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Chapter 4
“Atomic Structure”
Defining the Atom
 OBJECTIVES:
Democritus’s ideas
about atoms.
Describe
Defining the Atom
 OBJECTIVES:
Explain
theory.
Dalton’s atomic
Defining the Atom
 OBJECTIVES:
Identify
what instrument is
used to observe individual
atoms.

Democritus’s Atomic
Philosophy
The Greek philosopher Democritus (460
B.C. – 370 B.C.) was among the first to
suggest the existence of atoms (from
the Greek word “atomos”)
 He
believed that atoms were indivisible and
indestructible
 His ideas did agree with later scientific
theory, but did not explain chemical
behavior, and was not based on the
scientific method – but just philosophy
Dalton’s Atomic Theory (experiment based!)
John Dalton
(1766 – 1844)
1) All elements are composed of
tiny indivisible particles called
atoms
2) Atoms of the same element are
identical. Atoms of any one
element are different from
those of any other element.
3) Atoms of different elements combine in
simple whole-number ratios to form
chemical compounds
4) In chemical reactions, atoms are combined,
separated, or rearranged – but never
changed into atoms of another element.
4.1
Defining the Atom
> Early Models of the Atom
All elements are composed of tiny indivisible
particles called atoms.
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4.1
Defining the Atom
> Early Models of the Atom
Atoms of the same element are identical. The
atoms of any one element are different from
those of any other element.
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4.1
Defining the Atom
> Early Models of the Atom
Atoms of different elements can physically mix
together or can chemically combine in simple
whole-number ratios to form compounds.
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4.1
Defining the Atom
> Early Models of the Atom
Chemical reactions occur when atoms are
separated, joined, or rearranged. Atoms of one
element are never changed into atoms of
another element in a chemical reaction.
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Sizing up the Atom
 Elements are able to be subdivided into
smaller and smaller particles – these are
the atoms, and they still have properties
of that element
If you could line up 100,000,000
copper atoms in a single file, they
would be approximately 1 cm long
Despite their small size, individual
atoms are observable with instruments
such as scanning tunneling (electron)
microscopes
4.1
Defining the Atom
> Sizing up the Atom
Iron Atoms Seen Through a Scanning
Tunneling Microscope
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Defining the Atom
>
Chapter 4
Atomic Structure
4.1 Defining the Atom
4.2 Structure of the Nuclear
Atom
4.3 Distinguishing Among Atoms
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Defining the Atom
>
CHEMISTRY
& YOU
How did scientists determine the
structures that are inside an atom?
Doctors often use X-rays
to see bones and other
structures that cannot be
seen through your skin.
Scientists use many
methods to “see” inside
an atom.
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Defining the Atom
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Subatomic Particles
Much of Dalton’s atomic theory is
accepted today.
• One important change, however, is that
atoms are now known to be divisible.
• They can be broken down into even
smaller, more fundamental particles,
called subatomic particles.
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Defining the Atom
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Subatomic Particles
Subatomic Particles
What are three kinds of subatomic
particles?
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Defining the Atom
>
Subatomic Particles
Three kinds of subatomic particles
are electrons, protons, and neutrons.
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Defining the Atom
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Subatomic Particles
Electrons
In 1897, the English physicist J. J.
Thomson (1856–1940) discovered the
electron.
• Electrons are negatively charged
subatomic particles.
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Defining the Atom
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Subatomic Particles
Electrons
Thomson performed experiments that
involved passing electric current through
gases at low pressure.
• He sealed the gases in glass tubes fitted at
both ends with metal disks called
electrodes.
• The electrodes were connected to a source
of electricity.
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Defining the Atom
>
Subatomic Particles
Electrons
• One
electrode,
the anode
became
positively
charged.
• The other electrode, the cathode, became
negatively charged.
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Defining the Atom
>
Subatomic Particles
Electrons
The result was a
glowing beam, or
cathode ray,
that traveled from
the cathode to
the anode.
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Defining the Atom
>
Subatomic Particles
Electrons
Thomson found
that a cathode
ray is deflected
by electrically
charged metal
plates.
• A positively charged plate attracts the cathode
ray, while a negatively charged plate repels it.
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Defining the Atom
>
Subatomic Particles
Electrons
Thomson knew that opposite charges
attract and like charges repel, so he
hypothesized that a cathode ray is a
stream of tiny negatively charged
particles moving at high speed.
• Thomson called these particles corpuscles.
• Later they were named electrons.
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Defining the Atom
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Subatomic Particles
Electrons
The U.S. physicist Robert A. Millikan
(1868–1953) carried out experiments to
find the quantity of an electron’s charge.
• In his oil-drop experiment, Millikan
suspended negatively charged oil droplets
between two charged plates.
• He then changed the voltage on the plates
to see how this affected the droplets’ rate of Slide
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fall.
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Subatomic Particles
Electrons
The U.S. physicist Robert A. Millikan
(1868–1953) carried out experiments to
find the quantity of an electron’s charge.
• From his data, he found that the
charge on each oil droplet was a
multiple of 1.60  10–19 coulomb,
meaning this must be the charge of
an electron.
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Defining the Atom
>
Subatomic Particles
Electrons
The U.S. physicist Robert A. Millikan
(1868–1953) carried out experiments to
find the quantity of an electron’s charge.
• Using this charge and Thomson’s chargeto-mass ratio of an electron, Millikan
calculated an electron’s mass.
• Millikan’s values for electron charge and
mass are similar to those accepted today.
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Defining the Atom
>
Subatomic Particles
Electrons
An electron has one unit of negative
charge, and its mass is 1/1840 the mass
of a hydrogen atom.
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Defining the Atom
>
Subatomic Particles
Protons and Neutrons
If cathode rays are electrons given off by
atoms, what remains of the atoms that
have lost the electrons?
• For example, after a hydrogen atom (the
lightest kind of atom) loses an electron, what
is left?
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Defining the Atom
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Subatomic Particles
Protons and Neutrons
You can think through this problem using four
simple ideas about matter and electric charges.
1. Atoms have no net electric charge; they are
electrically neutral.
2. Electric charges are carried by particles of matter.
3. Electric charges always exist in whole-number
multiples of a single basic unit; that is, there are no
fractions of charges.
4. When a given number of negatively charged particles
combines with an equal number of positively charged Slide
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particles, an electrically neutral particle is formed.
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Subatomic Particles
Protons and Neutrons
It follows that a particle with one unit of
positive charge should remain when a
typical hydrogen atom loses an electron.
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Defining the Atom
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Subatomic Particles
Protons and Neutrons
In 1886, Eugen Goldstein (1850–1930)
observed a cathode-ray tube and found
rays traveling in the direction opposite to
that of the cathode rays.
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Defining the Atom
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Subatomic Particles
Protons and Neutrons
In 1886, Eugen Goldstein (1850–1930)
observed a cathode-ray tube and found
rays traveling in the direction opposite to
that of the cathode rays.
• He concluded that they were composed of
positive particles.
• Such positively charged subatomic particles
are called protons.
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Defining the Atom
>
Subatomic Particles
Protons and Neutrons
In 1932, the English physicist James
Chadwick (1891–1974) confirmed the
existence of yet another subatomic
particle: the neutron.
• Neutrons are subatomic particles with no
charge but with a mass nearly equal to that
of a proton.
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Defining the Atom
>
Interpret Data
The table below summarizes the
properties of these subatomic particles.
Properties of Subatomic Particles
Particle
Symbol
Relative
charge
Relative mass
(mass of proton = 1)
Actual mass
(g)
Electron
e–
1–
1/1840
9.11  10–28
Proton
p+
1+
1
1.67  10–24
Neutron
n0
0
1
1.67  10–24
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Defining the Atom
>
Subatomic Particles
Although protons and neutrons are
extremely small, theoretical physicists
believe that they are composed of yet
smaller subnuclear particles called quarks.
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Defining the Atom
> The Atomic Nucleus
When subatomic particles were discovered,
scientists wondered how the particles were
put together in an atom.
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Defining the Atom
>
When subatomic particles were discovered,
scientists wondered how the particles were
put together in an atom.
• Most scientists—including J. J. Thompson—
thought it likely that the electrons were evenly
distributed throughout an atom filled uniformly
with positively charged material.
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Defining the Atom
> The Atomic Nucleus
When subatomic particles were discovered,
scientists wondered how the particles were
put together in an atom.
• Most scientists—including J. J. Thompson—
thought it likely that the electrons were evenly
distributed throughout an atom filled uniformly
with positively charged material.
– In Thomson’s atomic model, known as the “plumpudding model,” electrons were stuck into a lump
of positive charge, similar to raisins stuck in
dough.
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Defining the Atom
>
The Atomic Nucleus
This model of the atom turned out to be
short-lived, however, due to the work of a
former student of Thomson, Ernest
Rutherford (1871–1937).
• Born in New Zealand,
Rutherford was
awarded the Nobel
Prize for Chemistry in
1908. His portrait
appears on the New
Zealand $100 bill.
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Defining the Atom
>
The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
In 1911, Rutherford and his co-workers
wanted to test the existing plum-pudding
model of atomic structure.
• They devised the gold-foil experiment.
• Their test used alpha particles, which are helium
atoms that have lost their two electrons and
have a double positive charge because of the
two remaining protons.
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Defining the Atom
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The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
In the experiment, a narrow beam of alpha particles
was directed at a very thin sheet of gold.
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Defining the Atom
>
The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
In the experiment, a narrow beam of alpha particles
was directed at a very thin sheet of gold.
• According to the prevailing
theory, the alpha particles
should have passed easily
through the gold, with only
a slight deflection due to
the positive charge thought
to be spread out in the gold
atoms.
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Defining the Atom
>
The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
Rutherford’s results were that most alpha particles
went straight through, or were slightly deflected.
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Defining the Atom
>
The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
Rutherford’s results were that most alpha particles
went straight through, or were slightly deflected.
• What was surprising is
that a small fraction of
the alpha particles
bounced off the gold foil
at very large angles.
• Some even bounced
straight back toward the
source.
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Defining the Atom
>
The Atomic Nucleus
The Rutherford Atomic Model
Based on his experimental results,
Rutherford suggested a new theory of the
atom.
• He proposed that the atom is mostly empty
space.
– Thus explaining the lack of deflection of most
of the alpha particles.
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Defining the Atom
>
The Atomic Nucleus
The Rutherford Atomic Model
Based on his experimental results,
Rutherford suggested a new theory of the
atom.
• He concluded that all the positive charge and
almost all of the mass are concentrated in a
small region that has enough positive charge to
account for the great deflection of some of the
alpha particles.
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Defining the Atom
>
The Atomic Nucleus
The Rutherford Atomic Model
The Rutherford atomic model is known as
the nuclear atom.
In the nuclear atom, the protons and
neutrons are located in the positively
charged nucleus. The electrons are
distributed around the nucleus and occupy
almost all the volume of the atom.
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Defining the Atom
>
The Atomic Nucleus
The Rutherford Atomic Model
According to this model, the nucleus is tiny
and densely packed compared with the
atom as a whole.
• If an atom were the size of a football stadium,
the nucleus would be about the size of a
marble.
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Defining the Atom
>
The Atomic Nucleus
The Rutherford Atomic Model
Rutherford’s model turned out to be
incomplete.
• The Rutherford atomic model had to be revised
in order to explain the chemical properties of
elements.
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Defining the Atom
>
What evidence from Rutherford’s
Gold-Foil experiment disproves J.J.
Thompson’s “plum-pudding model”?
Rutherford observed that most of the
particles passed through the foil with no
deflection, and a small fraction were
deflected at large angles or reflected directly
back. If the plum-pudding model was true,
most alpha particles would have been
deflected at small angles by the evenlySlide
spaced electrons.
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Defining the Atom
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Key Concepts
• Three kinds of subatomic particles are
electrons, protons, and neutrons.
• In the nuclear atom, the protons and
neutrons are located in the nucleus. The
electrons are distributed around the
nucleus and occupy almost all the volume
of the atom.
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Defining the Atom
>
Glossary Terms
• electron: a negatively charged subatomic
particle
• cathode ray: a stream of electrons produced
at the negative electrode (cathode) of a tube
containing a gas at low pressure
• proton: a positively charged subatomic
particle found in the nucleus of an atom
• neutron: a subatomic particle with no charge
and a mass of 1 amu; found in the nucleus of
an atom
• nucleus: the tiny, dense central portion of an
atom, composed of protons and neutrons
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Defining the Atom
>
BIG IDEA
Electrons and the Structure of Atoms
Atoms have positively-charged protons
and neutral neutrons inside a nucleus,
and negatively-charged electrons outside
the nucleus.
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Defining the Atom
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Chapter 4
Atomic Structure
4.1 Defining the Atom
4.2 Structure of the Nuclear Atom
4.3 Distinguishing Among
Atoms
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Defining the Atom
>
CHEMISTRY
& YOU
How can there be different varieties of
atoms?
Just as there are
many types of dogs,
atoms come in
different varieties
too.
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Defining the Atom
> Atomic Number and Mass Number
Atomic Number and Mass Number
What makes one element different from
another?
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Defining the Atom
> Atomic Number and Mass Number
Atomic Number
Elements are different because they
contain different numbers of protons.
• An element’s atomic number is the
number of protons in the nucleus of an
atom of that element.
• The atomic number identifies an
element.
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Defining the Atom
>
Interpret Data
For each element listed in the table below, the
number of protons equals the number of electrons.
Atoms of the First Ten Elements
Name
Symbol
Atomic
number
Protons
Neutrons
Mass
number
Electrons
Hydrogen
H
1
1
0
1
1
Helium
He
2
2
2
4
2
Lithium
Li
3
3
4
7
3
Beryllium
Be
4
4
5
9
4
Boron
B
5
5
6
11
5
Carbon
C
6
6
6
12
6
Nitrogen
N
7
7
7
14
7
Oxygen
O
8
8
8
16
8
Fluorine
F
9
9
10
19
9
20
10
Neon
Ne
10
10
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Defining the Atom
> Atomic Number and Mass Number
Atomic Number
Remember that atoms are
electrically neutral.
• Thus, the number of electrons (negatively
charged particles) must equal the number
of protons (positively charged particles).
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Defining the Atom
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Sample Problem 4.1
Understanding Atomic Number
The element nitrogen (N) has an atomic
number of 7. How many protons and
electrons are in a neutral nitrogen atom?
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Defining the Atom
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Sample Problem 4.1
1 Analyze Identify the relevant concepts.
The atomic number gives the number of
protons, which in a neutral atom equals
the number of electrons.
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Defining the Atom
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Sample Problem 4.1
2 Solve Apply the concepts to this problem.
• Identify the atomic number.
• Then use the atomic number to find
the number of protons and electrons.
The atomic number of nitrogen is 7.
So, a neutral nitrogen atom has 7 protons
and 7 electrons.
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Defining the Atom
> Atomic Number and Mass Number
Mass Number
The total number of protons and
neutrons in an atom is called the mass
number.
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Defining the Atom
> Atomic Number and Mass Number
Mass Number
If you know the atomic number and
mass number of an atom of any
element, you can determine the atom’s
composition.
The number of neutrons in an atom is the
difference between the mass number and
atomic number.
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Defining the Atom
> Atomic Number and Mass Number
Mass Number
If you know the atomic number and
mass number of an atom of any
element, you can determine the atom’s
composition.
Number of neutrons = mass number – atomic number
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Defining the Atom
> Atomic Number and Mass Number
Mass Number
The composition of any
atom can be
represented in
shorthand notation
using atomic number
and mass number.
Au is the
chemical
symbol for
gold.
• The atomic number is the subscript.
• The mass number is the superscript.
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Defining the Atom
> Atomic Number and Mass Number
Mass Number
You can also refer to
atoms by using the
mass number and the
name of the element.
•
197
79
Au may be written as gold-197.
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Au is the
chemical
symbol for
gold.
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Defining the Atom
>
Sample Problem 4.2
Determining the Composition of an Atom
How many protons, electrons, and neutrons
are in each atom?
9
4
a. Be
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20
10
b. Ne
23
11
c. Na
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Sample Problem 4.2
1 Analyze List the knowns and the unknowns.
Use the definitions of atomic number and mass
number to calculate the numbers of protons,
electrons, and neutrons.
KNOWNS
Beryllium (Be)
atomic number = 4
mass number = 9
Neon (Ne)
atomic number = 10
mass number = 20
Sodium (Na)
atomic number = 11
mass number = 23
UNKNOWNS
protons = ?
electrons = ?
neutrons = ?
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Sample Problem 4.2
2 Calculate Solve for the unknowns.
Use the atomic number to find the number of
protons.
atomic number = number of protons
a. 4
b. 10
c. 11
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Sample Problem 4.2
2 Calculate Solve for the unknowns.
Use the atomic number to find the number of
electrons.
atomic number = number of electrons
a. 4
b. 10
c. 11
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Defining the Atom
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Sample Problem 4.2
2 Calculate Solve for the unknowns.
Use the mass number and atomic number
to find the number of neutrons.
number of neutrons = mass number – atomic number
a. number of neutrons = 9 – 4 = 5
b. number of neutrons = 20 – 10 = 10
c. number of neutrons = 23 – 11 = 12
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Defining the Atom
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Sample Problem 4.2
3 Evaluate Do the results make sense?
• For each atom, the mass number
equals the number of protons plus the
number of neutrons.
• The results make sense.
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What information is needed to determine
the composition of a neutral atom of any
element?
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What information is needed to determine
the composition of a neutral atom of any
element?
The atomic number and mass number are needed
to determine an atom’s composition. The atomic
number gives the number of protons, which equals
the number of electrons. The number of neutrons
is the difference between the mass number and
the atomic number.
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Defining the Atom
> Isotopes
There are three different kinds of neon
atoms.
• How do these atoms differ?
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Defining the Atom
> Isotopes
• All have the same number of protons (10).
• All have the same number of electrons (10).
• But they each have different numbers of
neutrons.
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Defining the Atom
> Isotopes
Isotopes are atoms that have the same
number of protons but different numbers
of neutrons.
• Neon-20, neon-21, and neon 22 are three
isotopes of neon.
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Isotopes
Because isotopes of an element have
different numbers of neutrons, they
also have different mass numbers.
• Despite these differences, isotopes are
chemically alike because they have
identical numbers of protons and
electrons, which are the subatomic
particles responsible for chemical
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behavior.
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Defining the Atom
> Isotopes
Remember the dogs at the beginning of
the lesson.
• Their color or size doesn’t
change the fact that they are
all dogs.
• Similarly, the number of
neutrons in isotopes of an
element does not change
which element it is because the
atomic number does not
change.
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Defining the Atom
>
CHEMISTRY
& YOU
How are the atoms of one element
different from the atoms of another
element? How are isotopes of the same
element different?
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CHEMISTRY
& YOU
How are the atoms of one element
different from the atoms of another
element? How are isotopes of the same
element different?
Atoms of different elements are different because
they contain different numbers of protons.
Isotopes of the same element are different
because they have different numbers of neutrons,
and thus different mass numbers.
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Sample Problem 4.3
Writing Chemical Symbols of
Isotopes
Diamonds are a naturally occurring form of
elemental carbon. Two stable isotopes of
carbon are carbon-12 and carbon-13. Write
the symbol for each isotope using
superscripts and subscripts to represent the
mass number and the atomic number.
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Defining the Atom
>
Sample Problem 4.3
1 Analyze Identify the relevant concepts.
Isotopes are atoms that have the same
number of protons but different numbers
of neutrons. The composition of an atom
can be expressed by writing the chemical
symbol, with the atomic number as a
subscript and the mass number as a
superscript.
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Sample Problem 4.3
2 Solve Apply the concepts to this problem.
Use Table 4.2 to identify the symbol
and the atomic number for carbon.
The symbol for carbon is C.
The atomic number of carbon is 6.
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Defining the Atom
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Sample Problem 4.3
2 Solve Apply the concepts to this problem.
Look at the name of the isotope to find
the mass number.
For carbon-12, the mass number is 12.
For carbon-13, the mass number is 13.
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Defining the Atom
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Sample Problem 4.3
2 Solve Apply the concepts to this problem.
Use the symbol, atomic number, and
mass number to write the symbol of the
isotope.
For carbon-12, the symbol is
12
6
C.
For carbon-13, the symbol is
13
6
C.
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Defining the Atom
>
Atomic Mass
Atomic Mass
How do you calculate the atomic mass
of an element?
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Defining the Atom
> Atomic Mass
The mass of even the largest atom is
incredibly small.
• Since the 1920s, it has been possible to
determine the tiny masses of atoms by
using a mass spectrometer.
• The mass of a fluorine atom was found to
be 3.155 x 10–23 g.
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Defining the Atom
> Atomic Mass
Such data about the actual masses of
individual atoms can provide useful
information, but in general these values
are inconveniently small and impractical
to work with.
• Instead, it is more useful to compare the
relative masses of atoms using a reference
isotope as a standard.
• The reference isotope chosen is carbon-12.
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Defining the Atom
> Atomic Mass
This isotope of carbon has been
assigned a mass of exactly 12 atomic
mass units.
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Defining the Atom
> Atomic Mass
In nature, most elements occur as a
mixture of two or more isotopes.
• Each isotope of an element has a fixed
mass and a natural percent abundance.
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Defining the Atom
>
Interpret Data
Natural Percent Abundance of
Stable Isotopes of Some Elements
Name
Hydrogen
Symbol
1
1
2
1
H
H
3
1H
Helium
3
2
4
2
Carbon
12
6
13
6
He
He
C
C
16
8
Oxygen
O
17
8O
18
O
8
Chlorine
35
17
37
17
Cl
Cl
Natural percent
abundance
Mass (amu)
Atomic mass
99.985
0.015
negligible
1.0078
2.0141
3.0160
1.0079
0.0001
99.9999
3.0160
4.0026
4.0026
98.89
1.11
12.000
13.003
12.011
99.759
0.037
0.204
15.995
16.995
17.999
15.999
75.77
34.969
24.23
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35.453
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Defining the Atom
> Atomic Mass
Chlorine occurs as two isotopes: chlorine35 and chlorine-37.
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Defining the Atom
> Atomic Mass
Chlorine occurs as two isotopes: chlorine35 and chlorine-37.
• If you calculate
the arithmetic
mean of these
two masses
((34.968 amu + 36.966 amu)/2), you get an
average atomic mass of 35.986.
• However, this value is higher than the actual
value of 35.453.
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Defining the Atom
> Atomic Mass
Chlorine occurs as two isotopes: chlorine35 and chlorine-37.
• To explain this
difference, you
need to know the
natural percent
abundance of the isotopes of chlorine.
• Chlorine-35 accounts for 75 percent of the
naturally occurring chlorine atoms; chlorine-37
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accounts for only 24 percent.
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Defining the Atom
> Atomic Mass
Because there is more chlorine-35 than chlorine37 in nature, the atomic mass of chlorine, 35.453
amu, is closer to 35 than to 37.
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Defining the Atom
> Atomic Mass
The atomic mass of an element is a
weighted average mass of the atoms in a
naturally occurring sample of the
element.
• A weighted average mass reflects both
the mass and the relative abundance of
the isotopes as they occur in nature.
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Defining the Atom
>
Sample Problem 4.4
Understanding Relative
Abundance of Isotopes
The atomic mass of copper is
63.546 amu. Which of copper’s
two isotopes is more abundant:
copper-63 or copper-65?
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Defining the Atom
> Atomic Mass
• To calculate the atomic mass of an
element, multiply the mass of each
isotope by its natural abundance,
expressed as a decimal, and then add
the products.
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Defining the Atom
> Atomic Mass
Carbon has two stable isotopes: carbon-12,
which has a natural abundance of 98.89 percent,
and carbon-13, which has a natural abundance
of 1.11 percent.
• The mass of carbon-12 is 12.000 amu; the mass
of carbon-13 is 13.003 amu.
• The atomic mass of carbon is calculated as
follows:
Atomic mass of carbon = (12.000 amu x 0.9889) + 13.003 amu x 0.0111)
= (11.867 amu) + (0.144 amu)
= 12.011 amu
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Defining the Atom
>
Sample Problem 4.5
Calculating Atomic Mass
Element X has two naturally occurring isotopes.
The isotope with a mass of 10.012 amu (10X)
has a relative abundance of 19.91 percent. The
isotope with a mass of 11.009 amu (11X) has a
relative abundance of 80.09 percent. Calculate
the atomic mass of element X.
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Sample Problem 4.5
2 Calculate Solve for the unknowns.
Use the atomic mass and the decimal form
of the percent abundance to find the mass
contributed by each isotope.
for 10X: 10.012 amu x 0.1991 = 1.993 amu
for 11X: 11.009 amu x 0.8009 = 8.817 amu
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Sample Problem 4.5
2 Calculate Solve for the unknowns.
Add the atomic mass contributions for all
the isotopes.
For element X, atomic mass = 1.953 amu + 8.817 amu
= 10.810 amu
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Sample Problem 4.5
3 Evaluate Does the result make sense?
The calculated value is closer to the
mass of the more abundant isotope, as
would be expected.
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Why is the atomic mass of an element
usually not a whole number?
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Why is the atomic mass of an element
usually not a whole number?
The atomic mass of an element is usually
not a whole number because it is a
weighted average of the masses of the
naturally occurring isotopes of the
element.
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