Helium (He): Bohr Model, Protons, Neutrons & Electrons
Helium is element number 2, a noble gas in group 18, period 1. Its atom has 2 protons, 2 neutrons and 2 electrons arranged in 1 shell: 2.
Protons, neutrons and electrons in helium
The atomic number of helium is 2, so every helium atom has 2 protons. A neutral atom has the same number of electrons, 2. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For helium-4, that is 4 − 2 = 2 neutrons. Helium has 2 stable isotopes, with 1, 2 neutrons.
How to draw the Bohr model of helium
- Draw the nucleus and write 2 p⁺ and 2 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Check: 2 = 2 electrons, the same as the number of protons.
Electron configuration
The electron configuration of helium is 1s². Its 2 outer electrons decide how it reacts: its outer shell is full, so it hardly reacts at all.
Key facts about helium
| Symbol | He |
|---|---|
| Atomic number | 2 |
| Relative atomic mass | 4.0026 |
| Group · period · block | group 18 · period 1 · s-block |
| Type | Noble Gas |
| State at 25 °C | gas |
| Electron shells | 2 |
| Electron configuration | 1s² |
| Valence electrons | 2 |
| Common ion | none: its outer shell is already full |
| Electronegativity (Pauling) | — |
| First ionisation energy | 24.59 eV (2373 kJ/mol) |
| Atomic radius (empirical) | — |
Where helium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does helium have?
Helium has 2 protons and 2 electrons (atomic number 2). Its most common isotope, helium-4, has 4 − 2 = 2 neutrons.
What is the electron configuration of helium?
1s². In shells that is 2.
How many valence electrons does helium have?
2. Helium is in group 18, and its outer shell holds 2 electrons.
Where is helium on the periodic table?
In group 18, period 1, in the s-block. It is a noble gas.
