 First, remember that a single covalent bond is between two atoms and it involves one pair of shared electrons. For instance, in this methane molecule there are four bonds, each joining one hydrogen atom with the central carbon. Now have a look at these three other molecules. Each is a compound of hydrogen with another non-metal. Why is it that in each molecule the central atom has a different number of bonds? As you know different atoms have different electron structures and in particular different numbers of valence electrons. It's this that determines the number of bonds that an atom typically forms. So to work out the number of bonds typically formed by atoms of a particular element we look at its valence. And remember we're only dealing with non-metals here because it's covalent bonding. So that's groups 4, 5, 6, 7 and 8 along with hydrogen in group 1. Elements in group 7 have a valence of minus 1. That's because they have 7 valence electrons which means they need to gain one electron to get a full outer shell. These elements typically form one covalent bond. Elements in group 6 have a valence of minus 2. They have six valence electrons so they need to gain two electrons to get a full outer shell to achieve a noble gas configuration. These elements typically form two covalent bonds. Likewise elements in group 5 have a valence of minus 3 and form 3 bonds and elements in group 4 have a valence of minus 4 and form 4 bonds. What about elements in group 8? Well what distinguishes the noble gases is that they have noble gas configurations that is full outer shells and so they don't need to gain any extra electrons. As a consequence they don't usually form any bonds at all.