Covalent bonds usually form between atoms of elements that share protons valence electrons nuclei oppositely charged ions

If the blank is after 'share' in the original question and Protons, valence electrons, nuclei, and oppositely charged ions are answers, then valence electrons are the answer. Covalent bonds share electrons. a simple rule for protons is "change the protons (number) change the element". When a bond forms the element does not change therefore the protons do not change. 

Theoretical temperature at which molecules stop moving and no more heat can be removed from the object. kelvin: absolute zero degrees. none of the above.