water. A molecule of water is made of two atoms of hydrogen and one
atom of oxygen: HO. A typical living human body is composed of ap-
proximately 65 percent water. The elemental abundances in a living
human body, by weight, would look something like this, with oxygen
being in first place because of all the water:
oxygen ~ 65%
carbon ~ 18.5%
hydrogen ~ 9.5%
nitrogen ~ 3.2%
calcium ~ 1.5%
phosphorus ~ 1.0%
potassium ~ 0.4%
sulfur ~ 0.3%
sodium ~ 0.2%
chlorine ~ 0.2%These are the top ten chemical elements (with abundance represented
as percentage of body weight) that make up a living human body
and, indeed, that make up most living organisms on Earth. Ten other
elements are also present and essential for healthy human life—
magnesium, manganese, iron, cobalt, nickel, copper, zinc, selenium,
molybdenum, and iodine—but these are found in much smaller
amounts than the top ten.
If we represent the list of elements in terms of numbers of atoms
(rather than percent weight) present in a living human body, then hy-
drogen would be number one, because there are vastly more hydrogen
atoms than any other type of atom in a living organism. If we were
to remove all the water and dry out the body, it would first of all no
longer be living, because water is, as far as we know, essential for life.