1. Drag out the atoms you need 2. Drag them so their shells overlap 3. Click near a shell to add that atom's own electron - click an electron to remove it
Drag an atom here to delete it
An atom's outer shell is only complete when 2 electrons (H) or 8 electrons (everything else here) are visible around it - its own electrons, plus any of a neighbour's electrons that fall inside its shell because you've overlapped them. A shared pair must have one dot and one cross - one electron from each atom. Two dots or two crosses sitting together isn't a normal covalent bond, it's one atom donating both electrons (a dative/coordinate bond), which isn't part of GCSE and won't count as correct here.