State the octet rule. Using the electronic configurations of neon and sodium, explain why neon does not react but sodium does.

1. 1 · State the rule. Atoms combine to get 8 electrons in their outer shell, like a noble gas; hydrogen needs 2.
2. 2 · Neon. Neon (10) is 2, 8. Its outer shell is full, so it is stable and does not react.
3. 3 · Sodium. Sodium (11) is 2, 8, 1. It loses its one outer electron to become Na⁺ (2, 8), like neon.
4. 4 · Energy. Bonding lowers the energy, so the compound formed is more stable than the separate atoms.
5. 5 · Real life. Neon in shop signs never reacts, which shows how stable a full shell is.
6. 6 · Close in one line. So reaching a noble gas configuration is the basic cause of chemical bonding.

Scoring tip: Write each configuration with commas (2, 8, 1). The examiner checks the number in the outer shell.