What determines how atoms interact to form molecules?

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Valence electrons determine bonding patterns because the outer-shell electrons are the ones that actually participate in forming bonds. The number of valence electrons an atom has dictates how many bonds it can form and how many electron domains will surround the atom, which in turn sets the molecule’s geometry through how those electrons arrange themselves to minimize repulsion. Atoms bond in order to reach a more stable, lower-energy arrangement—typically by filling their outer shell through sharing (covalent bonds) or transferring electrons (ionic bonds). The resulting bond types and shapes come from this available pool of valence electrons and how they can be arranged around the atoms.

The other factors don’t determine bonding in the same way. The mass of an atom doesn’t fix bonding patterns, the color of an atom is unrelated to how it bonds, and while the distance from the nucleus to outer electrons affects energy levels, it’s ultimately the valence electron count and their arrangement that decide how atoms interact and what the most stable distribution of electrons looks like.