Astatine (At): Bohr Model, Protons, Neutrons & Electrons
Astatine is element number 85, a halogen in group 17, period 6. Its atom has 85 protons, 125 neutrons and 85 electrons arranged in 6 shells: 2, 8, 18, 32, 18, 7.
Protons, neutrons and electrons in astatine
The atomic number of astatine is 85, so every astatine atom has 85 protons. A neutral atom has the same number of electrons, 85. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For astatine-210, that is 210 − 85 = 125 neutrons. Every isotope of astatine is radioactive; 210 is the mass number of its most stable one.
How to draw the Bohr model of astatine
- Draw the nucleus and write 85 p⁺ and 125 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 7 electrons on it.
- Check: 2 + 8 + 18 + 32 + 18 + 7 = 85 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, 7) is the one to draw.
Electron configuration
The electron configuration of astatine is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵. Its 7 outer electrons decide how it reacts: it tends to gain 1 more, forming At⁻ ions.
Key facts about astatine
| Symbol | At |
|---|---|
| Atomic number | 85 |
| Relative atomic mass | [210] |
| Group · period · block | group 17 · period 6 · p-block |
| Type | Halogen |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 32, 18, 7 |
| Electron configuration | [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵ |
| Valence electrons | 7 |
| Common ion | At⁻ |
| Electronegativity (Pauling) | 2.20 |
| First ionisation energy | 9.32 eV (899 kJ/mol) |
| Atomic radius (empirical) | — |
Where astatine sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does astatine have?
Astatine has 85 protons and 85 electrons (atomic number 85). Its most common isotope, astatine-210, has 210 − 85 = 125 neutrons.
What is the electron configuration of astatine?
[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵. In shells that is 2, 8, 18, 32, 18, 7.
How many valence electrons does astatine have?
7. Astatine is in group 17, and its outer shell holds 7 electrons.
Where is astatine on the periodic table?
In group 17, period 6, in the p-block. It is a halogen.
