Physical Science
Atomic Physics
Slides subject to change
Modern Physics
Focus on the atomic, microscopic scale.
 Properties of electrons, and the electron
cloud around each nucleus.


Properties of the nucleus,
subatomic particles.

Nature of light.
Also Quantization

At the level of atoms, many quantities are
found only in integer multiples of
elementary amounts (for example, electric
charge e).

These microscopic quantities are said to
be quantized − it’s as tiny as you can get,
cannot be subdivided.
Photons

In 1905 Einstein proposed that light
exists as tiny packets, or particles,
which he called photons. Light is quantized.

“Can be absorbed or generated only as a
whole."

We live in a quantum universe, one built out
of tiny, discrete chunks of energy and matter.
Light Energy
A photon has no mass, yet it has energy.
 Energy of one photon equals Planck’s
constant h times the frequency f of the light
wave.
E = hf


Planck’s constant” h = 6.626x10–34 J-s
Green Laser

Energy of one photon from the green laser:

f = 5.64x1014 Hz
Ephoton= hf
 = (6.626x10–34)(5.64x1014)
 = 3.74x10–19 J

Photons Interact with Atoms

When light is absorbed by an object, the
absorption occurs in the atoms of the
object. Emission of light comes from the
atoms.
Emission
Absorption
Things That Are Invisible

Hebrews 11:3 By faith we understand that
the universe was formed at God's
command, so that what is seen was not
made out of what was visible.
Dalton Model 1807
An atom is ...
 tiny
 indivisible
 uniformly dense
Billiard Ball Model
 solid sphere
 Each chemical element is composed of
tiny, indivisible particles called atoms,
which are identical for that element.

J.J. Thompson Model
Electron discovered in 1897 by English
physicist J.J. Thompson.
 But atoms are electrically neutral, so he
proposed the negative charges are spread
throughout the atom (1903).
 Perhaps electrons are like raisins in
Christmas “plum pudding?”

Thompson’s “Plum
Pudding” Model
Ernest Rutherford Model
Ernest Rutherford (New Zealand
chemist) discovers atoms have small
charged nucleus (1911).
 His students Ernest Marden and Hans
Geiger aim alpha particles (helium nuclei) at
thin gold foil.
 1 out of every 8,000 particles were reflected.

Rutherford’s Model
All the positive charge is crammed inside a
tiny, massive nucleus.
 Nucleus is equivalent to a marble in the
middle of a football stadium.

The much lighter electrons are
well outside the nucleus.
 Atoms are almostly entirely
empty space.

Rutherford’s
“Planetary” Model
Nuclear Dimensions
Hydrogen Emission

When light is emitted from a pure gas
(say, in a gas discharge tube) there are a
limited number of frequencies in the line
spectrum. Why?
Bohr Model
Niels Bohr model (1913) looks like a
planetary model.
 Orbit radius, and total energy of each
electron orbit has a quantum value.
 Result: electrons only orbit at certain
radiuses.
 It has lowest energy when it is closest to
the nucleus, higher energy farther from the
nucleus.

Bohr Model for Hydrogen

An electron’s energy in its orbit is
characterized by a whole number value,
principal quantum number n = 1, 2, 3 ...

When hydrogen electron is
in the lowest state (closest
to the nucleus), n = 1, it’s in
the ground state.
Bohr Model

Hydrogen emits a photon when the electron
drops from a higher energy level to a lower
energy level.
Emission
Bohr Model

Hydrogen absorbs a photon when E=hf is
exactly the right energy to enable a jump
from a lower energy level to a higher energy
Absorption
level.
Bohr Characteristics
Bohr hydrogen orbit radii
 r1 = “Ground State” radius
= 0.053x10−9 m = 0.053 nanometer.
 r2 = 4 r1
 r3 = 9 r1

 rn
= n 2 r1
Energy in Orbit
Bohr hydrogen orbit energy
 E1 = “Ground State” energy
= − 13.6 electron volts (approximately
2.18x10−18 joules.
 E2 = E1/4 = −3.4 eV
 E3 = E1/9 = −1.5 eV


En = E1/n2
Hydrogen
a
hydrogen
electron’s
energy is drawn
like this:
Ground State at this energy level
Electron volts (eV)
 Sometimes
When atom
absorbs
–1.5 E3
energy,
electron
–3.4 E2
goes to a
higher
energy level.
–13.6E1
1 electron volt (eV) is 1.6x10–19 J
Hydrogen

Electron needs energy to jump to higher
state. May come from heat (bumped by
another atom), light (photons), or
electricity (bumped by electrons).

Enables it to jump from E1 to say, E2 or
higher.

Energy must be exact amount.
Hydrogen
Electrons give up energy to jump to a
lower state.
 May jump from E2 down to E1.
 Will give up exact amount with a photon of
E = hf.
When atom
Sometimes a hydrogen
electron’s energy is drawn
like this
Ground State at this energy level
Electron volts (eV)

–1.5
–3.4
emits
energy,
E2 electron
goes to
lower level.
E3
E1
–13.6
1 electron volt (eV) is 1.6x10–19 J
Energy Transitions
If an H atom is in excited state E2, it will
normally spontaneously emit a photon to
go from E2 → E1.
 ΔE = E1 – E2 = (− 13.6) − (−3.4)
= –10.2 eV ·1.6x10−19 J/1 eV
= –1.63x10–18 J


The minus sign indicates the atom lost
energy (the energy goes into a photon).
Energy Transitions

From Einstein and Bohr, loss of energy
goes into a emission of a photon of
specific frequency.
 E = hf = 1.63x10–18 J

f = 1.63x10–18 / 6.634x10–34
 f2→1

= 2.46x1015 Hz
This is a photon in the ultraviolet range.
Energy States

The characteristic red line in the hydrogen
spectrum represents an electron transition
from the n = 3 state to n = 2 state.
More on Light
In 1905 Einstein proved that energy and
matter are linked in the most famous
relationship in physics: E = mc2.
 The energy content of a body is equal to
the mass of the body times the speed of
light squared.
 He suggested that the heat produced by
radium could mark the conversion of tiny
amounts of the mass of the radium salts
into energy.

Even More on Light
Mass can be converted into energy and
energy into mass.
 Example, there are two particles with the
same mass, an electron and a positron (an
“anti-particle”).
 Their mass is each 9.11x10−31 kg.
 Total equivalent energy is 2mec2 =
1.64x10−13 J.
 Equal but opposite charge −e, +e.

Even More on Light





What is a photon of that energy?
E = hf
1.64x10−13 = (6.626x10−34) f
f = 2.47x1020 Hz, a gamma ray.
An electron-positron collision conserves total
charge, and annilates both particles and creates
a gamma ray.
Historical Perspectives
Up to 1900, “Classical Physics”
 Laws of motion Galileo, Newton
 Gravitation Newton
 Electricity Coulomb
 Magnetism, Faraday, Orsted
 Electromagnetic waves Maxwell
 Optics
 Post-1900, “Modern Physics”

Lasers
Light Amplification by Stimulated Emission of
Radiation (LASER).
 Some atoms can stay in certain “metastable”
energy states for a long period of time.
 Sending a photon through the material will
“stimulate” transition from metastable state to
a lower state
 Creates an avalanche of photons that bounce
off mirrors at each end.
 Keep pumping energy into lasing medium.

Heisenberg Uncertainty
At the microscopic level, it is impossible to
know a particle’s exact position and
velocity simultaneously.
 There is a limit on measurement precision
at microscopic levels.


The measurement process
itself disturbs what you are
measuring in this
environment.
Absorption and Emission
When visible light of all wavelengths pass
through a cool sample of the gas, we get
an absorption spectrum.
 High voltage across a sample, get an
emission spectrum.

Absorption
Emission
Electron Cloud
Erwin Schrödinger model (1926).
 The electron’s position is is only a
probability.

Hydrogen
Ground state,
most probable
radius ~0.053 nm
Exactly the same
as Bohr’s model.
Matter Waves
If light can be a wave and act like a particle,
can particles act like waves?
 Louis deBroglie proposed:

λparticle = h/mv
Example: Wavelength of small ball, 0.5 kg at 26 m/s
λparticle = h/mv = (6.634x10−34)/(0.5)(26) = 5.1x10−35 m
Too short to see any effect.
Electron Diffraction

Electron beam through a crystal does
show diffraction effects.
Light, As Bullets

You expect light to hit the target as shown.
Double Slit

However, …
Split Personality

The particles form a diffraction pattern.
CCD camera
detects single
photons
Double Slit
Diffraction
pattern