How many pairs of electrons are shared when hydrogen and oxygen unite to form a molecule of water?
1 per Hydrogen Atom for a total of 2 Hydrogen HAS only one electron so it can only share one. Oxygen is a molecule that needs 8 electrons in its outer shell to become stable. It has 2 in its inner shell (just like Helium) and 6 more in its outer shell. It wants to add a pair of electrons to reach that magic number of 8 so it is willing to grab and share 2 hydrogen electrons.