What type of atoms tend to form covalent bonds?
Covalent bonds form between atoms of nonmetallic elements.
In general, bonds are considered to be covalent if the electronegativity difference between the two atoms bonding is less than 2.0 Pauling units. (Some sources list slightly lower values as the cut off. I've seen 1.7 or 1.8 in a few places.)
A H-H bond is covalent because the electronegativity difference is 0 (2.1 - 2.1)
An O-H bond is covalent because the electronegativity difference is 1.4 (3.5 - 2.1).
Metals may bond together and have electronegativity differences of less than 2.0 Pauling units. Since the electrons involved in the bonding of metallic atoms are free to move from atom to atom the bonding type is called metallic bonding.
When metals bond to nonmetals, the electronegativity difference tends to be large. This results in the formation of an ionic bond. Ex. IN NaCl the difference is 2.1 (Cl=3.0 and Na=0.9)