Atomic Structure and Radioactive Decay
(A) The Atomic Model
(1) Rutherford’s model of the atom
In 1911, H. Geiger and E. Marsden carried out an experiment bombarding a thin gold foil with α
particles in a vacuum. The experimental set-up and the results are shown in the following.
Base on these experimental results, Rutherfold proposed the following atomic model:
 All positive charge of atom and most of the mass were concentrated in a tiny core (nucleus).
 Electrons orbit around the nucleus.
(2) Atomic number and mass number
The number of protons in the nucleus of an atom is called its proton number or atomic number.
The protons and neutrons in the nucleus are collectively called nucleons.
The total number of nucleons (protons + neutrons) in a nucleus is called nucleon number or mass
number.
(3) Nuclides and Isotopes
A nucleus with a particular mass number and atomic number is called a nuclide. A nuclide of
element X with atomic number Z and mass number A is represented in the following way: ZA X .
Nuclides which are radioactive are called radionuclides.
Nuclides of an element with the same atomic number but different mass numbers are called
isotopes. Isotopes which are radioactive are called radioisotopes.
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Atomic Structure and Radioactive Decay
(B) Radioactive decay and uses of radioisotopes
(1) Radioactive decay
Radioactive decay (or disintegration) is due to unstable nuclei, which emit α particles,
particles or γ rays in the process, and go to a more stable state. It is a spontaneous process.
When a nucleus decays, it changes into a nucleus of another element.
Daughter nucleus
Parent nucleus
+
 (the nucleus that parent
(the nucleus that decays)
nucleus decays into)
α-particle,
β-particle
or γ-radiaion
Decay products (atoms/particles formed after decay)
There are three types of radioactive decays:
Type
Equation
α-decay
β-decay
γ-emission
What happens?
A helium nucleus (α-particle) is released from the
parent nucleus
One of the neutrons in the parent nucleus
changes into a proton and an electron (β-particle)
After emitting an α or a β particle, some nuclei
are left with more energy than normal. This extra
energy is emitted asγ-rays to make the nuclei
more stable. (There is no change in A an Z)
Radioactive decay usually occurs in a series (decay series), e.g.
(2) Random Nature of Radioactive Decay
Activity is the number of disintegrations per second and is measured in becquerel (Bq).
Half-life of a radioactive nuclide is the time taken for half of the nuclei present in any given sample
to decay. It is also the time taken for the activity of an given sample to fall to half of its original
value.
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Atomic Structure and Radioactive Decay
Radioactive decay is random in nature.
In a radioactive decay, the number of undecayed nuclei and hence the activity halved at a regular
interval. In other words, the half-life of a radioactive source is a constant.
(3) Uses of Radioisotopes
Few naturally-occurring substances are radioactive. But, radioisotopes can be man-made by
bombarding some nuclei with fast-moving particles. Such process is called transmutation.
Radioisotopes can be used in medicine, industry, agriculture and archaeology.
Medicine
1. Radiotherapy
Gamma rays are used to kill malignant cancer cells. Theγsource rotates around the cancerous
area and destroys the cancer cells.
2. Tracers
Weak radioisotopes are injected into the body as tracers.
e.g. β ray is used to check the thyroid andγray is used for producing brain scan
3. Sterilization
Syringes and other equipment are sterilized by exposing them to g rays.
Industry
1. Thickness gauge
Tyre cord or metal sheet moves between a β source on one side and a detector on the other.
The change in the thickness is reflected by the change in the GM count rate.
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Atomic Structure and Radioactive Decay
2. Static eliminator
α particles eliminated ionize adjacent air. The air ions attract loose electrons and reduce static
charge.
3 Smoke detector (fire alarm)
α particle ionize the air and form a current. When there is a fire, smoke particles collide with the
ions and the current drops. The alarm will be triggered
4. Tracers detecting a leaking system
The leaked γ rays from the pipes can be detected from the ground above.
Agricultural
Tracer
Phosphorus-30 is added to the fertilizer to study its intake by the plants.
Archaeological
Carbon-14 dating
Carbon-14 in a dead plant decays by β emission, with a half-life of 5700 years. By measuring
the activity of carbon-14 in dead plants, their age can be found.
(C) Radiation safety
Nuclear radiation is potentially very dangerous. It can destroy or damage living cells. It can
change the DNA structure and cause cancer. It can also cause incurable radiation sickness.
Gamma rays are the most dangerous because they can penetrate tissues deep in the body.
Alpha particles can be stopped by skin, but can become hazardous if alpha-emitting substance is
taken into the body
Effective dose is measured in sievert (Sv). The average annual dose from background radiation
in Hong Kong is about 2 mSv.
Radioactive sources should be handled with great care.
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Atomic Structure and Radioactive Decay