Actinium (Ac): Bohr Model, Protons, Neutrons & Electrons
Actinium is element number 89, an actinide in the f-block (the actinide row drawn below the table), period 7. Its atom has 89 protons, 138 neutrons and 89 electrons arranged in 7 shells: 2, 8, 18, 32, 18, 9, 2.
Protons, neutrons and electrons in actinium
The atomic number of actinium is 89, so every actinium atom has 89 protons. A neutral atom has the same number of electrons, 89. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For actinium-227, that is 227 − 89 = 138 neutrons. Every isotope of actinium is radioactive; 227 is the mass number of its most stable one.
How to draw the Bohr model of actinium
- Draw the nucleus and write 89 p⁺ and 138 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 32 electrons on it.
- Draw the fifth shell and put 18 electrons on it.
- Draw the sixth shell and put 9 electrons on it.
- Draw the seventh shell and put 2 electrons on it.
- Check: 2 + 8 + 18 + 32 + 18 + 9 + 2 = 89 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, 32, 18, 9, 2) is the one to draw.
Electron configuration
The electron configuration of actinium is [Rn] 7s² 6d¹. As an actinide it fills an inner f-subshell, and its common ion is Ac³⁺.
Key facts about actinium
| Symbol | Ac |
|---|---|
| Atomic number | 89 |
| Relative atomic mass | [227] |
| Group · period · block | f-block · period 7 |
| Type | Actinide |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 32, 18, 9, 2 |
| Electron configuration | [Rn] 7s² 6d¹ |
| Valence electrons | varies (outer shell: 2) |
| Common ion | Ac³⁺ |
| Electronegativity (Pauling) | 1.10 |
| First ionisation energy | 5.17 eV (499 kJ/mol) |
| Atomic radius (empirical) | 195 pm |
Where actinium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does actinium have?
Actinium has 89 protons and 89 electrons (atomic number 89). Its most common isotope, actinium-227, has 227 − 89 = 138 neutrons.
What is the electron configuration of actinium?
[Rn] 7s² 6d¹. In shells that is 2, 8, 18, 32, 18, 9, 2.
How many valence electrons does actinium have?
Actinium is an actinide, so its valence electrons are spread over two shells; its outermost shell holds 2.
Where is actinium on the periodic table?
In the f-block (the actinide row drawn below the table), period 7, in the f-block. It is an actinide.
