The outer shell decides
Helium, neon and argon hardly react. Sodium reacts fiercely. Why?
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The idea
A full outer shell is stable. One outer electron is easily lost. Helium, neon and argon have full outer shells. Sodium has just 1 outer electron.
The outermost shell that holds electrons is the valence shell. Its electrons are valence electrons.
If the outer shell has 8 electrons, it is an octet. For helium, 2 electrons fill the only shell. An atom with a full outer shell is stable and hardly reacts. Stable means it does not easily change.
Other atoms react more. They lose, gain or share electrons to get a full outer shell.
Generally, atoms with fewer than 4 outer electrons tend to lose them. With more than 4, they tend to gain electrons. With exactly 4, they tend to share. Hydrogen is different, because its only shell fills at 2.
The number of electrons lost, gained or shared is the valency.
| Atom | Shells | What it does | Valency |
|---|---|---|---|
| Magnesium | 2, 8, 2 | Loses 2 electrons | 2 |
| Chlorine | 2, 8, 7 | Gains 1 electron | 1 |
| Silicon | 2, 8, 4 | Shares 4 electrons | 4 |
Watch out!
Helium and magnesium both have 2 outer electrons. Helium's two fill its only shell, so it is stable. Magnesium's two leave its shell far from full, so it loses them.