What type of atoms tend to form covalent bonds?

Atoms of nonmetallic elements can form covalent bonds with one another.

If the electronegativity difference between the two atoms bonding is less than 2.0 Pauling units, then a bond is generally considered covalent. (Some sources list slightly lower values as the cut off. I've seen 1.7 or 1.8 in a few places.)

Since there is no difference in electronegativity between 2.1 and 2.1, an H-H bond is covalent.

Because of the electronegativity difference of 1.4 (3.5 - 2.1), an O-H bond is covalent.

Metallic bonding is a type of bonding in which the electrons involved are free to move from one metallic atom to another. Metals can bond together and have electronegativity differences of less than 2.0 Pauling units.

An ionic bond forms when metals form large electronegativity differences with nonmetals. For example, in NaCl, the difference is 2.1 (Cl=3.0 and Na=0.9).

The majority of non-metal atoms create covalent bonds.