Why does hydrogen have an ionic bond with flourine but only a covalent with carbon??
Electronegativity. (How are there five answers and nobody has used that word yet?) There are different ways to measure and define that term, and different ways to rationalize the effect, but qualitatively it’s this: the ability of an element to attract electrons to itself. If you have two elements of equal electronegativity, they share electrons equally, and you’ll have a perfectly covalent bond. If you have two elements of slightly different electronegativity, the electron cloud associated with the bond will be polarized toward the element with the higher EN value, so one atom bears a partial negative charge and the other a partial positive. That leads to a small polarity across the bond, but not so much that electrons are not still being shared — they’re just shared somewhat unequally, resulting in a polar covalent bond. If you have two elements of dramatically different electronegativity, you completely transfer electrons from one atom to another, form a cation and an anion, and fo
