COMPOSITION OF EARTH'S CRUST, SEA WATER & HUMAN BODY

Hydrogen, oxygen, carbon, and nitrogen constitute more than 99% of the atoms in the human body, with most of the H and O occurring as H₂O. Oxygen, silicon, aluminum, and iron are the most abundant atoms in the earth’s crust, with hydrogen, carbon, and nitrogen being relatively rare (less than 0.2% each). Nitrogen as dinitrogen (N₂) is the predominant gas in the atmosphere, and carbon dioxide (CO₂) is present at a level of 0.05%, a small but critical amount. Oxygen is also abundant in the atmosphere and in the oceans. What property unites H, O, C, and N and renders these atoms so suitable to the chemistry of life? It is their ability to form covalent bonds by electron-pair sharing. Furthermore, H, C, N, and O are among the lightest elements of the periodic table capable of forming such bo