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Lesson 1: Atoms—Basic Units of Matter
Lesson 2: Discovering Parts of the Atom
Lesson 3: Elements, Isotopes, and Ions—
How Atoms Differ
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4.1 Atoms—Basic Units of Matter
matter
atom
nucleus
proton
neutron
electron
4.1 Atoms—Basic Units of Matter
What is the current atomic model?
• Matter is everything that has mass and takes
up space, such as gases, solids, and liquids.
• Matter is not sound, heat, or light—these
are forms of energy.
• An atom is a very small particle that
makes up all matter.
4.1 Atoms—Basic Units of Matter
Parts of the Atom
• Atomic-force microscopes show the surfaces
of atoms.
• The nucleus is the region located in the
center of the atom.
• A particle with a positive charge is a proton.
• A particle with a negative charge is an
electron.
• A neutron has no charge.
4.1 Atoms—Basic Units of Matter
Parts of the Atom (cont.)
4.1 Atoms—Basic Units of Matter
Parts of the Atom (cont.)
4.1 Atoms—Basic Units of Matter
The Size of Atoms
• Protons, neutrons, and electrons are all
smaller than the atom.
4.1 Atoms—Basic Units of Matter
Historical Evidence of Atoms
• Democritus (460–370 B.C.) was the first to
propose that atoms were indivisible solid
spheres with no holes.
4.1 Atoms—Basic Units of Matter
The Law of Conservation of Mass
• A chemical reaction rearranges atoms of
one substance into another substance
with different properties.
• The total mass of the starting materials is
always equal to the total mass of the
product.
4.1 Atoms—Basic Units of Matter
The Law of Definite Proportions
• Any pure compound always contains the
same elements in the same proportion.
– Water from your kitchen is the same as
water in a glacier on Mars.
– H2O: two hydrogen atoms and one
oxygen atom
4.1 Atoms—Basic Units of Matter
Dalton’s Atomic Model
1. All matter is made up of atoms.
2. Atoms are neither created nor destroyed
in chemical reactions.
3. Atoms of different elements combine in
whole-number ratios.
4. Each element is made of a different kind
of atom.
5. The atoms of different elements have
different masses and properties.
4.1 Atoms—Basic Units of Matter
Dalton’s Atomic Model (cont.)
4.1 Atoms—Basic Units of Matter
Which is NOT a particle in an atom?
A positron
B neutron
C electron
D proton
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D
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C
0%
B
A
B
C
D
A
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2.
3.
4.
4.1 Atoms—Basic Units of Matter
Which law states that the total mass of
the starting materials equals the total
mass of the product in a chemical
reaction?
A Dalton’s atomic model
B the law of conservation
of mass
C the law of definite
proportions
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0%
D
D Democritus’ law
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C
0%
B
A
B
C
D
A
1.
2.
3.
4.
4.1 Atoms—Basic Units of Matter
____ make up the nucleus of
an atom.
A Protons and electrons
B Neutrons and electrons
C Protons and neutrons
D Neutrons and photons
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D
0%
C
0%
B
A
B
C
D
A
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2.
3.
4.
4.2 Discovering Parts of the Atom
spectral lines
energy level
electron cloud
4.2 Discovering Parts of the Atom
How were electrons discovered?
• Scientists have put together a detailed
model of atoms and their parts.
• Cathode-ray tubes were used in early
experiments on atoms.
4.2 Discovering Parts of the Atom
How were electrons discovered? (cont.)
• A cathode ray is a stream of particles that
can be seen when an electric current
passes through a vacuum tube.
• The ray travels
from a negatively
charged disk at
one end to a
positively
charged disk at
the other end.
4.2 Discovering Parts of the Atom
Thomson’s Experiments
• Thomson discovered the cathode rays did
not travel in a straight line, but bent towards
the positively charged plate.
4.2 Discovering Parts of the Atom
Thomson’s Experiments (cont.)
• Opposite charges attract each other.
• Thomson concluded the cathode ray must
have a negative charge and named the
particles electrons.
4.2 Discovering Parts of the Atom
Thomson’s Atomic Model
• Thomson proposed that an atom was a
positively charged sphere.
• Electrons mixed in to
balance the charge.
4.2 Discovering Parts of the Atom
Discovering the Nucleus
• In Rutherford’s gold foil experiment,
particles were shot through a thin sheet of
gold into a detector behind the foil.
4.2 Discovering Parts of the Atom
Discovering the Nucleus (cont.)
• Rutherford predicted the path
of the particles would bend
only slightly because the
particles would not come
upon a charge large enough
to strongly repel them.
4.2 Discovering Parts of the Atom
Discovering the Nucleus (cont.)
• Most of the particles did pass straight though.
• Some particles
were strongly
bounced to
the side.
• One in about
8000 bounced
completely
backwards.
4.2 Discovering Parts of the Atom
Discovering the Nucleus (cont.)
• If the positive charge was
spread evenly, all the particles
would have passed through
the foil with only a small
direction change.
• Only something with a larger
mass and positive charge
could cause some of the
particles to bounce
backwards.
4.2 Discovering Parts of the Atom
Rutherford’s Atomic Model
4.2 Discovering Parts of the Atom
Rutherford’s Atomic Model (cont.)
• The positively charged nucleus is in
the center of an atom.
• Electrons with a
negative charge
travel around empty
space surrounding
the nucleus.
4.2 Discovering Parts of the Atom
Completing Rutherford’s Model
• Rutherford also discovered the proton, a
particle with a positive charge.
• Rutherford knew the mass of a proton,
but could not account for the total mass
of an atom.
• Rutherford’s theory was later confirmed when
the existence of the neutron—a neutral
atomic particle with a mass similar to a
proton but without a charge—was proved.
4.2 Discovering Parts of the Atom
Weakness in the Rutherford Model
• How are electrons arranged?
• Why do different elements have different
chemical properties?
• Why are some elements more reactive
than others?
4.2 Discovering Parts of the Atom
Bohr and the Hydrogen Atom
• Rutherford thought the electrons
moved around the nucleus like a
ball swinging on a rope at any
distance.
• Bohr thought electrons traveled in
circles with a certain diameter.
4.2 Discovering Parts of the Atom
Bohr and the Hydrogen Atom (cont.)
• Bohr studied hydrogen because it has only
one electron.
• When atoms are excited, they absorb and
release energy as light.
4.2 Discovering Parts of the Atom
The Spectrum of Hydrogen
• Light passing through a prism is broken
into a continuous spectrum of light—red,
orange, yellow, green, blue, and violet
blend into each other.
4.2 Discovering Parts of the Atom
The Spectrum of Hydrogen (cont.)
• Ultraviolet rays have shorter wavelengths
and higher energies than visible light.
• Infrared light has longer wavelengths and
lower energies than visible light.
4.2 Discovering Parts of the Atom
The Spectrum of Hydrogen (cont.)
• When excited, hydrogen and neon give off
unique narrow bands of light on the
spectrum that are called spectral lines.
4.2 Discovering Parts of the Atom
Spectral Lines and Energy Levels
• Each color in a spectral line is a different
wavelength and different energy.
• Electrons can have only certain amounts
of energy.
• Electrons can only move at a certain distance
from the nucleus that corresponds to that
amount of energy.
4.2 Discovering Parts of the Atom
Spectral Lines and Energy Levels (cont.)
• The region in
space that an
electron can move
about the nucleus
is called the
energy level.
4.2 Discovering Parts of the Atom
Electrons in the Bohr Atom
• The electrons can move only in an orbit that
is a set distance from the nucleus.
• Each energy level can hold a certain
number of electrons.
Bohr’s Model
4.2 Discovering Parts of the Atom
Electrons in the Bohr Atom (cont.)
• Electrons fill the energy levels in order.
– The lowest level is filled first.
– The second level has no electrons until
the first level is full.
– The first level holds 2 electrons, the
second level holds 8 electrons.
– The last energy level may or may not
be filled.
4.2 Discovering Parts of the Atom
Electrons in the Bohr Atom (cont.)
• This diagram shows how electrons are
placed in the elements with atomic
numbers 1–10.
4.2 Discovering Parts of the Atom
Bohr’s Model and Chemical Properties
• Elements have different chemical properties
because they have different numbers of
electrons in their outer energy level.
4.2 Discovering Parts of the Atom
Bohr’s Model and Chemical Properties
(cont.)
• Unreactive elements have the exact
number of electrons needed to fill their
outer energy level.
• Elements with incomplete outer energy
levels are likely to form compounds.
4.2 Discovering Parts of the Atom
Limitations of Bohr’s Model
• Energy levels were like circular orbits.
• Bohr’s theory works for the simple
hydrogen atom, but not for more complex
elements.
Atomic Model
4.2 Discovering Parts of the Atom
The Electron Cloud Model
• The electron cloud is the region surrounding
an atomic nucleus where an electron is most
likely to be found.
• Electrons are more likely to
be near the nucleus
because they are attracted
to the positive charge of
the protons.
Electron Cloud
Model
4.2 Discovering Parts of the Atom
An excited hydrogen atom emits
narrow bands of light called ____.
A energy lines
B wave lines
C spectral lines
D wavelengths
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D
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C
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B
A
B
C
D
A
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2.
3.
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4.2 Discovering Parts of the Atom
Elements that do not react with
other elements must have ____.
A completely filled
energy levels
B excited electrons
C empty energy levels
0%
0%
0%
D
0%
C
D the same number of
protons and neutrons
B
A
B
C
D
A
1.
2.
3.
4.
4.2 Discovering Parts of the Atom
In the gold foil experiment, why did some
particles pass straight through the foil?
A because they were
repelled by the protons in
the foil
B because they were
attracted by the protons
in the foil
C because atoms have no
effect on charged particles
0%
0%
D
D because atoms are
mostly empty spaces
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
element
atomic number
mass number
isotope
average atomic mass
ion
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Different Elements—
Different Numbers of Protons
• An element is a pure substance made from
atoms that all have the same number of
protons.
• Atoms of a particular element always have
the same number of protons.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Different Elements—
Different Numbers of Protons (cont.)
• The number of protons in an atom of an
element is the element’s atomic number.
How can you tell
which elements form
chemical bonds?
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Atomic Number and
the Periodic Table
• The periodic table shows elements that
increase in atomic number horizontally.
• Elements in the same column have similar
properties.
• Most of the elements on the periodic table
are metals.
Atomic Number and
the Periodic Table (cont.)
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Atomic Number and
the Periodic Table (cont.)
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Isotopes—Different Numbers
of Neutrons
• Atoms of the same element always have
the same number of protons, but they may
have different numbers of neutrons.
Isotopes
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Protons, Neutrons, and
Mass Number
• An atom’s mass number is the sum of the
number of protons and the number of
neutrons.
• To calculate the number of neutrons,
subtract the atomic number from the mass
number.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Isotopes
• Atoms of the same element that have
different numbers of neutrons are called
isotopes.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Isotopes (cont.)
• The average atomic mass of an element
is the weighted average mass of the
mixture of an element’s isotopes.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Isotopes (cont.)
• Radioactive isotopes are unstable and
break down releasing particles, radiation,
and energy.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Isotopes of Hydrogen
• Hydrogen has three isotopes.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Isotopes of Hydrogen (cont.)
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Ions—Gaining or Losing Electrons
• Atoms can gain or lose electrons.
• When an electron is gained or lost, the atom
no longer has the same number of electrons
as protons.
• The atom no longer has a neutral charge.
• An atom that has gained or lost an electron
and is no longer neutral is called an ion.
Ions—Gaining or Losing Electrons (cont.)
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Positive Ions—
Losing Electrons
• When an atom loses an electron, it has
more protons than electrons.
• The atom has a positive charge and is
called a positive ion.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
Negative Ions—
Gaining Electrons
• When an atom gains an electron, it has
more electrons than protons.
• The atom has a negative charge and is
called a negative ion.
• Positive ions and negative ions attract
each other and form compounds.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
The carbon element has several
isotopes. How is carbon-14 different from
carbon-13?
A Carbon-14 has a
positive charge.
B Carbon-13 has a
positive charge.
C Carbon-14 has one more
neutron than carbon-13.
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D
0%
C
D Carbon-14 has one less
neutron than carbon-13.
B
A
B
C
D
A
1.
2.
3.
4.
4.3 Elements, Isotopes, and Ions—
How Atoms Differ
What is the number of protons in
an element called?
A isotope
B atomic number
C atomic mass
D radiotope
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D
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C
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B
A
B
C
D
A
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2.
3.
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4.3 Elements, Isotopes, and Ions—
How Atoms Differ
How does a neutral atom become a
negative ion of the same element?
A It gains a proton.
B It loses a proton.
C It loses an electron.
D It gains an electron.
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D
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C
0%
B
A
B
C
D
A
1.
2.
3.
4.
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Atoms of the same element always
have the same number of ____.
A electrons
B neutrons
C ions
D protons
0%
0%
D
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
0%
D
0%
C
An electron moves into a higher
energy level. What must have
happened?
A The electron
released energy.
B The electron
absorbed energy.
C The atom gained a
negative charge.
0%
0%
D The atom gained a
positive charge.
B
A
B
C
D
A
1.
2.
3.
4.
An atom contains 10 electrons, 11
protons, and 11 neutrons. What is the
charge on the atom?
A 1−
B 1+
C 2−
D 2+
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D
0%
C
0%
B
A
B
C
D
A
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2.
3.
4.
Elements with the same number of
protons but differing numbers of
neutrons are called ____.
A ions
B electrons
C tritium
D isotopes
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D
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C
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B
A
B
C
D
A
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3.
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A spectral line is caused by ____.
A an excited electron
releasing energy and
falling to a lower
energy level
B an excited electron
absorbing electricity and
jumping to a higher
energy level
0%
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0%
D
D an atom losing a proton
0%
C
C an atom gaining an
ionic charge
B
A
B
C
D
A
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3.
4.
SCI 3.a
The atomic mass of an atom is
equal to ____.
A the number of protons
B the number of neutrons
C the number of protons
and neutrons
D the number of electrons
and protons
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D
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C
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B
A
B
C
D
A
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SCI 7.b
The atomic number of carbon is 6.
Carbon-14 contains ____.
A 7 protons and 7 neutrons
B 6 protons and 8 neutrons
C 8 protons and 6 neutrons
D 6 protons and 6 neutrons
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D
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C
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B
A
B
C
D
A
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3.
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SCI 3.a
D
C
What happens when an atom gains a
negative charge?
A The atom gains
an electron.
B The atom gains
a proton.
C The atom loses
an electron.
0%
0%
0%
0%
D The atom loses
a proton.
B
A
B
C
D
A
1.
2.
3.
4.
SCI 3.a
Which particle has a positive
charge?
A nucleus
B proton
C electron
D neutron
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D
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
SCI 3.a
Who thought that atoms were very
small, solid spheres?
A Dalton
B Rutherford
C Thompson
D Democritus
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D
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C
0%
B
A
B
C
D
A
1.
2.
3.
4.
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