Water is the most abundant molecule in which atomic oxygen is found on earth. Water is composed of two atoms of hydrogen (the simplest of the elements) and one atom of oxygen covalently bonded (four valence electrons are shared). Water covers about 71% of the land mass on earth and by weight water is approximately 0.33% of the atmosphere. It is the only element that is present in the earth-like temperature range as liquid, solid and gas. There is an earth-water cycle, and it nourishes, cleans and sustains the land with transpiration to clouds and condensation as rain, snow, sleet and hail. Our bodies vary in oxygen content, but the average is around 50%; slightly less in women than in men, and more in the skinny than in the obese. Oxygen has physical and chemical properties that are unusual and not predicted, compared to other small, simple compounds. It has maximum density at about 4 degrees Centigrade which causes lakes and ponds to freeze from the top down with interesting consequences. It is a good solvent for many chemicals and it forms the milieu of the cells of our bodies. It forms hydrogen bonds with itself which introduces unexpected structure in collected water molecules. Without it, life as we know it (and what other kind of life might there be?) would not be possible– we even search for it in space and on other worlds as a sign there might be life there.
Keywords: Atmosphere, Aquifer, Boiling point, Covalent bond, Diatomic, Dipole, Freezing point, Heat of vaporization, Hydrogen bond, Hydrophilic, Hydrophobic, Ocean, pH, Sea, Surface tension, Thales, Transpiration, Universal solvent, Water, Water cycle.