What is a antonym for neutron?

Answer 1

The concept of "antonym" is used for properties, not for objects (or, in grammatical terms, for adjectives and adverbs, not for nouns).

There is a particle called an "anti-neutron", but that can hardly be considered an antonym.

Answer 2

The antonym for neutron is proton. Neutrons and protons are subatomic particles that make up the nucleus of an atom, with neutrons carrying no charge and protons carrying a positive charge.