Elements are made of only one kind of atom. Example: Cu Al When two or more elements combine chemically, a compound is formed. Example: NaCl H 2 O AlK(SO 4 ) 2 NaHCO 3 ZnO CuSO 4 Atoms must form bonds between them to form compounds. Atoms use their valence electrons to form bonds. Valence electrons are the electrons in an atoms last energy level You can determine valence electrons by determining the group the element belongs to. (Subtract 10 from groups 13 18 to determine valence electrons.) An atom with fewer than 8 valence electrons is more likely to form bonds. Atoms use the magic number 8 to make their outer energy levels stable. (Except for H and He. Their magic number is 2.) Atoms of elements in group 18 dont usually form compounds because they already have 8 valence electrons. (Helium either because its outer energy level only holds two electrons and thats how many it has.) Two kinds of chemical bonds: Ionic bonds form when an atom loses or gains electrons from a nearby atom the oppositely charged ions are attracted to each other Ionic bonds usually form between a metal and a nonmetal the new compounds formed are called ionic compounds Covalent bonds A covalent bond is formed when atoms share electrons to reach the magic number 8 in their outer energy level Covalent bonds usually form between two nonmetals. Covalent compounds are made of individual particles called molecules