Basics and Discovery of Atmospheric Chemicals - Air Pollution and Global Warming: History, Science, and Solutions - page 2

Table 1.1. Characteristics of the first ten elements in the periodic table

Number of
protons

Number of

(atomic neutrons in Atomic mass Number of

(g mol −1 ) Element Symbol number) main isotope electrons

Hydrogen H 1 0 1.00794 1

Helium He 2 2 4.00206 2

Lithium Li 3 4 6.941 3

Beryllium Be 4 5 9.01218 4

Boron B 5 6 10.811 5

Carbon C 6 6 12.011 6

Nitrogen N 7 7 14.0067 7

Oxygen O 8 8 15.9994 8

Fluorine F 9 10 18.9984 9

Neon Ne 10 10 20.1797 10

The periodic table of the elements ,developed in
1869 by Russian chemist Dmitri Mendeleev (1834-
1907), lists elements in order of increasing atomic num-
ber. Table 1.1 identifies the first ten elements of the peri-
odic table and some of their characteristics. The atomic
mass of an element in the periodic table is the sum, over
all isotopes of the element, of the percentage occurrence
in nature of the isotope multiplied by the atomic mass
of the isotope.
The simplest element in the periodic table is hydro-
gen (H), which contains one proton, no neutrons, and
one electron. Hydrogen occurs in three natural isotopic
forms. The most common is protium (one proton and
one electron), shown in Figure 1.1. The other two are
deuterium ,which contains one proton, one neutron,
and one electron, and tritium ,which contains one pro-
ton, two neutrons, and one electron. Helium (He), also
shown in Figure 1.1, is the second simplest element and
contains two protons, two neutrons, and two electrons.

When one atom bonds to another atom of either the
same or different atomic number, it forms a molecule.
A molecule is a group of atoms of like or different
elements held together by chemical forces. When a
molecule consists of different elements, it is a com-
pound. A compound is a substance consisting of atoms
of two or more elements in definite proportions that
cannot be separated by physical means.

1.1.2. Gases and Particles
Gases are distinguished from particles in two ways.
First, a gas consists of individual atoms or molecules
that are separated, whereas a particle consists of an
aggregate of atoms or molecules bonded together. Thus,
a particle is larger than a single gas atom or molecule.
Second, whereas particles contain liquids or solids,
gases are in their own phase state. Particles may be
further segregated into aerosol particles and hydro-
meteor particles.
An aerosol is an ensemble of solid, liquid, or mixed-
phase particles suspended in air. An aerosol particle is
asingle liquid, solid, or mixed-phase particle among an
ensemble of suspended particles. The term aerosol was
coined by British physicochemist Frederick George
Donnan (1870-1956) near the end of World War I
(Green and Lane, 1969).
A hydrometeor is an ensemble of liquid, solid, or
mixed-phase water particles suspended in or falling
through the air. A hydrometeor particle is a single
such particle. Examples of hydrometeor particles are

(a) Hydrogen (b) Helium

Figure 1.1. Simplified configuration of protons,
neutrons, and electrons in (a) protium, an isotope of
the hydrogen atom, and (b) a helium atom.