In chemical compounds covalent bonds form when -?

when....

1- a metal forms bond with metals.

2- a nonmetal with nonmetal nonmetal.

3- or atoms having electronegativities difference less than 1.7 (with exeption of HF having E.N diff. 1.9)......

or Jefferson lab says: when pairs of electrons are shared between two nonmetal atoms

or when pairs of electrons are shared between two nonmetl atoms