In general what determines whether atoms will form chemical bonds

The number of electrons in its outermost shell.

Explanation:

Atoms react to form bonds when the number of electron in the outermost shell is not maximum i. e the shell can still accommodate electrons. A fluorine atom combines with itself to form a fluorine molecule because the number of electrons in its outermost shell is not complete ( which is seven instead of the supposed eight). So, both atoms end up sharing an electron pair form a covalent bond.

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