A lone pair is a pair of outer electrons that belongs to one atom and is not shared in a bond, for example the two lone pairs on the oxygen atom in water.
A lone pair is a pair of outer electrons that belongs to one atom and is not shared in a bond. Malay: pasangan sunyi · Chinese: 孤对电子.
When an atom bonds, not all of its outer electrons are necessarily used to form bonds. The valence electrons that are left over pair up and stay on that single atom as lone pairs. They do not join the two atoms together, so they are not bonds, but they still count towards the atom’s stable octet and they still occupy space around the atom. Lone pairs matter because they can be donated to form a dative bond and because a hydrogen bond forms between a slightly positive hydrogen and a lone pair on a neighbouring molecule.
Example. In a water molecule, H2O, the oxygen atom uses two of its six outer electrons to form two bonds with hydrogen; the remaining four electrons stay as two lone pairs on the oxygen. In ammonia, NH3, the nitrogen atom has one lone pair, which is the pair it later donates when the ammonium ion forms.
Confusion to avoid. Do not confuse a lone pair with a bonding pair. A bonding pair is shared between two atoms and holds them together; a lone pair belongs to one atom only and forms no bond. When you draw a dot-and-cross diagram, show the lone pairs as well as the bonds, because leaving them out loses marks and hides where a dative or hydrogen bond can form.
The lone pair is a key detail in the covalent structures of this chapter, and identifying it correctly is what makes dative-bond and hydrogen-bond answers work in SPM Chemistry.
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