Upper secondary · 12 min

Electron shells and configurations, H to Ar

Step the atomic number from 1 to 18 and watch electrons fill the shells, layer by layer.

Goal

Count electrons per shell for H–Ar, write configurations like $2.8.1$ for Na, and link full outer shells to the noble gases.

Apparatus and reagents

Electron-shells widget with atomic-number slider.

Procedure

Set $Z = 1$ (H): one electron in the first shell. Note the maximum capacity $2n^2$ of shell $n = 1$ .
Raise Z slowly to 10 (Ne): count how the first shell fills to 2, then the second to 8.
Continue to $Z = 18$ (Ar): the third shell reaches 8 and the atom is again a noble gas.
Write the configuration $n_1.n_2.n_3$ for Na, Mg and Cl, and predict which will lose or gain electrons.

What to observe

Shells fill in order: 2 electrons on the first, 8 on the second, 8 on the third for Z = 1 → 18.
He, Ne and Ar all end with a complete outer shell — the signature of noble-gas stability.

Explanation

Electrons occupy shells of increasing energy. Each shell $n$ holds at most $2n^2$ electrons, and a full outer shell (8 electrons, the octet) confers the chemical inertness of the noble gases. The Bohr picture here is a teaching model — orbitals refine it — but it correctly predicts why Na loses one electron and Cl gains one.