Question

an atom becomes an ion with a - 1 charge because the atom gains one electron. loses one electron. gains one proton. loses one proton.

Explanation:

An atom has equal numbers of protons (positive charge) and electrons (negative charge). A - 1 charge means there is one more negative charge than positive. Electrons are gained or lost to form ions. Protons are in the nucleus and not easily gained or lost in ion - formation (except in nuclear reactions, which is not relevant here). Gaining one electron (negative) gives a net - 1 charge. Losing an electron would give a positive charge. Gaining or losing a proton changes the element (as the number of protons defines the element), and it's not a common ion - formation process.

Answer:

gains one electron.