Which substance is a combination of different atoms?

A compound is a combination of one or more elements that are bonded chemically. Examples are ozone (three oxygen atoms), water (two hydrogens and one oxygen), carbon dioxide (one carbon and two oxygens), and nitrogen gas (two nitrogen atoms) 

Be careful of some elements that combine with themselves in pairs to form compounds. Hydrogen, for example, is an element which you can see in compounds like water and methane, but when hydrogen atoms are floating around by themselves they pair up to make a diatomic molecule (H2). Hydrogen gas is made of pairs of hydrogen atoms (a compound), not individual free-floating atoms. Other elements behave like this too (helium, chlorine, oxygen, nitrogen, and iodine are examples).