Valency is determined by the number of electrons gained, lost, or shared to achieve a full outer shell (usually eight electrons, known as the octet rule).
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Atoms react in order to achieve an octet in their outermost shell, either by gaining, losing, or sharing electrons with other atoms.
An outermost shell possessing eight electrons is said to possess an octet.
It was explained as an attempt to attain a fully-filled outermost shell.
Aluminium has a valency of three, as it contains three electrons in its outermost shell.