An atom or molecule in which the total number of electrons is not equal to the total number of protons (3)

I believe the answer is:

ion

(Other definitions for ion that I've seen before include "Electrically-charged particle" , "Eletrically charged particle" , "Subatomic particle" , "Particle (phys.)" , "Charged atom/molecule" .)

I've seen this clue in The Independent.

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