What is the difference between atoms, molecules, diatomic molecules, and formula units

Please help, I can't find the definitions anywhere in my chemistry book. I have to group these items into their correct group (the numbers are all subscripts):

Copper
Xenon
H2
Ni
Oxygen
Chlorine
HgBr2
CoCl2
SO2
C3H8
KMnO4
Ca(OH)2

An atom is a "Kind" of elements. There about 118 different kinds They are the building block of nature.
Two or more atoms combined are called molecules. the atoms are: Cu, Xe, Ni, O, Cl,
The remainder are molecules. You could call H2 a diatomic molecule if you wish. an ionic .