contrast ionic covalent bonds. what happens to the valence electrons how does it affect the elements/compounds

The number of electrons in an atom's outermost valence shell governs its bonding behaviour. Elements whose atoms have the same number of valence electrons are grouped together in the Periodic Table. ... To form a covalent bond, one electron from the halogen and one electron from another atom form a shared pair.

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atp is used for energy.