Two neutral atoms, one being a nonmetal and the other a metal, form an ionic bond when the nonmetal uses energy to remove valence electrons from the metal.
Since a metal is not very electronegative but a nonmetal is, the nonmetal can remove the metal’s valence electrons.
In a perfect scenario, both atoms, now ions, will have octets. An octet is formed when an ion has a full valence shell (usually noted with 8 valence electrons).