Which part of an atom is gained, lost, or shared during a chemical reaction? Electron

Proton

Neutron

Nucleus

The gain, loss, or sharing of electrons occurs in every chemical bond.

Explanation:

Should electrons be lost by one particle, and gained by another particle, the now charged particles can be chemically bound by an ionic bond.

Should a pair of electrons form a region of high electron density between two positively charged atomic nuclei, a bond can occur in that mutual attraction of the positively charged nuclei towards the region of high electron density operates. This is a long-winded description of a covalent bond.