Selenium (Se): Bohr Model, Protons, Neutrons & Electrons
Selenium is element number 34, a nonmetal in group 16, period 4. Its atom has 34 protons, 46 neutrons and 34 electrons arranged in 4 shells: 2, 8, 18, 6.
Protons, neutrons and electrons in selenium
The atomic number of selenium is 34, so every selenium atom has 34 protons. A neutral atom has the same number of electrons, 34. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For selenium-80, that is 80 − 34 = 46 neutrons. Selenium has 6 stable isotopes, with 40, 42, 43, 44, 46, 48 neutrons.
How to draw the Bohr model of selenium
- Draw the nucleus and write 34 p⁺ and 46 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 6 electrons on it.
- Check: 2 + 8 + 18 + 6 = 34 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, 6) is the one to draw.
Electron configuration
The electron configuration of selenium is [Ar] 4s² 3d¹⁰ 4p⁴. Its 6 outer electrons decide how it reacts: it tends to gain 2 more, forming Se²⁻ ions.
Key facts about selenium
| Symbol | Se |
|---|---|
| Atomic number | 34 |
| Relative atomic mass | 78.971 |
| Group · period · block | group 16 · period 4 · p-block |
| Type | Nonmetal |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 6 |
| Electron configuration | [Ar] 4s² 3d¹⁰ 4p⁴ |
| Valence electrons | 6 |
| Common ion | Se²⁻ |
| Electronegativity (Pauling) | 2.55 |
| First ionisation energy | 9.75 eV (941 kJ/mol) |
| Atomic radius (empirical) | 115 pm |
Where selenium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does selenium have?
Selenium has 34 protons and 34 electrons (atomic number 34). Its most common isotope, selenium-80, has 80 − 34 = 46 neutrons.
What is the electron configuration of selenium?
[Ar] 4s² 3d¹⁰ 4p⁴. In shells that is 2, 8, 18, 6.
How many valence electrons does selenium have?
6. Selenium is in group 16, and its outer shell holds 6 electrons.
Where is selenium on the periodic table?
In group 16, period 4, in the p-block. It is a nonmetal.
