Basics and Discovery of Atmospheric Chemicals - Air Pollution and Global Warming: History, Science, and Solutions - page 1

Chapter 1

Basics and Discovery of Atmospheric
Chemicals

carries a charge of -1 and a tiny mass. 1 The nucleus
of an atom consists of 1 to 118 protons and 0 to 165
neutrons. Protons have a net charge of

The study of air pollution begins with the study of chem-
icals that comprise the air. These chemicals include
molecules in the gas, liquid, or solid phases. Because
the air contains so many different types of molecules,
it is helpful to become familiar with the more impor-
tant ones through the history of their discovery. Such a
history also gives insight into characteristics of atmo-
spheric chemicals and an understanding of how much
our knowledge of air pollution today relies on the scien-
tific achievements of alchemists, chemists, natural sci-
entists, and physicists of the past. This chapter begins
with some basic definitions, and then examines histori-
cal discoveries of chemicals of atmospheric importance.
Finally, types of chemical reactions that occur in the
atmosphere are identified, and chemical lifetimes are
defined.

1 and a mass
1,836 times that of an electron. Neutrons have zero net
charge and a mass 1,839 times that of an electron. For
the net charge of an atom to be zero, the number of
electrons must equal the number of protons. Positively
charged atoms have fewer electrons than protons. Nega-
tively charged atoms have more electrons than protons.
Positively or negatively charged atoms are called ions .
The average mass of protons plus neutrons in a
nucleus is called the atomic mass .Electrons are not
included in the atomic mass calculation because the
summed mass of electrons in an atom is small in com-
parison with the summed masses of protons and neu-
trons. The number of protons in an atomic nucleus is
the atomic number .
An element is a single atom or a substance composed
of several atoms, each with the same atomic number
(the same number of protons in its nucleus). Whereas
all atoms of an element have a fixed number of protons,
not all atoms of the element have the same number of
neutrons. Atoms of an element with the same number of
protons but a different number of neutrons are isotopes
of the element. Isotopes of an element have different
atomic masses but similar chemical characteristics.

+

1.1. Basic Definitions
Air is a mixture of gases and particles, both of
which are made of atoms. In this section, atoms, ele-
ments, molecules, compounds, gases, and particles are
defined.

1.1.1. Atoms, Elements, Molecules,
and Compounds
In 1913, Niels Bohr (1885-1962), a Danish physi-
cist, proposed that an atom consists of one or more
negatively charged electrons in discrete circular orbits
around a positively charged nucleus. Each electron

1

Mass is an absolute property of a material. Mass, multiplied by
gravity, equals weight, which is a force. Because gravity varies with
location and altitude, weight is a relative property of a material. A
person who is nearly “weightless” in space, where gravity is small,
has the same mass, whether in space or on the surface of the Earth.