Valency refers to the ability of an atom to bond with other atoms, determined by the number of electrons in its outer shell. Atoms with a full outer shell are generally stable and do not readily bond, while those with incomplete shells tend to react with other atoms to achieve stability. This property is crucial in forming molecules and compounds. In chemistry, valency is often represented by a number that indicates how many bonds an atom can form. For example, the valency of hydrogen is 1, meaning it can form one bond, while carbon has a valency of 4, allowing it to bond with four other atoms.