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Introduction to the Modern Concept
of Atomic Structure
Objective: To describe some important features of subatomic particles.
Nucleus
∼10−13cm
∼10−8cm
Figure 3.9
A nuclear atom viewed in cross
section. (The symbol means
approximately.) This drawing
does not show the actual scale.
The nucleus is actually much
smaller compared with the size
of an atom.
C H E M I S T RY
In this model the atom is called
a nuclear atom because the
positive charge is localized in a
small, compact structure (the
nucleus) and not spread out
uniformly, as in the plum pudding view.
C H E M I S T RY
The chemistry of an atom arises
from its electrons.
W H AT I F ?
The average diameter of an
atom is 1.3 1010 m.
What if the average diameter of
an atom were 1 cm? How tall
would you be?
I
n the years since Thomson and Rutherford, a great deal has been learned
about atomic structure. The simplest view of the atom is that it consists
of a tiny nucleus (about 1013 cm in diameter) and electrons that move
about the nucleus at an average distance of about 108 cm from it (Figure
3.9). To visualize how small the nucleus is compared with the size of the
atom, consider that if the nucleus were the size of a grape, the electrons
would be about one mile away on average. The nucleus contains protons,
which have a positive charge
equal in magnitude to the elecTABLE 3.4
trons’ negative charge, and
The Mass and Charge of the
neutrons, which have almost the
Electron, Proton, and Neutron
same mass as protons but no
Relative
Relative
charge. The neutrons’ function in
Particle
Mass*
Charge
the nucleus is not obvious. They
may help hold the protons (which
electron
1
1
repel each other) together to form
proton
1836
1
the nucleus, but we will not be
neutron
1839
none
concerned with that here. The rel*The electron is arbitrarily assigned a mass of 1
ative masses and charges of the
for comparison.
electron, proton, and neutron are
shown in Table 3.4.
An important question arises at this point: If all atoms are composed of
these same components, why do different atoms have different chemical properties?
The answer lies in the number and arrangement of the electrons. The space
in which the electrons move accounts for most of the atomic volume. The
electrons are the parts of atoms that “intermingle” when atoms combine to
form molecules. Therefore, the number of electrons a given atom possesses
greatly affects the way it can interact with other atoms. As a result, atoms of
different elements, which have different numbers of electrons, show different chemical behavior. Although the atoms of different elements also differ
in their numbers of protons, it is the number of electrons that really determines chemical behavior. We will discuss how this happens in later chapters.
CHEMISTRY in ACTION
How Big Is an Atom?
1. Get a strip of paper 11 by 1.
2. Cut the paper in half. Discard one piece.
3. Repeat step 2 until you can no longer cut the paper. How many times
could you cut it?
4. How many times would you need to cut the paper to have a piece of
paper remaining that is the same width as an atom? (Average atom
diameter 1.3 1010 m.)
3.6
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