Structure of the Nuclear
Atom
What is everything made
of?
The Atom
A. The atom is the smallest particle of an
element that retains its properties of the
element.
What culture was the first
to come up with the idea
of an atom?
History of the Atom
A. Democritus
1. Democritus (460 B.C. –
370 B.C) was the first to
suggest the existence of
atoms
2. Democritus believed
atoms were indivisible and
indestructible
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What are some ideas that he missed?
B. Dalton’s Atomic Theory
1. All elements composed of indivisible
particles called atoms
2. Atoms of the same element are
identical
3. Atoms of different elements can
physically mix together or chemically
combine in simple whole-number ratios
to form compounds.
4. Chemical reactions occur when atoms
are separated, joined, or rearranged.
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How big is an atom?
What are things we use to see small things?
What are some different types?
lll. Size of the Atom
A. The radii of most
atoms range from 5 x
10-11m to 2 x 10-10m
B. Individual atoms are
observable with
instruments such as a
scanning tunneling
microscope.
Question Time
• What is an atom?
• Who is Democritus?
• What are the four ideas of Dalton’s
Atomic Theory?
• What is the size of most atoms?
• What do scientists use to see an atom?
Does an atom normally have a charge?
What are subatomic particles?
Can atoms have a charge?
lV. Atoms and Their Charges
A. Atoms have no net electrical charge, they are
electrically neutral
B. Electric charges are carried by particles of
matter (subatomic particles)
C. Electric charges always exist in whole-number
multiples. (no fractions )
D. When a given number of negatively charged
particles combines with an equal number of
positively charged particles, an electrically
neutral particle is formed
What are the subatomic particles?
What are their charges?
V. Subatomic Particles
A. There are three kinds of subatomic
particles. These particles make up the
atom.
1. Electrons (negative)
2. Protons (positive)
3. Neutrons (neutral)
What do we know about electrons?
B. Electrons
1. Electrons are negatively charged
2. One electron carries one unit of negative charge
3. Has a mass of 1/1840 of a hydrogen atom
4. Symbol for and electron is e or e-
What do we know about protons?
C. Protons
1. Protons are positively charged subatomic particles
2. Each proton has a mass about 1840 times that of an
electron
3. Symbol for a proton is p or p+
What do we know about neutrons?
D. Neutrons
1. Neutrons are subatomic particles with no charge
2. Has a mass equal to that of a proton (1840 times of
an election)
3. Symbol for a neutron is n or n0
Question Time
• What charge are atoms?
• Are there fraction of charges?
• What if there were nine positively charged particles
and eight negatively charged particles, what would
the net charge of the atom be?
• What are the three subatomic particles and what
are their charges?
• What is the mass of an electron compared to a
hydrogen atom?
• What is the mass of a proton compared to a
neutron?
Vl. J.J. Thompsons’s Plum Pudding
Model
A. When subatomic particles were
discovered, scientist wondered how
these particles were put together in
an atom.
B. J.J. Thompson thought that
electrons were evenly distributed
throughout. An atom was filled
uniformly with positively charged
material called the “plum-pudding”
model or “chocolate cookie dough”
model
Vll. Rutherford’s Gold Foil
Experiment
A. In 1911 Ernest Rutherford wanted to study the
structure of the atom as well. The plum-pudding
model was the prevailing theory back then
B. In his experiment he used alpha particles, which
are helium atoms that have lost their two
electrons.
C. Then, directed the alpha particles at a very thin
sheet of gold foil. The particles should have
passed easily through the gold with a slight
deflection due to the positive charge thought to
be spread out in the gold atoms
What do you think happened?
D. The great majority of alpha particles passed
straight through the gold atoms, without
deflection
E. A small fraction of the alpha particle bounced
off the gold foil at large angles.
F. Based on the experimental results
Rutherford suggested that the atoms were
mostly empty space, but had a dense center
called the nucleus.
What does that mean?
G. Gold Foil Experiment Conclusion
1. All the positive charge and all the mass of the
atoms was concentrated in a small region called
the nucleus
2. “This is almost as incredible as if you fired a 15-inch
shell at a piece of tissue paper and it came back and
hit you.” (Actual Quote)
What do we call the center of an atom?
What is in the center?
Vlll. The Nuclear Atom
A. The nucleus is the central core of an atom
1. composed of protons and neutrons.
B. The nucleus is much smaller than the atom, yet
contains most of its mass
C. The electrons are distributed around the nucleus
1. occupy almost all the volume of the atom
2. Most of the atom is made up of empty space.
lX. Fun Facts
A. The volume of a hydrogen nucleus is a trillion
times smaller than the volume of a hydrogen
atom, yet the nucleus contains most of the mass.
B. If the nucleus (proton) of a hydrogen atom were
as large as the width of a human thumb, the
electron would be on the average about one
kilometer away in a great expanse of empty
space.
Question Time
• What is the plum-pudding model and who
thought of it?
• Is the plum-pudding model correct?
• What happened in the gold foil experiment?
• What did the gold foil experiment prove?
• Where is the positive charge located in the
atom?
• What contains most of the mass of the atom?
• What occupies most of the volume of the atom?
• What is the atom is mostly made up of?