Barium (Ba): Bohr Model, Protons, Neutrons & Electrons
Barium is element number 56, an alkaline earth metal in group 2, period 6. Its atom has 56 protons, 82 neutrons and 56 electrons arranged in 6 shells: 2, 8, 18, 18, 8, 2.
Protons, neutrons and electrons in barium
The atomic number of barium is 56, so every barium atom has 56 protons. A neutral atom has the same number of electrons, 56. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For barium-138, that is 138 − 56 = 82 neutrons. Barium has 7 stable isotopes, with 74, 76, 78, 79, 80, 81, 82 neutrons.
How to draw the Bohr model of barium
- Draw the nucleus and write 56 p⁺ and 82 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 18 electrons on it.
- Draw the fifth shell and put 8 electrons on it.
- Draw the sixth shell and put 2 electrons on it.
- Check: 2 + 8 + 18 + 18 + 8 + 2 = 56 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, 18, 8, 2) is the one to draw.
Electron configuration
The electron configuration of barium is [Xe] 6s². Its 2 outer electrons decide how it reacts: it tends to lose them, forming Ba²⁺ ions.
Key facts about barium
| Symbol | Ba |
|---|---|
| Atomic number | 56 |
| Relative atomic mass | 137.33 |
| Group · period · block | group 2 · period 6 · s-block |
| Type | Alkaline Earth Metal |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 18, 8, 2 |
| Electron configuration | [Xe] 6s² |
| Valence electrons | 2 |
| Common ion | Ba²⁺ |
| Electronegativity (Pauling) | 0.89 |
| First ionisation energy | 5.21 eV (503 kJ/mol) |
| Atomic radius (empirical) | 215 pm |
Where barium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does barium have?
Barium has 56 protons and 56 electrons (atomic number 56). Its most common isotope, barium-138, has 138 − 56 = 82 neutrons.
What is the electron configuration of barium?
[Xe] 6s². In shells that is 2, 8, 18, 18, 8, 2.
How many valence electrons does barium have?
2. Barium is in group 2, and its outer shell holds 2 electrons.
Where is barium on the periodic table?
In group 2, period 6, in the s-block. It is an alkaline earth metal.
