Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
7.1.1 Describe a model of the atom that features
a small nucleus surrounded by electrons.
7.1.2 Outline the evidence that supports a
nuclear model of the atom.
7.1.3 Outline one limitation of the simple model
of the nuclear atom.
7.1.4 Outline evidence for the existence of
atomic energy levels.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Describe a model of the atom that features a
small nucleus surrounded by electrons.
In 1897 British physicist J.J. Thomson discovered
the electron, and went on to propose a "plum
pudding" model of the atom in which all of the
electrons were embedded in a spherical positive
charge the size of the atom.
atomic
diameter
+7
The “Plum
pudding”
model of
the atom
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Describe a model of the atom that features a
small nucleus surrounded by electrons.
In 1911 British physicist Ernest
Rutherford conducted experiments on
the structure of the atom by
sending alpha particles through
gold leaf.
Gold leaf is like tin foil, but it
can be made much thinner so that
the alpha particles only travel
through a thin layer of atoms.
FYI
An alpha () particle is a double-positively
charged particle emitted by radioactive materials
such as uranium.
Atomic structure
Describe a model of the atom that features a
small nucleus surrounded by electrons.
Rutherford proposed that alpha particles would
travel more or less straight through the atom
without deflection if Thomson’s “Plum pudding”
model was correct:



FYI
Instead of observing minimal scattering as in
the plum pudding, Rutherford observed the
scattering as shown on the next slide:
scintillation screen
Topic 7: Atomic and nuclear physics
7.1 The atom
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline the evidence that supports a nuclear
model of the atom.
Here we see that the deflections are much more
scattered...
nucleus
The atom
The
Rutherford
Model
Rutherford proposed that the positive charge of
the atom was located in the center, and he coined
the term nucleus.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline the evidence that supports a nuclear
model of the atom.
Expected
Results
Actual Results
FYI
IBO requires you to qualitatively understand the
Geiger-Marsden scattering experiment.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline the evidence that supports a nuclear
model of the atom.
Only by assuming a concentration of positive
charge at the
center of the
atom, as opposed
Geiger
to “spread out”
as in the plum
pudding model,
could Rutherford
and his team explain the results of the
Marsden
experiment.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline the evidence that supports a nuclear
model of the atom.
PRACTICE: In the Geiger-Marsden experiment 
particles are scattered by gold nuclei. The
experimental results indicate that most 
particles are
A. Scattered only at small angles.
B. Scattered only at large angles.
C. Absorbed by the target.
D. Scattered back along the original path.
SOLUTION:
Observing the image…
Most  particles
scatter at small angles.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline the evidence that supports a nuclear
model of the atom.
PRACTICE: In 1913 Geiger and Marsden fired alpha
particles at gold foil. The diagram shows two
such alpha particles () at A and B and two gold
nuclei within the foil. Sketch
in the likely paths for each
A
alpha particle within the box. 
SOLUTION:
B
Since  particles and nuclei

are both (+) the  particles will be repelled.
From A the particle will scatter at a small
angle. Remember it is repulsed, not attracted.
From B the particle will scatter at a large
angle, and perhaps even right back.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
When a gas in a tube is subjected to a voltage,
the gas ionizes, and emits light.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
We can analyze that light by
looking at it through a spectroscope.
A spectroscope acts similar to a prism,
in that it separates the incident light
into its constituent wavelengths.
For example, heated barium gas will produce an
emission spectrum that looks like this:
4000
4500
5000
5500
6000
6500
7000
7500
 / 10-10 m
An emission spectrum is an elemental fingerprint.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
Each element also has an absorption spectrum,
caused by cool gases between a source of light
and the scope.
continuous
spectrum
light
source
light
source
compare…
cool
gas
X
hot
gas
X
absorption
spectrum
emission
spectrum
Same fingerprint!
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
In the late
1800s a Swedish
physicist by
the name of
J.J. Balmer
observed the
spectrum of
hydrogen - the
simplest of all
the elements:
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
In reality, there are many additional natural
groupings for the hydrogen spectrum, two of which
are shown here:
0
200
Lyman
Series
(UV)
400
600
Balmer
Series
(Visible)
800
1000
1200
1400
Paschen
Series
(IR)
1600
1800
2000
 / 10-10 m
These groupings led scientists to imagine that
the hydrogen’s single electron could occupy many
different energy levels, as shown next:
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
The first 7 energy levels
for hydrogen are shown here:
7
6
The energy levels are
5
4
labeled from the lowest to
3
2
the highest as n = 1 to
1
n = 7 in the picture.
n is called the principal
quantum number and goes all
the way up to infinity ()!
In its ground state or
unexcited state, hydrogen’s
single electron is in the 1st
energy level (n = 1):
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
As we will see later, light
energy is carried by a particle
7
6
called a photon.
5
4
If a photon of just the
3
2
right energy strikes a
1
hydrogen atom, it is
absorbed by the atom and
stored by virtue of the
electron jumping to a new
energy level:
The electron jumped from the
n = 1 state to the n = 3 state.
We say the atom is excited.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
When the atom de-excites the
electron jumps back down to a
7
6
lower energy level.
5
4
When it does, it emits a
3
2
photon of just the right
1
energy to account for the
atom’s energy loss during
the electron’s orbital drop.
The electron jumped from the
n = 3 state to the n = 2 state.
We say the atom is de-excited,
but not quite in its ground state.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
The table shown here accounts for all of the
observed
hydrogen
emission
spectra.
The excitation illustrated looked
Infrared
like this:
The deVisible
excitation
looked
like this:
Ultraviolet
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
The human eye is only sensitive to the Balmer
series of
photon
energies (or
wavelengths):
Infrared
Visible
Ultraviolet
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
The previous energy level diagram was NOT to
scale. This one is. Note that none of the energy
drops of the other
0.00 eV
n=
n=5
-0.544 eV
series overlap
Second
n=4
-0.850 eV
Excited
those of the
State  n = 3
-1.51 eV
Balmer series, and First
thus we cannot see Excited
Paschen Series (IR)
State  n = 2
-3.40 eV
any of them.
[ HEAT ]
Balmer Series (Visible)
But we can still
sense them!
FYI
Energy can be
measured in eV.
Ground
State  n = 1
[ SUNBURN ]
Lyman Series (UV)
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
Later when we study of Option B – Quantum
Physics, we will find out that light not only
acts like a wave,
0.00 eV
n=
-0.544 eV
having a wavelength and nn == 54
-0.850 eV
a frequency, but it acts
n=3
-1.51 eV
like a particle (called
a photon) having an
Paschen Series (IR)
n=2
-3.40 eV
energy given by
[ HEAT ]
Balmer Series (Visible)
E = hf
energy of a
photon
Where
h = 6.6310-34 J s
n=1
-13.6 eV
and is called
[ SUNBURN ]
Planck’s constant.
Lyman Series (UV)
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
EXAMPLE:
An electron jumps from energy level n = 3 to
n=
energy level n = 2 in the
n=5
hydrogen atom.
n=4
(a) What series is this
n=3
de-excitation in?
Paschen
SOLUTION:
n=2
Find it on the energy diagram:
Balmer
This jump is contained in the
Balmer Series, and produces a
visible photon.
n=1
Lyman
0.00 eV
-0.544 eV
-0.850 eV
-1.51 eV
-3.40 eV
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
EXAMPLE:
An electron jumps from energy level n = 3 to
n=
energy level n = 2 in the
n=5
hydrogen atom.
n=4
(b) Find the atom’s change
n=3
in energy in eV and in J.
Paschen
SOLUTION:
n=2
∆E = Ef – E0
Balmer
= 3.40 - 1.51 = 1.89 eV.
∆E = (-1.89 eV)(1.6010-19 J / eV)
n=1
= -3.0210-19 J.
Lyman
0.00 eV
-0.544 eV
-0.850 eV
-1.51 eV
-3.40 eV
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
EXAMPLE:
An electron jumps from energy level n = 3 to
n=
energy level n = 2 in the
n=5
hydrogen atom.
n=4
(c) Find the energy (in J) of
n=3
the emitted photon.
Paschen
SOLUTION:
n=2
The hydrogen atom lost
Balmer
-19
3.0210
J of energy.
From conservation of energy a
photon was created having
n=1
-19
E = 3.0210
J.
Lyman
0.00 eV
-0.544 eV
-0.850 eV
-1.51 eV
-3.40 eV
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
EXAMPLE:
An electron jumps from energy level n = 3 to
n=
energy level n = 2 in the
n=5
hydrogen atom.
n=4
(d) Find the frequency of the
n=3
emitted photon.
Paschen
SOLUTION:
n=2
From E = hf we have
Balmer
-19
-34
3.0210
= (6.6310 )f, or
f = 3.0210-19/6.6310-34
n=1
f = 4.561014 Hz.
Lyman
0.00 eV
-0.544 eV
-0.850 eV
-1.51 eV
-3.40 eV
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
EXAMPLE:
An electron jumps from energy level n = 3 to
n=
energy level n = 2 in the
n=5
hydrogen atom.
n=4
(e) Find the wavelength (in nm) n = 3
of the emitted photon.
Paschen
SOLUTION:
n=2
From v = f where v = c we have
Balmer
8
14
3.0010 = (4.5610 )
 = 6.5810-7 m.
n=1
Then  = 6.5810-7 m
= 65810-9 m = 658 nm.
Lyman
0.00 eV
-0.544 eV
-0.850 eV
-1.51 eV
-3.40 eV
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
PRACTICE: Which one of the following provides
direct evidence for the existence of discrete
energy levels in an atom?
A. The continuous spectrum of the light emitted
by a white hot metal.
B. The line emission spectrum of a gas at low
pressure.
C. The emission of gamma radiation from
radioactive atoms.
D. The ionization of gas atoms when bombarded by
alpha particles.
SOLUTION:
Dude, just pay attention!
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
PRACTICE: A spectroscopic
examination of glowing
hydrogen shows the presence
of a 434 nm blue emission
line.
(a) What is its frequency?
SOLUTION:
Use c = f
where c = 3.00108 m s-1 and  = 434 10-9 m:
 3.00108 = (43410-9)f
f = 3.00108/43410-9
= 6.911014 Hz.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
PRACTICE: A spectroscopic
examination of glowing
hydrogen shows the presence
of a 434 nm blue emission
line.
(b) What is the energy
(in J and eV) of each of
its blue-light photons?
SOLUTION: Use E = hf:

E = (6.6310-34)(6.911014)
E = 4.5810-19 J.
E = (4.5810-19 J)(1 eV/ 4.5810-19 J)
E = 2.86 eV.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
n=
PRACTICE: A spectroscopic
n=5
n=4
examination of glowing
n=3
hydrogen shows the presence
of a 434 nm blue emission
Paschen
n
=
2
line.
(c) What are the energy levels
Balmer
associated with this photon?
SOLUTION:
Because it is visible use the
n=1
Balmer Series with
∆E = -2.86 eV.
Lyman
Note that E2 – E5 = -3.40 - -0.544 = -2.86 eV.
Thus the electron jumped from n = 5 to n = 2.
0.00 eV
-0.544 eV
-0.850 eV
-1.51 eV
-3.40 eV
-13.6 eV
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
PRACTICE: The element helium was first identified
by the absorption spectrum of the sun.
(a) Explain what is meant by the term absorption
spectrum.
continuous
SOLUTION:
spectrum
An absorption spectrum is produced when a cool
gas is between a source having a continuous
spectrum and an observer with a spectroscope.
The cool gases absorb their signature wavelengths
and remove them from the continuous spectrum.
Where the wavelengths have been absorbed by the
gas there will be black lines.
absorption
emission
spectrum
spectrum
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
PRACTICE:
One of the wavelengths of the absorption spectrum
for helium occurs at 588 nm.
(b) Show that the energy of a photon having a
wavelength of 588 nm is 3.3810-19 J.
SOLUTION:
E = hf but c = f so that f = c/.
Thus E = hc/ so that
E = (6.6310-34)(3.00108)/58810-9)
E = 3.3810-19 J.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
PRACTICE: The diagram represents a
few energy levels of the helium atom.
(c) Use the information in the
diagram to explain how absorption
at 588 nm arises.
SOLUTION: We need the difference in
energies between two levels to be
3.3810-19 J.
Note that 5.80 – 2.42 = 3.38.
Since it is an absorption the atom
stored the energy by jumping an electron from the -5.8010-19 J level to
the -2.4210-19 J level, as illustrated.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline evidence for the existence of atomic
energy levels.
In a later option we will discover that the most
intense light reaching us from the sun is between
500 nm and 650 nm in wavelength.
Evolutionarily our eyes have developed in such a
way that they are most sensitive to that range of
wavelengths, as shown in the following graphic:
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline one limitation of the simple model of the
nuclear atom.
By classical electromagnetic theory, charged
particles in accelerated motion radiate energy in
the form of electromagnetic radiation.
Electrons in atoms are in uniform circular
motion, thus having a centripetal acceleration.
But if an electron in an atom
1 Is not
radiates energy, it will
observed
ultimately lose its energy and
fall into its nucleus.
Since this is not observed,
1 Is
the simple model of the
observed
atom is flawed.
Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
Outline one limitation of the simple model of the
nuclear atom.
Another limitation is that the “orbitals” are
really not as simple as circular.
On the following slide are images of what the
orbitals really look like surrounding a hydrogen
atom:
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
7.1.5 Explain the terms nuclide, isotope and
nucleon.
7.1.6 Define nucleon number A, proton number Z
and neutron number N.
7.1.7 Describe the interactions in a nucleus.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Explain the terms nuclide, isotope and nucleon.
Recall the mass spectrometer in which an atom is
stripped of its electrons and accelerated
through a voltage into
a magnetic field.
Scientists determined
through the use of such
a device that hydrogen
nuclei came in three
different masses:
Since the charge of
the hydrogen nucleus is e, scientists
postulated the
existence of a neutral
particle called
the neutron.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Explain the terms nuclide, isotope and nucleon.
The proton and neutron are called nucleons.
Proton [ Charge = 1e or just +1 ]
Nucleons
Neutron [ Charge = 0e or just 0 ]
For the element hydrogen, it was found that its
nucleus existed in three forms:
Isotopes
Hydrogen
Deuterium
Tritium
A set of nuclei for a single element having
different numbers of neutrons are called
isotopes.
A particular isotope of an element is called a
species or a nuclide.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Explain the terms nuclide, isotope and nucleon.
An element’s chemistry is determined by the
number of electrons surrounding it.
The number electrons an element has is determined
by the number of protons in that element’s
nucleus.
Therefore it follows that isotopes of an element
have the same chemical properties.
For example there is water, made of hydrogen H
and oxygen O, with the molecular structure H2O.
But there is also heavy water, made of deuterium
D and oxygen O, with the molecular formula D2O.
Both have exactly the same chemical properties.
But heavy water is slightly denser than water.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Define nucleon number A, proton number Z and
neutron number N.
A species or nuclide of an element is described
by three integers:
The nucleon number A is the total number of
protons and neutrons in the nucleus.
The proton number Z is the number of protons in
the nucleus. It is also the atomic number.
The neutron number N is the number of neutrons in
the nucleus.
It follows that the relationship between all
three numbers is just
A = Z + N
nucleon relationship
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Define nucleon number A, proton number Z and
neutron number N.
In nuclear physics you need to be able to
distinguish the different isotopes.
NUCLEAR PHYSICS
CHEMISTRY
H
Mass Number = A
H
H
Protons = Z
1
1
H
0
hydrogen
hydrogen-1
2
1
N = Neutrons
1
deuterium
hydrogen-2
3
1
H
2
tritium
hydrogen-3
FYI
Since A = Z + N, we need not show N.
And Z can be found on any periodic table.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Define nucleon number A, proton number Z and
neutron number N.
PRACTICE: Which of the following gives the
correct number of electrons, protons and neutrons
in the neutral atom 6529Cu?
A = 65, Z = 29, so N = A – Z = 65 – 29 = 36.
Since it is neutral, the number of electrons
equals the number of protons = Z = 29.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Define nucleon number A, proton number Z and
neutron number N.
PRACTICE: Ag-102, Ag-103 and Ag-104 are all
isotopes of the element silver. Which one of the
following is a true statement about the nuclei of
these isotopes?
A. All have the same mass.
B. All have the same number of nucleons.
C. All have the same number of neutrons.
D. All have the same number of protons.
SOLUTION: Isotopes of an element have different
masses and nucleon totals.
Isotopes of an element have the same number of
protons, and by extension, electrons. This is why
their chemical properties are identical.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Define nucleon number A, proton number Z and
neutron number N.
PRACTICE:
B-field Source
Track X shows the deflection of a
singly-charged carbon-12 ion in the
deflection chamber of a mass
A
spectrometer. Which path best shows
B
the deflection of a singly-charged
X
carbon-14 ion? Assume both ions
C
travel at the same speed.
SOLUTION:
D
Since carbon-14 is heavier, it will
have a bigger radius than carbon-12.
Since its mass is NOT twice the mass of carbon12, it will NOT have twice the radius.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Describe the interactions in a nucleus.
Given that a nucleus is roughly 10-15 m in
diameter is should be clear that the Coulomb
repulsion between protons within the nucleus must
be enormous.
Given that most nuclei do NOT spew out their
protons, there must be a force that acts within
the confines of the nucleus to overcome the
Coulomb force.
We call this force the strong force.
In a nutshell, the strong force…
(1)counters the Coulomb force to prevent nuclear
decay and therefore must be very strong.
(2)is very short-range, since protons located far
enough apart do, indeed, repel.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Describe the interactions in a nucleus.
PRACTICE:
The nucleus of an atom contains protons. The
protons are prevented from flying apart by
A. The presence of orbiting electrons.
B. The presence of gravitational forces.
C. The presence of strong attractive nuclear
forces.
D. The absence of Coulomb repulsive forces at
nuclear distances.
SOLUTION:
It is the presence of the strong force within the
nucleus.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Describe the interactions in a nucleus.
PRACTICE:
Use Coulomb’s law to find the repulsive force
between two protons in a helium nucleus. Assume
the nucleus is 1.0010-15 m in diameter and that
the protons are as far apart as they can get.
SOLUTION:
From Coulomb’s law the repulsive force is
F = ke2/r2 = 9109(1.610-19)2/(1.0010-15)2
F = 230 N.
FYI
From chemistry we know that atoms can be
separated from each other and moved easily.
This tells us that at the range of about 10-10 m
(the atomic diameter), the strong force is zero.
Topic 7: Atomic and nuclear physics
7.1 The atom
Nuclear structure
Describe the interactions in a nucleus.
STRONG
ELECTROMAGNETIC
WEAK
ELECTRO-WEAK
GRAVITY
+
+
nuclear
force
STRONGEST
Range:
Extremely Short
light, heat
and charge
radioactivity
freefall
Range:
Short
WEAKEST
Range:

Range:

Force Carrier:
Force Carrier:
Force Carrier:
Gluon
Photon
Graviton