A lone nitrogen atom has one pair of electrons and three unpaired electrons in its outer shell available for making chemical bonds. The outer electronic shell, when full, has its eight electrons arranged in four electron pairs.

Oxygen (O 2,6) has six outer electrons, and as the electron diagrams suggest, they occur as two electron pairs and two unpaired electrons. Diatomic gas makes up the other 20% of the atmosphere of our planet, with other gases being present only in small amounts.

Oxygen combines readily with carbon compounds and releases large amounts of heat when it does so. This is why oxygen was selected during the evolutionary process as the main energy source for living organisms. However, it is wrong to think of oxygen chiefly as an atmospheric gas involved in respiration. Nearly half the atoms in our planet are oxygen atoms, locked with metals and silicon atoms in the silicate minerals beneath our feet.