Shells and valency

How electrons fill the shells (the Bohr–Bury rules, from two scientists, Bohr and Bury):

• Shell n holds at most 2n² electrons: K (n = 1) holds 2, L (n = 2) holds 8, M (n = 3) holds 18.
• The outermost shell holds at most 8.
• Shells fill in order, K then L then M: the next one starts only when the one before is full.

The way electrons are arranged in shells is the electronic configuration:

• hydrogen 1; helium 2;
• carbon 2, 4;
• sodium 2, 8, 1;
• argon 2, 8, 8.

The outermost shell is the valence shell, and its electrons are valence electrons. Eight electrons there make an octet. Atoms with a full octet (or helium, with its full K shell of 2) are stable: they sit quietly and hardly react with anything.

Combining capacity. Count how many hydrogen or chlorine atoms one atom joins with:

• in H₂O, oxygen joins 2 H atoms, so its capacity is 2;
• in NH₃, nitrogen joins 3;
• in MgCl₂, magnesium joins 2 Cl atoms.

Valency is the number of electrons an atom loses, gains or shares to get a full octet. Think of it like hunger: if you have only one roti and need four, it's easier to give that one away. Atoms do the same:

• Fewer than 4 valence electrons: it gives them away. Sodium (2, 8, 1) loses 1, so its valency is 1.
• More than 4: it takes more. Oxygen (2, 6) gains 2, so its valency is 2.
• Exactly 4: it shares. Carbon (2, 4) shares 4, so its valency is 4.

Valencies of the first 18 elements: