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The Atom
VOCABULARY
atom
excited state
neutron
atomic mass
ground state
nucleus
atomic mass unit
heterogeneous
orbital
atomic number
homogeneous
proton
compound
isotope
pure substance
electron
mass number
element
mixture
wave-mechanical
model
Chemistry is the study of matter, which is anything that has mass and volume. You are made up
of matter, as is your desk. Even the gases that
make up air are matter. Chemistry deals with the
composition of matter and changes that occur in it.
themselves made of fire, wind, earth, or water. As
Boyle's ideas were slowly accepted, additional elements were discovered, and the Greek concept of
what makes up matter faded.
Dalton's Theory
Early Studies of Matter
The study of matter has roots in ancient Greece
more than 2000 years ago. The Greeks proposed
that matter is made up of particles, but not particles of elements, as we know them today. Instead
they viewed matter as being composed of four elemental substances-fire, earth, wind, and wateras shown in Figure 1-1.
D
Air
d
Fire
Water
The work of Boyle led John Dalton to propose his
revolutionary theory in the 1700s. He theorized
that the basic unit of matter is a tiny particle
called an atom.
DALTON'S ATOMIC THEORY Dalton's theory of
the atom can be summarized by the following
points.
• All elements are composed of indivisible atoms.
• All atoms of a given element are identical.
• Atoms of different elements are different; that
is, they have different masses.
• Compounds are formed by the combination of
atoms of different elements.
Although we now know that some of Dalton's
theory was not correct, it laid the important
groundwork for the current concept of the atom.
Parts of the Atom
Figure 1-1. The four elements:
Fire,air,earth,and water
A Change of Thought
This view of the nature of matter lasted until the
1600s when Robert Boyle identified gold and silver
as themselves being elemental; that is, they are not
Experimental studies of the atom soon showed
that it was not indivisible but was made up of even
smaller parts.
ELECTRONS J. J. Thomson used a cathode ray
tube to show one of these smaller units that make
up an atom. Because the ray produced in the tube
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1
was deflected a certain way by an electrical or
magnetic field, he concluded that the ray was
formed by particles and that the particles were
negatively charged. The only source available for
the particles was the atoms present. Thus,
Thompson theorized that an atom contains small,
negatively charged particles, which he named
Scattered
()(particles
electrons.
A concept of the atom developed in which these
negatively charged particles were visualized as
being embedded in atoms, just as we might find
raisins in bread. This model was called the "plum
pudding" model. In this model, the mass of the rest
of the atom was evenly distributed and positively
charged, taking up all of the space not occupied by
the electrons.
THE NUCLEUS If electrons are present in atoms,
what makes up the rest of the atom? One scientist
who studied this question was Ernest Rutherford.
A group of scientists that included Rutherford conducted the following experiment, with surprising
results. Look at Figure 1-2A.They directed alpha
particles, which are positively charged particles
that are much smaller than an atom, at a thin
piece of gold foil. If the plum pudding model of the
atom were correct, all of the alpha particles would
pass through the foil with just a few being slightly
deflected.
As the scientists expected, most of the particles
passed straight through the foil, and a few were
slightly deflected. But to their amazement, some of
the alpha particles were greatly deflected, and
some even bounced back, as shown in Figure 1-2B.
From this experiment, Rutherford concluded that
atoms have a dense central core, called a nucleus,
while the remainder of the atom is essentially
empty space. Because alpha particles are positively
charged and were repelled by the nucleus, the
nucleus must also be positively charged because
like charges repel each other.
Modern Atomic Theory
Scientists used the information derived from this
and other experiments to further describe atomic
structure.
THE BOHR ATOM Based on these recent developments, Neils Bohr proposed a model of the atom
showing a dense nucleus with electrons found in
surrounding orbits. Look at the Bohr model shown
in Figure 1-3. It shows a nucleus surrounded by
electrons in circular orbits. To stay in such an orbit,
2
Topic 1
Circular
fluorescent
screen
Thin gold foil
Most particles are
undeflected
A
)))))))))))))))))
))))))))))))))))))
Incident
()(particles
e-(i
))))))))))))))))))
)))))))))))))))))))))
8
Figure 1-2. The Rutherford gold-foil experiment:
(A) Scattering of alpha particles (8) The deflections of the alpha
particles showed that atoms have a dense, positively charged
center. Experimental results showed that only about one alpha
particle out of each 10,000 was dramatically deflected.
each electron in an atom must possess just the
right amount of energy to keep it in place around
the nucleus. Note that the maximum number of
electrons in the first energy level is two. The second level has a maximum of eight electrons.
This model, often called the planetary model,
remained the basis for the structure of the atom for
some time. While this model did show the separation of the nucleus and the electrons, it had serious
flaws because certain experimental evidence did
not support it.
THE WAVE-MECHANICAL MODEL Advances in
the study of energy aided in modifying the atomic
model. Energy had been viewed as being waves,
Most probable
electron
location
/.~--;; ~.,-. /
/{:.-~..
I;
\ •
\\
/
Electron
/'
A
"~~\
.;:.)'
,rs)1
-\:Z~.:::: .:o~j:(/'
Nucleus
Figure 1-3. A Bohr model of the element lithium
and matter as particles. By the 1900s, energy and
matter were both viewed as acting as both waves
and particles. The wave aspect of nature was
expanded, and it was also proposed that energy
was made up of tiny packets called quanta. These
energy packets acted like particles.
When it was later determined that the electron
not only has properties of mass but also has wavelike properties, this concept of a dual nature was
incorporated into the current model of the atom,
the wave-mechanical model. This modern model of
the atom pictures the atom as having a dense, positively charged nucleus as proposed in the planetary model. The major difference between the
wave-mechanical model and the Bohr model is
found in the manner in which the electrons are
pictured. Instead of moving in definite, fixed orbits
around the nucleus as suggested in the Bohr
model, the wave-mechanical model portrays electrons with distinct amounts of energy moving in
areas called orbitals. An orbital is described as a
region in which an electron of a particular amount
of energy is most likely to be located. Models of
orbitals are shown in Figure 1-4.
Thus, the modern model of the atom is not the
invention of a single scientist, but rather one that
Figure 1-4. Electron cloud model of the atom: The modern
model of the atom shows a dense nucleus. (A) In this diagram of
hydrogen, each dot represents a possible location for the electron.
The ring shows the most probable location of an electron. (B) In
this model of a cross-section of a multielectron atom, the dots represent probable locations of electrons. Each of the darker circles
represents an orbital.
has evolved over a long period of time. Figure 1-5
summarizes some of the atomic models involved in
the evolution of the current atomic model.
+
+
Dalton
Thomson
Rutherford
Bohr
Present
Figure 1-S. Representations of cannonball, plum pudding, nuclear,
planetary, and wave-mechanical atomic models
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Review Questions
10. What are the major differences between the plum
pudding and planetary models of the atom?
1. The concept that matter is composed of tiny, discrete
particles is generally attributed to the (1) Greeks
(2) Romans (3) English (4) Germans
11. Why did Rutherford conclude that the atom was
mostly empty space?
12. What is a major difference between the Rutherford
2. The first subatomic particle discovered was the
(1) proton
(2) neutron
(3) electron
and the wave-mechanical
models of the atom?
(4) photon
3. According to the law of the conservation of mass, if
the products of a reaction have a mass of 64.0 g, then
the total mass of the reactants (1) must be less than
64.0 g (2) must be equal to 64.0 g (3) must be more
than 64.0 g (4) is not related to the mass of the
products
4. The gold-foil experiment led scientists to conclude
that an atom's (1) positive charge is evenly distributed throughout its volume (2) negative charge is
mainly concentrated in its nucleus (3) mass is evenly
distributed throughout its volume (4) volume is
mainly unoccupied
5. The model of the atom that pictured the atom with
electrons stuck randomly throughout the mass of
the atom was called the (1) cannonball model
(2) plum pudding model (3) planetary model
(4) wave-mechanical model
6. After bombarding
a gold foil sheet with alpha particles, scientists concluded that atoms mainly consist
of (1) electrons (2) empty space (3) protons
(4) neutrons
7. Experimental evidence indicates that the nucleus of
an atom (1) contains most of the mass of the atom
(2) contains a small percentage of the mass of the
atom (3) has no charge (4) has a negative charge
8. Dalton's atomic theory states that (1) all atoms of an
element are positively charged (2) different elements can have the same mass (3) atoms of a given
element must be identical (4) all the atoms in a
compound are identical
9. Modern theory pictures an electron as (1) a particle
only (2) a wave only (3) both a particle and a wave
(4) neither a particle nor a wave
The Structure of the Atom
You have seen that all atoms are composed of a
small, dense, positively charged nucleus surrounded by a large space occupied by electrons.
Research has shown that the nucleus contains two
types of particles-protons with a positive charge,
and neutrons with no charge.
Subatomic Particles
Protons have a mass of only 1.67 X 10-24 g.
Because the mass of a proton is so small, it is more
convenient to use a different scale whose units are
called atomic mass units to represent its mass. A
proton is assigned 1.0 atomic mass unit (amu). A
neutron has approximately the same mass as a
proton.
Each atom of a specific element must contain
the same number of protons as each other atom of
that element. The number of protons in the
nucleus of an atom is the atomic number of that
element. For example, chlorine has an atomic
number of 17. Each chlorine atom contains 17
protons in its nucleus.
Electrons occupy the space of an atom outside
the nucleus and have a charge equal to, but opposite of, R proton. Electrons are much less massive
than either the proton or neutron, having a mass of
only 1/1836 amu. Table 1-1 summarizes information about each of the particles that make up an
atom.
It has been mentioned that the mass of an
atom is extremely small. The atomic mass scale
replaces grams as the unit used to describe the
masses of atoms. The nucleus of a carbon atom
containing 6 protons and 6 neutrons is taken as
Table 1-1. Some Subatomic Particles
Particle
Topic 1
Mass
Location
Symbol
1
1
1
Proton
+1
1amu
nucleus
Neutron
0
1amu
nucleus
on
1/1836 amu
outside
-le
Electron
4
Charge
-1
H or vp
1
0