Question

when the elements copper (cu) and oxygen (o) chemically combine, they form...
an ion
a mixture
a gas
a compound

Explanation:

When two elements (copper and oxygen) chemically combine, they form a compound. A compound is a substance made from two or more different elements that have been chemically bonded together. An ion is a charged particle, a mixture is a combination of substances without chemical bonding, and a gas is a state of matter, not the result of this chemical combination.

Answer:

a compound