If chlorine (nonmetal) has seven valence electrons while barium (metal) has
two to donate, how many chlorine atoms are needed to form an ionic bond
with barium?

Two chlorine atoms

Explanation:

Chlorine is a nonmetal belonging to group 7 of the periodic table. It has an atomic number of 17 distributed as (2, 8, 7), so it has 7 valence electrons and needs JUST ONE to complete its outermost shell.

Cl + e- ---> Cl-

On the other hand, barium is a metal belonging to group 2 on the periodic table. It has an atomic number of 56 with 2

valence electrons in its outermost shell, and gives off the two electrons to form a stable octet structure.

Ba > Ba2+ + 2e-

Thus, barium loses its 2 outermost electrons to form a Ba2+ ion, while TWO chlorine atoms receive them to form 2Cl- resulting in an ionic bond in the compound, BaCl2 (Barium Chloride)

Ba2+ + 2Cl- ---> BaCl2

it is called acid bonding