Electrons cluster around atomic nuclei in distinct shells. These shells are made of subshells, each of which can hold two electrons. The first shell has one s orbital, which holds two. The second and suceeding shells also have three p orbitals, which each hold two. Three p orbitals and an s orbital make eight electrons. An atom tends to be most stable when all four of these orbitals are either empty or full. Therefore, molecules will tend to form with each atom except hydrogen and helium (first period elements without a second shell) having eight outer electrons. This may mean four different bonds (a bond counts as two electrons for each atom in the bond), or maybe some double bonds, or even lone pairs on the atom.
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