Which change to an atom occurs when it forms a positive ion

loss of electrons

Explanation:

An atom is made up of three subatomic particles named protons neutrons and electrons. An atom forms positive ion with the loss of at least one electron.

For instance: Calcium with atomic number 20, and electronic configuration 1s2, 2s2 2p6, 3s2 3p6, 4s2 will lose the two outermost electrons to form a divalent positive ion (Ca2+).

Ca --> Ca2+ + 2e-

Thus, loss of electrons is the change to an atom that occurs when it forms a positive ion