I am imagining that the occasional oxygen might be floating close enough to a certain calcium atom and take the two electrons, which will ionize them and they will now 'glue'. Maybe the same thing happens in a hydrogen and chlorine, and a covalent bond is smoothly and calmly formed.
I expect these cases to be the exception, because if atoms have speeds of hundreds of meters per second, violent collisions would be the norm for their encounter. The question is: Is it the case that electron transfer occurs in collisions, but the kinetic energy overcomes the electrostatics and the atoms fly apart, and then they eventually find other partners to bond as they wander around? Even in covalent bonds? (It was the latter part that sparked the question, because in covalent 'no complete electron transfer occurs', but I couldn't picture atoms nicely slowing down to share electrons). Or is it always (collision=bonding-and-becoming-molecule-in-one-shot)? Or both?