Nitrogen and oxygen together make up about 99% of the air we breathe. The vast majority of our atmosphere is nitrogen. Oxygen is ten times as abundant as nitrogen in the universe, but it makes up only about 21 percent of our atmosphere. So, the less common element is the most abundant in our atmosphere. What does that mean to us? The bottom line is that the atmosphere is fine-tuned for life. Let’s examine that more carefully.
An atom of oxygen and an atom of nitrogen differ by only one proton and one electron. That may not seem like much, but it makes a world of difference. Both of those elements form diatomic molecules, meaning that two atoms bond together to make one molecule of oxygen or nitrogen.
Covalent bonding is the chemical bonding of atoms by equal sharing of electrons. That bond gives atoms stability in their outer, or valence, electron shells. Atomic stability requires eight valence electrons. The only elements with that number are the so-called “noble gases”–helium, neon, argon, krypton, and radon. For that reason, they are inert, refusing to combine with other elements. All other elements need electrons to complete the octet in their valence shells.
An oxygen atom has six electrons in its valence shell, so it needs to share two electrons to become stable. When an oxygen atom shares two electrons with another oxygen atom, they both become stable. Nitrogen, on the other hand, has only five valence electrons. Therefore, by forming a covalent bond with another nitrogen atom, sharing three electrons, both atoms complete their outer shell. In this way, our atmosphere is made up of stable diatomic oxygen and nitrogen molecules.
However, not all molecules are equally stable. That is where we see the atmosphere is fine-tuned for life. For example, oxygen molecules have a double bond sharing two electrons, but nitrogen atoms have a triple bond sharing three electrons for more stability. That difference may seem insignificant, but it is essential to make life possible. Come back tomorrow when we will explain what a difference it makes.
— Roland Earnst © 2022