The Foundations of Chemistry

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he Dalton theory of the atom and related ideas were the basis for our study of
composition stoichiometry(Chapter 2) and reaction stoichiometry(Chapter 3), but that
level of atomic theory leaves many questions unanswered. Whydo atoms combine
to form compounds? Whydo they combine only in simple numerical ratios? Whyare
particular numerical ratios of atoms observed in compounds? Whydo different elements
have different properties? Whyare they gases, liquids, solids, metals, nonmetals, and so
on? Whydo some groups of elements have similar properties and form compounds with
similar formulas? The answers to these and many other fascinating questions in chem-
istry are supplied by our modern understanding of the nature of atoms. But how can we
study something as small as an atom?
Much of the development of modern atomic theory was based on two broad types of
research carried out by dozens of scientists just before and after 1900. The first type dealt
with the electrical nature of matter. These studies led scientists to recognize that atoms
are composed of more fundamental particles and helped them to describe the approxi-
mate arrangements of these particles in atoms. The second broad area of research dealt
with the interaction of matter with energy in the form of light. Such research included
studies of the colors of the light that substances give off or absorb. These studies led to
a much more detailed understanding of the arrangements of particles in atoms. It became
clear that the arrangement of the particles determines the chemical and physical proper-
ties of each element. As we learn more about the structures of atoms, we are able to
organize chemical facts in ways that help us to understand the behavior of matter.
We will first study the particles that make up atoms and the basic structure of atoms.
Then we will take a look at the quantum mechanical theory of atoms and see how this
theory describes the arrangement of the electrons in atoms. Current atomic theory is
considerably less than complete. Even so, it is a powerful tool that helps us describe the
forces holding atoms in chemical combination with one another.

SUBATOMIC PARTICLES

FUNDAMENTAL PARTICLES

In our study of atomic structure, we look first at the fundamental particles.These are
the basic building blocks of all atoms. Atoms, and hence allmatter, consist principally of
three fundamental particles: electrons, protons,and neutrons.Knowledge of the nature and
functions of these particles is essential to understanding chemical interactions. The rela-
tive masses and charges of the three fundamental particles are shown in Table 5-1. The

5-1

5-1 Fundamental Particles 177

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 7.2, Structure of Atoms.

Many other subatomic particles, such

as quarks, positrons, neutrinos, pions,

and muons, have also been discovered.

It is not necessary to study their

characteristics to learn the

fundamentals of atomic structure that

are important in chemical reactions.

TABLE 5-1 Fundamental Particles of Matter

Charge

Particle Mass (relative scale)

electron (e
) 0.00054858 amu 1 


proton (por p) 1.0073 amu 1 

neutron (nor n^0 ) 1.0087 amu none

Accounts of some important

developments in atomic theory appear

on the World Wide Web; for example,

The National Museum of Science and

Industry can be found at

http://www.nmsi.ac.uk/on-line/

Electron