Protons and neutrons are packed tightly in
the nucleus, where you find the majority of
the atom’s mass.
The Strong Force
Since protons repel each other, the strong
force allows protons and neutrons to be
attracted to each other. This is 4 x
stronger than electric force! Unlike
electricity, the strong force is a short-
range force. When protons and neutrons
move away from each other, the force
Strong Force vs. Large Nuclei
Because there are so many protons and neutrons in
large nuclei, the electric repulsion of all the protons
is stronger than then short-range strong force.
Therefore, protons and neutrons are held together
less tightly in a large nucleus.
Instability and Radioactivity
When the strong force is not strong enough to hold
the nucleus permanently together, the nucleus
begins to decay and give off matter and energy
(radioactivity.) All nuclei containing more than 83
protons are radioactive (although there are smaller
radioactive nuclei.) Synthetic elements, produced
in a lab, contain a large number of protons (92+), so
they decay soon after they are created.
You already know that an isotope is an atom with a
different number of neutrons to protons. Isotopes of
heavier elements are stable when the ratio of
neutrons to protons is about 3 to 2. If the ratios are
different (less or greater), than the nucleus is
considered unstable. This makes those isotopes
You can tell if an atom is radioactive by comparing
the mass number (p + n) to the atomic number (p):
mass number 12 mass number 14
atomic number 6 atomic number 6
Which atom of carbon is radioactive?
The Discovery of Radioactivity
In 1896, Henri Becquerel left uranium salt in a desk
drawer with a photographic plate. When he opened
the plate later, he found an outline of clumps of the
uranium salt. He realized that the saltss must have
emitted some unknown invisible rays, or radiation,
that darkened the film (it was an x-ray of the salt!)
In 1898, Marie and Pierre Curie discovered 2 new
elements (polonium and radium), that were also
radioactive. They were able to obtain .1 g of radium
for several tons of the mineral pitchblende after
more than 3 years of experiments.
The 3 types of nuclear radiation
are alpha, beta and gamma
radiation. Alpha and beta are
particles, and gamma radiation
is the resulting electromagnetic
An alpha particle is made up of 2 protons and
2 neutrons that are emitted from the decaying nucleus.
It is the same as a He nucleus and has a charge of +2 and
an atomic mass of 4. It actually
becomes 4 He
Alpha particles are the largest with the most charge.
They lose energy quickly when reacting with matter.
When they go through matter, the electrons in the
matter’s atoms react and are pulled away. This leaves
behind positively charged ions. Because alpha particles
quickly lose energy, they are the least penetrating form
of nuclear radiation. They can be stopped by a sheet of
Alpha particles can be dangerous inside the human
body, as a single alpha particle can damage fragile
Alpha particles can be used in some smoke detectors.
The detectors ionize the surrounding air. An
electric current flows through this air to form a
circuit, unless smoke particles break the circuit,
causing the alarm to sound.
An atom loses 2 protons when it emits alpha
particles, so it forms into a different element.
Transmutation is the process of changing one
element to another through nuclear decay.
The new element has 2 fewer protons and decreases
the mass number by 4. The charge of the original
nucleus = the sum of the charges of the nucleus
and the alpha particle that are formed.
Polonium into Lead
Polonium (Po) loses 2 protons (out of 84) and the
mass number is decreased by 4:
210Po 206Pb + 4He
+84 +82 +2
Sometimes an unstable nucleus in a neutron decays
into a proton, and then it emits an electron. This
electron is called a beta particle. Beta decay is
caused by a weak force.
Now the atom has one more proton than it did before
the decay. This means that it goes through
transmutation. However, the mass number doesn’t
change during beta decay, so the mass number is
the same as the original element.
Beta Decay: Iodine
Changing into Xenon
I + Xe
+53 -1 +54
Beta Particles (with a charge of -1) are faster and
more penetrating than alpha particles. They can go
through paper (but not a sheet of aluminum foil.)
They can also cause damage to biological cells.
The most penetrating form of nuclear radiation (EM
wave with highest frequency, shortest wavelength.)
They are energy (no mass or charge.) They are
emitted from the nucleus when alpha or beta decay
occurs. They can penetrate almost all solids
except for exceptionally dense materials such as
lead or concrete. However, because they produce
no charge, they can actually due less damage than
A measure of the time required by the nuclei of an isotope
to decay is called a half-life. The half-life of a radioactive
isotope is the amount of time takes for half the nuclei in
a sample of the isotope to decay.
Carbon Dating: Used to date once-living things. C14 has a
half-life of 5,730 years, found in molecules of CO2. This is
found in plants and in plant-eating animals.
Uranium Dating: Some rocks contain uranium isotopes.
These isotopes decay into lead isotopes. The ratio of the
uranium isotopes and the daughter nuclei (lead isotopes)
is measured and the number of half-lives since the rock
was formed can be calculated.
Cloud Chambers: filled with water or ethanol vapor, in
which a radioactive sample is placed. It gives off
charged alpha or beta particles, which travel
through the chamber. The particle knocks
electrons of the air atoms in the chamber, creating
ions. The water/ethanol vapor condenses around
the ions, creating a visible path of droplets along
the track of the particle (alpha = short, thick trails;
beta = long, thin trails.)
A bubble chamber holds superheated liquid (pressure
prevents boiling.) When a moving particle leaves
ions behind, the liquid boils along the trail.
The electroscope has leaves at the bottom. If the
scope gets an electric charge, the leaves repel
each other. When an positive charge is introduced,
then the excess negative charge is neutralized.
Nuclear radiation moving through the air can remove
electrons from some air molecules, causing other
air molecules to gain electrons. These positive air
ions come in contact with the electroscope and
attract the electrons from the leaves, so the leaves
move back together.
A Geiger counter measures the amount of radiation by
producing an electric current when it detects a
It has a tube with a positive charge running through
the center of a negatively charged copper cylinder.
The tube is filled with gas at a low pressure. When
radiation enters the tube at one end, It knocks
electrons from the atoms of the gas. Then causes
a chain reaction among the gas atoms, creating an
“electron avalanche”. When a large number of
electrons reaches the wire, a current is produced.
The energy is turned into sound (clicking) and
flashing light. The intensity of these energies
measures the intensity of the radiation.
Low-level radiation is naturally emitted by Earth’s
rocks, soil and atmosphere. Traces can be found in
building materials, plants and animals.
Most of the background radiation comes from the
decay of radon gas. High levels can be very
dangerous when it is trapped in homes. Sometimes
radiation from cosmic rays can infiltrate the Earth’s
atmosphere. Living organisms contain C14.
Background radiation can never be completely
• Enrico Fermi thought that, by bombarding nuclei
with neutrons, the neutrons would be absorbed
(creating larger nuclei.) Instead, when a neutron
struck a uranium-235 nucleus, it split apart into
smaller nuclei. Fission means to divide.
• Only large nuclei can undergo nuclear fission. The
products of fission includes both smaller nuclei and
random neutrons. Some of the mass after fission is
missing, because it turns into extreme energy.
Einstein was the first to realize that the Law of
Conservation of Mass and the Law of Conservation
of Energy are actually tied to each other.
Fission: Mass and
Einstein’s Theory of Relativity proposed that mass
can be converted into energy, and energy could be
converted into mass:
Energy (J) = mass (kg) x 300,000 km/s2 (speed of light)
E = mc2
If one gram of mass is converted into energy, then
approximately 100 trillion joules of energy is
Fission: Chain Reactions
During nuclear fission, the neutrons emitted can strike other
nuclei and cause them to split, thus releasing more
neutrons. A series of repeated fission reactions cause
the release of neutrons during each reaction.
Critical Mass is the amount of material required so that
each fission reaction produces at least one more fission
reaction. If there is not enough material, critical mass is
not reached, and there is no chain reaction.
If the chain reaction isn’t controlled, then an enormous
amount of energy is released. Chain reactions can be
controlled by adding materials that absorb neutrons.
Then the reaction continues at a constant rate.
Even more energy can be released when 3 nuclei with
low masses are combined to form one nucleus with
a larger mass. Fusion fuses atomic nuclei together.
An example would be 2 hydrogen (H) atoms combining
to form a helium (He) atom with a larger nucleus.
Fusion and Temperature
Nuclear fusion requires positively charged nuclei to
get close together. If they are moving very fast,
then they would have the kinetic energy to
overcome the repulsive electrical force.
Only at temperatures of millions of degrees Celsius
are nuclei able to get close enough to fuse. The
example below is the fusing of deuterium and
Nuclear Reactions and
When a radioactive atom is introduced into the body,
it can travel to specific parts and join other
molecules, where it can be easily found.
These radioactive isotopes (radioisotopes) are called
tracers. They can be followed through the body
and show how particular organs are functioning.
Examples are how problems are detected in the
thyroid, heart or gall bladder.
Gamma rays can used with radioisotopes to target
and destroy the fastest growing cells (tumors.)