The octet rule is the tendency of atoms to lose, gain or share electrons so that each atom reaches a stable outer shell of eight electrons, like a noble gas.
The octet rule is the tendency of atoms to lose, gain or share electrons during bonding so that each atom ends up with a stable outer shell of eight electrons, the same arrangement as a noble gas. Malay: petua oktet · Chinese: 八隅体规则.
An atom is most stable when its outermost shell is full. For most of the elements you meet in Form 4, a full outer shell means eight electrons, the configuration of the nearest Group 18 noble gas such as neon or argon. Atoms that do not already have this arrangement are reactive, and they reach it by bonding. A metal atom empties its outer shell by losing electrons, a non-metal atom completes its shell by gaining electrons, and two non-metals complete their shells by sharing electrons. The octet rule is therefore the single idea behind both ionic and covalent bonding, it tells you why a bond forms at all.
Example. A sodium atom has the arrangement 2.8.1. By losing its single outer electron it becomes Na+ with 2.8, the octet of neon. A chlorine atom has 2.8.7; by gaining one electron it becomes Cl− with 2.8.8, the octet of argon. In water, an oxygen atom (2.6) shares two electrons with two hydrogen atoms so that it too reaches eight outer electrons.
Confusion to avoid. The octet is not universal. Small atoms near the start of the periodic table are stable with a duplet of two electrons, hydrogen and helium reach stability at two, not eight. Do not force an octet onto hydrogen when you draw a dot-and-cross diagram.
Across this chapter the octet rule is the starting point: every ionic and covalent structure you draw exists so that its atoms can reach a stable octet, and stating this reasoning clearly is often worth a mark in SPM Chemistry.
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