Topic 7 Atomic,
nuclear and
particle physics
7.1 – Discrete energy and
radioactivity
Essential idea: In the microscopic world energy is
discrete.
Quantum model of the atom

With the discovery of elementary particles, the model
of the atom had to change

Previously unexplained phenomena began to clear up
with greater understanding of the composition of atoms
and the properties of subatomic particles
Blackbody Radiation

One phenomena was that hot object emitted a specific spectra of
light

This radiation is typified for an idealized blackbody

As the temperature of a blackbody changes, so does the intensity
of the frequencies and wavelengths of light it emits

The wavelengths of light emitted by a blackbody is defined by
Wien’s Law
2.90 × 10−3 m ∙ 𝐾
λ=
𝑇
Blackbody Spectrum
Sample

Estimate the temperature of the Sun given that it emits
light at peak intensity in the viable spectrum at around
500 nm.
Discrete Energy

In 1900, Max Planck proposed that the energy of the
oscillations of atoms is not continuous

The energy is a multiple of a minimum value of energy,
defined by the frequency of the oscillaion
𝐸 = ℎ𝑓

Planck’s constant and has a value of
ℎ = 6.626 × 10−34 J ∙ s

Since this energy is not continuous, it is considered
quantized, or discrete
Photon Theory of Light

In 1905, Albert Einstein extended the idea of discrete
energy by proposing that when an oscillating molecule
emits light, its energy decreases by an integer multiple
of hf

By conservation of energy, the emitted light must be
emitted in quanta of energy, called photons
𝐸 = ℎ𝑓
λ=
ℎ𝑐
𝐸
Sample Problem
Calculate the energy of a photon of blue light with a
wavelength of 450 nm in both J and eV.
Photoelectric Effect

When light is incident on the surface of a metal,
electrons are emitted from the surface
Discrete energy levels

Electrons only exist in very specific energy states, which
differ for atoms of different elements

The ground state is the lowest energy state of an atom

An excited state is any state in which an atom has
higher energy than the ground state

Electrons can only move from the specific energy state
by gaining or losing a quantum of energy
Energy level transition
Going to excited
state
Going to ground
state
Energy level transition

When electrical current is passed through a sample of
matter, the atoms become excited

As the electrons cycle through going from an excited
state to the ground state the photons emitted have the
specific frequencies that correspond to the quantum of
energy between states

When the light emitted by excited atoms is separated
through a prism, the specific frequencies of light the
result make up the emission-line spectrum
Emission Spectrums
Hydrogen Spectra
Bohr’s Model

In 1913, Danish physicist Niels Bohr proposed that electrons
can only exist in specific orbits around the nucleus of an
atom

While in a given orbit, an electron neither gains nor loses
energy

Emission is the process by which an electron falls to a
lower energy state, and releases a photon

Absorption is the process by which a photon adds energy
to increase the energy state of an electron
Hydrogen Spectra
Homework!

p. 782 #4, 6, 10, 12, 16
Fundamental Forces

In addition to gravity and electromagnetic forces, there are
two other fundamental forces that exist within atoms

The strong nuclear force is an attractive force between all
nucleons

The weak nuclear force is responsible for the radioactive
decay of certain subatomic particles

The nuclear forces are considered short-range, while both
electromagnetic and gravitational forces are considered
long-range
Radioactive Decay

At the end of the 1800’s, chemist/physicists were
examining that unidentified radiation given off by
certain elements without an external stimulus

Through thorough experimentation, it became clear
that this radiation was a result of the disintegration of
unstable nuclei

Predominantly this is a result of unstable isotopes
Radioactive Decay


There are multiple ways in which isotopes can undergo
radioactive decay

Alpha decay

Beta Decay

Gamma Decay
Each type of decay has different characteristics and
products
Alpha Decay

Alpha decay occurs when an isotope emits an α particle
( 42He), resulting in a change in identity of the atom
226
88Ra
222
4
86Rn + 2He

Alpha decay occurs when the strong force is unable to
hold large nuclei together

While alpha particles are massive compared to the
other particles, they have the lowest energy, and are
therefore the easiest to be absorbed
Beta Decay

Beta decay can be either positive or negative

Beta negative decay occurs when a neutron turns into a
proton, releasing a β- particle (electron) and an
antineutrino (ν)
14
6C

14
7N +
𝑒− + ν
Beta positive decay occurs when a proton turns into a
neutron, releasing a β+ particle (positron) and a
neutrino (ν)
19
19
+
10Ne
9F + 𝑒 + ν
Beta Decay

The weak force is the crucial force for beta decay

The energy of beta decay is greater than alpha decay,
making it slightly more capable of penetrating materials
Gamma Decay

Like electrons, the nucleus of an atom can exist at an
excited state

Gamma decay occurs when a nucleus goes from an
excited state to a ground state and releases a photon

This photon is incredibly energetic and is referred to as
a gamma ray
𝐴 ∗
𝑍𝑁
𝐴
𝑍𝑁
+𝛾
Gamma Decay

Gamma decay is the most energetic of the types of
radioactive decay

Therefore, it is the least easily absorbed and is the most
dangerous
Half Life

As a sample of a radioactive isotope decays, it does so
at a certain rate

Since the radioactive material no longer exists in its
original state, it decays exponentially

The half life of a material is the time it takes for half of
the initial sample to decay
Half Life
Half-life of C-14

What is the half life
of C-14?

What percentage of
C-14 is left after 4
half-lives?

How much carbon is
left after 10,000
years?
Decay Series

Many radioisotopes decay into daughter nuclei that are also
radioisotopes

The successive decay of radioisotopes is called the decay series
Decay Series

http://atom.kaeri.re.kr/nuchart/?zlv=1