However, with all those H2O molecules gone, carbon would assume
first place in the list of elemental abundances represented as percent
(dry) body weight. Hydrogen would still be in first place if we are
counting atoms, as there are more hydrogen atoms than there are car-
bon atoms in the molecules that make up organisms.
Wet weight or dry weight, the top five most abundant elements in
the human body, by weight or by number, would be, in some order:
carbon, hydrogen, oxygen, nitrogen, and calcium. And the other five
members of the top ten are phosphorus, potassium, sulfur, sodium,
and chlorine. These are the atoms of life, as we know it. It is interest-
ing to speculate as to what other forms of life might be possible. Life
without water? Silicon as a structural element, replacing carbon? Life
as an integrated circuit, or a vast network of integrated circuits?
When discussing an atom, it is generally assumed that the atom isina
state of neutral electric charge, with the positive charge of the atomic
nucleus balanced by the negative charge (equal in magnitude and op-
posite in sign) of electrons in orbital clouds surrounding the nucleus.
Ions are charged atoms, formed when atoms either gain or lose one
or more electrons; ions thus have either net negative or net positive
charge. One of the beautiful things about the periodic table of chem-
ical elements is that an atom’s position on the table tells us whether
it is likely to give up electrons and become a positively charged ion
(called a cation) or take on electrons and become a negatively charged
ion (called an anion). Elements on the far left side of table will easily
give up electrons and become cations (sodium, potassium, and cal-
cium, for example), whereas elements on the far right side (except
for the last column) will tend to take on electrons and become anions
(chlorine, for example).
Elements in the rightmost column of the table have outer orbitals