The outermost electron shell in an atom - it usually takes up to 2 or 8 electrons
The number of electrons in the outer shell of an atom matches the group number for that element on the Periodic Table. For example, carbon is in group 4 and has 4 electrons in its outer shell. All atoms want to have a full outer shell (usually 8) so they form bonds in order to do this. The elements in group 8 already have 8 electrons in their outer shell so they do not form bonds - it is very rare to see group 8 elements in a compound.