on it.
- Draw the fifth shell and put 2 electrons on it.
- Check: 2 + 8 + 18 + 8 + 2 = 38 electrons, the same as the number of protons.
From potassium onwards the shells don’t simply fill as 2, 8, 8: the fourth shell starts before the third is full, so the real arrangement above (2, 8, 18, 8, 2) is the one to draw.
Electron configuration
The electron configuration of strontium is [Kr] 5s². Its 2 outer electrons decide how it reacts: it tends to lose them, forming Sr²⁺ ions.
Key facts about strontium
| Symbol | Sr |
|---|---|
| Atomic number | 38 |
| Relative atomic mass | 87.62 |
| Group · period · block | group 2 · period 5 · s-block |
| Type | Alkaline Earth Metal |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 8, 2 |
| Electron configuration | [Kr] 5s² |
| Valence electrons | 2 |
| Common ion | Sr²⁺ |
| Electronegativity (Pauling) | 0.95 |
| First ionisation energy | 5.69 eV (549 kJ/mol) |
| Atomic radius (empirical) | 200 pm |
Where strontium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does strontium have?
Strontium has 38 protons and 38 electrons (atomic number 38). Its most common isotope, strontium-88, has 88 − 38 = 50 neutrons.
What is the electron configuration of strontium?
[Kr] 5s². In shells that is 2, 8, 18, 8, 2.
How many valence electrons does strontium have?
2. Strontium is in group 2, and its outer shell holds 2 electrons.
Where is strontium on the periodic table?
In group 2, period 5, in the s-block. It is an alkaline earth metal.
