Atoms and

Radioactivity
Harshitha Harisha grade 10 IGCSE A*
A Simple Model of the
Atom

• Electrons are held in the orbit by the force of

attraction between opposite charges
• Protons and neutrons are bound tightly
together in the nucleus by strong nuclear
force.
Elements

• No of protons: atomic number

• No of protons and neutrons in a nucleus: mass
number
• Isotopes have same atomic number but
different mass number.
• Electrons orbits the nucleus at certain levels
called shells. 2e in K shell, 8e in the other shells
What is an
Unstable
Nuclei?
• When the atoms of an
element have extra
neutrons or protons it
creates extra energy in the
nucleus and causes the
atom to become
unbalanced or unstable.
• In a stable nuclei, nuclear
force is strong enough and
brings sufficient energy to
hold nucleus together.
 • Some materials contain atoms with unstable nuclei
Nuclear Radiation • Each unstable nucleus disintegrates by shooting out a
tiny particle as well as a burst of wave energy.
• Material that emits nuclear radiation is called
radioactive material.
• The disintegration is called radioactive decay.
Few Isotopes of an
Element are
Radioactive.
• This is better than saying
radioactive materials
Ionizing
Radiation
• Ions are charged atoms
• Atoms become ions when they lose or
gain electrons
• Nuclear radiation can remove
electrons from atoms in its path
• This has an ionizing effect.
• Eg: ultraviolet radiation, X-rays
• If a gas becomes ionizing, it will
conduct electric current
• Ionization can damage or destroy cells
Types of radiation

ALPHA PARTICLES BETA PARTICLE GAMMA RAYS

Radiation Dangers

• Damage or destroy living cells

• Cause mutation
• High risk of cancer
• Alpha radiation is the most harmful as it has the most
ionising effect
• Less risk from radioactive source outside the body
• Beta and gamma rays are potentially the most harmful
because they can penetrate to internal organs. Alpha
particles can be stopped by the skin.
 • Small amount of radiation in the surrounding caused by
Background radioactive materials in the environment.
• Mainly comes from natural sources
Radiation • Protection: houses may need extra underfloor ventilation to
stop the gas collecting, Sealed floor.
Geiger-Muller (GM) Tube
Detect alpha, beta, and gamma radiation.
Geiger Muller Tube
Thin Mica Window: radiation can enter

If alpha particle enters, it ionizes the gas inside which sets off a high voltage spark across the gas
and a pulse of current in the circuit. Same effect for other particles.

Ratemeter: gives the reading In counts per second.

Scaler: total no of particles (or bursts of gamma radiation) detected by tube

An amplifier and loudspeaker: makes a click when each particle or bursts of gamma radiation is
detected.
Safety in laboratory

Stored In a lead
Picked with tongs,
container in a
not by hand
locked cabinet

Kept well away Left out of its

from the body, and container for as
not pointed at short time as
other people possible
 Studying alpha particles
Cloud Chamber because it makes their tracks
visible.
 Radioactive Decay
• If an isotope is radioactive, it has an
unstable arrangement of neutrons
and protons in its nuclei.
• The emission of an alpha or beta
particle make the nucleus stable.
• This alters the no of P and N in it, so it
becomes the nucleus of a dif element.
• OG Nucleus: Parent Nucleus
• Nucleus formed: daughter Nucleus
• Decay Products: The daughter
nucleus and any emitted particles
The symbol system
Alpha decay
Radium-226
Beta Decay
Gamma Decay
• With some isotopes, the emissions of an alpha and
beta particle from the nucleus leaves the protons
and neutrons in an excited arrangement.
• As the protons and neutrons rearrange to become
more stable, they lose energy.
• This is emitted as a burst of gamma radiation
• Gamma emission by itself causes no change in mass
number or atomic number.
To Note

Radioactive decay happens spontaneously and at random.

We cannot predict when a particular nucleus will disintegrate or in which

direction a particle will be emitted.

The process is unaffected by pressure, temperature, chemical change

Some types of nucleus are more unstable than others and decay at a faster rate.
Rate of decay and half-life

• Half-life
• The time taken for the no of radioactive
nuclei in a sample to halve.
• The time taken for the no of decays, or
the activity to halve.
• Every radioactive isotopes have different
half-lives.
Activity and half-live
• Activity: The average no of disintegrations
per second.
• Unit is becquerel (Bq)
• 100 Bq means 100 nuclei are disintegrated
per second
• Can use a GM to obtain results.
• The no of counts per second recorded by the
ratemeter is adjusted to allow for
background radiation.
Stability of the nucleus

• If the no of neutrons is plotted against the no of protons

for all the different isotopes of all the elements.
• Features:
• Stable isotopes lie along the stability line
• Isotopes above the stability line have too many
neutrons to be stable.
• They decay by beta- (electron)emission
because this reduced the no of neutrons.
• Isotopes below the stability line have few neutrons
to be stable.
• They decay by beta+ (positron) emission
because this increases the number of
neutrons.
• The heaviest isotope ( proton numbers > 83) decay
by alpha emission.
Nuclear Reactors
When alpha or beta
The temperature rises as Fact: The energy released
particles are emitted by a
nuclear energy (potential per atom is around a million
radioactive isotope, they
energy stored int eh times greater than that from
collide with surrounding
nucleus) is transformed into a chemical change as
atoms and make them move
thermal energy. burning.
faster.

To make decay rate faster: Nuclear Reaction:

The rate of decay is very Nuclei made more unstable Whenever a particle
slow. by bombarding them with penetrates and changes a
neutrons. nucleus.
Fission
• The splitting of heavy nuclei into smaller
nuclei with the emission or energy.
• The result is a chain reaction, a huge and rapid
release of energy.
• For the chain reaction to be maintained, the
uranium-235 must be above a certain critical
mass, otherwise too many neutrons escape.
• Was used in the atomic bomb
• This reaction is uncontrolled.
Fission in a
nuclear reactor
• In a nuclear reactor, a controlled chain
reaction takes place and thermal
energy is released at a steady rate.
• Used: to make steam for turbines in a
power station.
• Maintaining the reaction: The
neutrons must be slowed down,
otherwise many of them get absorbed
by the uranium 238. This can be done
by using a moderator.
 Hazards of Nuclear Waste

Strontium-90 and Plutonium-239:

Safe storage is
iodine-131: easily when breathed in Highly toxic
needed.
absorbed by body. it can kill.
 Nuclear Safety

1 2 3
Shield people from Keep people’s time Prevent radioactive
direct nuclear of exposure to materials from
radiation radiation as short as getting into the
possible body.
Energy and Mass
• Energy itself has mass
• If an object gains energy, its mass increases
Nuclear Fusion

• Protons and neutrons are held together by the strong

nuclear force.
• However some nuclei are more tightly held than others
• To get eh nuclei to release energy: make protons and
neutrons regroup into more tightly held arrangements
than before
• Energy can be released by fussing very light nuclei
together to make heavier ones.
• Takes place in the stars.
• Fusion is difficult to achieve because the nuclei are
charged and repel each other so they must travel fast.
• The gas must be hotter.
Fusion in the stars
Tracers and Treatments Thickness Monitoring
• Checking functions of body organs
• Tracking a plants uptake of fertilizers from roots to leaves
• Detecting leaks in underground pipes.
• Gamma camera: detect the gamma rays coming from the
radioactive tracer in the patients body.
• Gamma radiation can kill cancer cells

Smoke detector Testing for cracks

Atoms
and
Particles
Thompson’s
Plum
pudding
model
Rutherford’s
Nuclear Model
James Chadwick

Contained uncharged particles: neutrons

Rutherford-
Bohr Model
• Energy cannot be divided into
ever smaller amounts
• It is only absorbed or emitted in
tiny packet (a quantum)
• Electrons in higher orbits have
more energy than those in lower
ones.