The shell that matters
Electrons sit in shells around the nucleus, and the outermost occupied shell is called the valence shell. It is the only one that takes part in chemistry.
Valence shell
The outermost shell of an atom in which electrons are present. The electrons in it are the valence electrons.
Four atoms, side by side
Put two noble gases next to two reactive elements and the difference shows up in a single column.
| Element | Electron configuration | Valence electrons | Reacts? |
|---|---|---|---|
| Neon (Ne) | 2, 8 | 8 | No |
| Argon (Ar) | 2, 8, 8 | 8 | No |
| Sodium (Na) | 2, 8, 1 | 1 | Vigorously |
| Chlorine (Cl) | 2, 8, 7 | 7 | Vigorously |
Figure to be added
Eight is the stable number
Neon and argon each carry eight electrons in the valence shell, and neither reacts. Sodium carries one and chlorine seven, and both react readily. The correlation is not a coincidence: an arrangement of eight valence electrons is unusually stable, and an atom that already has one has no reason to change.
Helium is the exception worth noting early. Its shell holds only two electrons, but that shell is full, so helium is as unreactive as the rest. Full, not eight, is the real condition — for every shell after the first, full happens to mean eight.
What makes an atom reactive
An atom reacts because its valence shell is incomplete. The noble gases are unreactive not because of anything special about them, but because they have nothing left to gain.