An equation shows the molecules a reaction starts with and the ones it makes. A curly arrow shows what happens in between: a pair of electrons on the move.
Every bond is a shared pair of electrons, and a lone pair is a pair an atom keeps to itself. When bonds break and form, these pairs move. Chemists draw each moving pair as a curved arrow with a full head.
Tail and head
The tail of a curly arrow sits where the pair is now: on a lone pair or on a bond. The head points to where the pair goes. Ammonia's nitrogen has a lone pair, and it can reach out to a hydrogen ion, H⁺. One curly arrow shows it, with its tail on the lone pair and its head on the H⁺.
A lone pair that moves toward an atom becomes a new bond. The nitrogen kept its lone pair to itself, and now it shares that pair with the hydrogen.
An arrow that starts on a bond
A curly arrow can also start on a bond. The bond breaks, and both of its electrons go where the head points. In water, hydrogen chloride splits this way into the ions of hydrochloric acid, the acid in your stomach.
Chlorine pulls on shared electrons harder than hydrogen does, so the pair goes to chlorine. Each atom brought one electron to the bond. The chlorine leaves with two, one more than it brought, so it carries a − charge. The hydrogen leaves with none, as H⁺.
The drawings in this lesson show the lone pairs that take part. A chloride ion has four lone pairs in all, and the amber one is the pair that came from the bond.