 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 atom’s 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 don’t usually form compounds because they already have 8 valence electrons. (Helium either because its outer energy level only holds two electrons and that’s 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