that are completely filled with electrons and tend to neither gain
nor lose electrons. Because chemical interactions between elemental
atoms depend on the gaining, losing, or sharing of electrons, these
rightmost column elements are not interested in interacting or
reacting with anything; they form a collection of unreactive gaseous
elements called the noble or inert gases. Because of their lack of re-
activity, they are not involved in any known way as part of the life
process.
The importance of water for life as we know it cannot be overem-
phasized. Water is the canvas upon which life is painted, the land-
scape or stage upon which the molecular drama of life is played out.
Water is an example of a molecule—a stable configuration of atoms
held together in a particular geometric shape by the sharing of elec-
trons between atoms.
water = H2O = H-O-HThis sharing of electrons between atoms is called a covalent chemical
bond. In water, each hydrogen atom contributes one electron and
the oxygen atom contributes two electrons for mutual sharing.
The shared electrons constitute a sort of glue that holds the atoms
together.
The molecules from which living organisms are built and the mole-
cules that interact with living organisms are often more complex than
the simple water molecule. Here is one such molecule, a relatively
small one, though substantially larger than water. This is a depiction
of the molecular structure of the fluoxetine molecule, a substance
synthesized by chemists and marketed as a treatment for the mood
disorder called depression (see Chapter 21). In the United States,
fluoxetine is associated with the brand name Prozac. It is drawn here
in a diagrammatic way that has been developed in organic chemistry