Nitrogen cannot make long chains in the way that carbon can, for two reasons. The lone electron pairs on adjacent nitrogen atoms in a -N-N-N-N- chain repel one another more strongly than bonding pairs in the C-H bonds of hydrocarbons do. This weakens the N-N bond and makes it only half as strong as a C-C bond. The second factor is the great stability of the pieces of the ruptured chain, N₂ molecules. Nitrogen has three unpaired electrons for bonding, and it can share all three with a neighboring atom to produce the NN molecule shown at the right. Pure nitrogen therefore is a diatomic gas, N₂ , similar to H₂. Because the triple bond is quite strong, N₂ is a stable and relatively unreactive molecule. If "quadruple bonds" were possible, with four electron pairs shared between two atoms, then perhaps carbon also would be a diatomic gas, C₂ . But this is not possible, so carbon bonds to more than one neighbor and forms the three-dimensional structures of graphite and diamond. The sudden change in properties between solid diamond and gaseous nitrogen is one of the most dramatic among the chemical elements.