Half equations at the anode and cathode

A half equation shows what happens at one electrode only. It uses e^- for an electron. Atoms must balance, and so must the total charge on each side of the arrow.

At the cathode, positive ions gain electrons, so the electrons are on the left of the arrow. For copper ions, Cu²⁺ + 2e^- → Cu. Each copper ion has a 2+ charge, so it needs two electrons to become a neutral copper atom. For hydrogen ions, 2H⁺ + 2e^- → H₂. For lead ions, Pb²⁺ + 2e^- → Pb.

At the anode, negative ions lose electrons, so the electrons are on the right of the arrow. For chloride ions, 2Cl^- → Cl₂ + 2e^-. The left side has a charge of 2-, and the right side has two electrons, also 2-, so the charge balances. For bromide ions, 2Br^- → Br₂ + 2e^-.