4.1
Defining the Atom
> Early Models of the Atom
Early Models of the Atom
• An atom is the smallest particle of an element
that retains its identity in a chemical reaction.
• Philosophers and scientists have proposed
many ideas on the structure of atoms.
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4.1
Defining the Atom
> Early Models of the Atom
Democritus’s Atomic Philosophy
How did Democritus describe atoms?
Democritus
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4.1
Defining the Atom
> Early Models of the Atom
Democritus believed that atoms were
indivisible and indestructible.
Democritus’s ideas were limited because
they didn’t explain chemical behavior and
they lacked experimental support
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4.1
Defining the Atom
> Early Models of the Atom
Dalton’s Atomic Theory
How did John Dalton further
Democritus’s ideas on atoms?
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4.1
Defining the Atom
> Early Models of the Atom
By using experimental methods, Dalton
transformed Democritus’s ideas on atoms
into a scientific theory.
The result was Dalton’s atomic theory.
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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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4.1
Defining the Atom
> Sizing up the Atom
Sizing up the Atom
What instruments are used to observe
individual atoms?
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4.1
Defining the Atom
> Sizing up the Atom
Despite their small size, individual atoms
are observable with instruments such as
scanning tunneling microscopes.
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4.1
Defining the Atom
> Sizing up the Atom
Iron Atoms Seen Through a Scanning
Tunneling Microscope
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4.2
Defining the Atom
>
Subatomic Particles
Subatomic Particles
What are three kinds of subatomic particles?
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4.2
Defining the Atom
> Subatomic Particles
Three kinds of subatomic particles are
electrons, protons, and neutrons.
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4.2
Defining the Atom
> 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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4.2
Defining the Atom
> Subatomic Particles
Thomson performed experiments that involved
passing electric current through gases at low
pressure.
The result was a glowing beam, or cathode ray,
that traveled from the cathode to the anode.
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4.2
Defining the Atom
> Subatomic Particles
Cathode Ray Tube
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4.2
Defining the Atom
> Subatomic Particles
A cathode ray is deflected by a magnet.
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4.2
Defining the Atom
> Subatomic Particles
A cathode ray is deflected by electrically
charged plates.
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4.2
Defining the Atom
> Subatomic Particles
Thomson concluded that a cathode ray is a
stream of electrons. Electrons are parts of the
atoms of all elements.
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4.2
Defining the Atom
> 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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4.2
Defining the Atom
> Subatomic Particles
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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4.2
Defining the Atom
> Subatomic Particles
Table 4.1 summarizes the properties of
electrons, protons, and neutrons.
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4.2
Defining the Atom
>
The Atomic Nucleus
The Atomic Nucleus
How can you describe the structure of the
nuclear atom?
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4.2
Defining the Atom
> The Atomic Nucleus
J.J. Thompson and others supposed the atom
was filled with positively charged material and
the electrons were evenly distributed throughout.
This model of the atom turned out to be shortlived, however, due to the work of Ernest
Rutherford (1871–1937).
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4.2
Defining the Atom
> The Atomic Nucleus
Ernest Rutherford’s Portrait
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4.2
Defining the Atom
> The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
In 1911, Rutherford and his coworkers at the
University of Manchester, England, directed a
narrow beam of alpha particles at a very thin
sheet of gold foil.
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4.2
Defining the Atom
> The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
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4.2
Defining the Atom
> The Atomic Nucleus
Alpha particles scatter from the gold foil.
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4.2
Defining the Atom
> The Atomic Nucleus
The Rutherford Atomic Model
Rutherford concluded that the atom is mostly
empty space. All the positive charge and almost
all of the mass are concentrated in a small region
called the nucleus.
The nucleus is the tiny central core of an atom
and is composed of protons and neutrons.
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4.2
Defining the Atom
> The Atomic Nucleus
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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4.3
Defining the Atom
>
Atomic Number
Atomic Number
What makes one element different from
another?
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4.3
Defining the Atom
> Atomic Number
Elements are different because they contain
different numbers of protons.
The atomic number of an element is the number of
protons in the nucleus of an atom of that element.
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4.3
Defining the Atom
> Atomic Number
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4.3
Defining the Atom
>
Mass Number
Mass Number
How do you find the number of neutrons in an
atom?
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4.3
Defining the Atom
> Mass Number
The total number of protons and neutrons in an
atom is called the mass number.
• The number of neutrons in an atom is the
difference between the mass number and
atomic number.
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4.3
Defining the Atom
> Mass Number
Au is the chemical symbol for gold.
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Defining the Atom
> for Sample Problem 4.1
Problem Solving 4.17 Solve Problem 17 with the help of an
interactive guided tutorial.
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4.3
Defining the Atom
>
Isotopes
Isotopes
How do isotopes of an element differ?
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4.3
Defining the Atom
> Isotopes
Isotopes are atoms that have the same number
of protons but different numbers of neutrons.
• Because isotopes of an element have
different numbers of neutrons, they also have
different mass numbers.
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4.3
Defining the Atom
> Isotopes
Despite these differences, isotopes are
chemically alike because they have identical
numbers of protons and electrons.
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4.3
Defining the Atom
>
Atomic Mass
Atomic Mass
How do you calculate the atomic mass of an
element?
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4.3
Defining the Atom
> Atomic Mass
It is useful to to compare the relative masses of
atoms to a standard reference isotope. Carbon12 is the standard reference isotope. Cabon-12
has a mass of exactly 12 atomic mass units.
An atomic mass unit (amu) is defined as one
twelfth of the mass of a carbon-12 atom.
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4.3
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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4.3
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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4.3
Defining the Atom
> Atomic Mass
For example, carbon has two stable isotopes:
• Carbon-12, which has a natural abundance of
98.89%, and
• Carbon-13, which has a natural abundance of
1.11%.
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Defining the Atom
> 4.2
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Defining the Atom
4.2>
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