REV Atomic and Nuclear

Atomic Physics
and
Quantum Effects
A photon is a particle of light. It has no mass, but can
transfer energy to and from electrons.
To determine the energy of a photon: E = hf
To determine the momentum of a photon: p = h
λ
To determine the wavelength of a photon:
start with E = hf, then use v = fλ
1

E = 0 eV
E3 = ­6 eV
E2 = ­7 eV
E1 = ­10 eV
Atomic Energy Levels
E1 is called the ground state
E = 0 eV
E3 = ­6 eV
E2 = ­7 eV
E1 = ­10 eV
Electrons in an atoms can have
only certain amounts of
energy. To move up levels an
electron must absorb an exact
amount of energy from a
photon and to drop down an
energy level, the electron emits
a photon with a specific
amount of energy.
How can an electron get
from the ground state to
level 3?
It could either absorb 4 eV
from a single photon OR it
could first absorb 3 eV from
one photon and then 1 eV
from a second photon.
What if an electron absorbs 10 eV of energy? It leaves the
atom. This is called ionization energy.
2

The Photoelectric Effect
Light energy striking a metal surface can cause the metal to
eject one of its electrons, but only if the frequency of the
incident light is above a certain value.
The minimum frequency of light needed is called the cutoff
frequency.
The metal surface is described by its work function (φ).
Work function (φ) is calculated by (φ) = hfo
The Photoelectric Effect
of the
electrons
KEmax = E ­ φ
Energy of the
incident photons
Work Function
of the metal
remember, Ephoton = hf and φ = hfo
KEmax = hf ­ hfo
3

Stopping Potential
KEmax = qVo
if KEmax is expressed in eV, then stopping potential is that
same value expressed in Volts and vice versa.
From here, we can calculate the velocity of the ejected
electrons: KEmax = 1/2mv 2
mass of electron
de Broglie Wavelength (Matter Waves)
λ = h
mv
Compton Scattering
Photons collide with electrons, transferring some energy
to the electron, electron now has velocity (KE), photon
loses energy, thus is wavelength is lengthened.
4

Three Types of Subatomic Particles:
(1) Alpha α 2 protons & 2 neutrons stuck together
(2) Beta β + or β ­
(3) Gamma γ a photon
The Three Decay Processes:
either + or ­ electron
(1) Alpha emits one alpha particle, mass number drops
by 4, charge drops by 2
4
2α
(2) Beta Decay either an electron or positron is emitted.
No change in mass, charge either increases or
decreases by 1.
e + 0
+1
e ­ 0
­1
(3) Gamma a photon is emitted which results in no
change in outward appearance. Emitted photon has KE,
where did it come from? Change in mass during the
decay process.
5

15
8 O decays via B + emission. Which of the
following is the resulting nucleus?
a)
b)
c)
15
9 F
16
8 O
15
7
N
d)
e)
16
9
16
7 N
F
6

Identify the missing particle in the nuclear reaction below:
13
6 C + ( ? )
a) electron
b) proton
c) deuteron
d) positron
e) gamma photon
13
7 N + n
Mass Defect
Think about each of the decay processes; in each process,
a particle is ejected (really, really fast). The remaining
nucleus has to recoil to conserve momentum. Now, both
particles are moving. Originally, there was no KE, now
there's a bunch. Where did it come from?
Before the decay, there was more mass than after. During
the decay some of the mass was converted into energy.
E = Δmc 2
7

from your handout in class:
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