5.1 UNDERSTANDING
THE NUCLEUS OF AN ATOM
1. Composition of the nucleus of
an atom
a) Each atom has a very small and very dense core called nucleus.
b) Most of the mass of atom is contained in the nucleus
c) A nucleus consists of a number of
protons and neutrons.
d) Protons and neutrons also known
as nucleons.
e) A proton has a unit positive
charge.
f)
A neutron is an uncharged particle of about the same mass as the proton.
g) The electrons move in orbits around the
nucleus.
h) An atom is neutral because it contains an equal
number of positively
charged (protons) and negatively charged (electrons). So the net
charge is zero
2. Proton number, Z,
a) is defined as the number of
protons in a nucleus
b) The number of electrons = the
number of protons in neutral atom.
c) An element is identified by its
proton number
3. Nucleon number, A
a) is defined as the total number of
protons and neutrons in a nucleus.
b) Number of neutrons, N = A - Z
4. Nuclides
a) Is an element with same number of
protons but different number
of neutrons.
b) Nuclide notation
c) Example
5. Isotopes
a) are atoms with the same proton number
but different nucleon number.
b)
Isotopes of an element contain the
i.
same number of protons
ii.
same number of electrons
iii.
have same chemical properties reactions (involve the electrons)
iv.
have different physical properties (mass is different.)
c)
Can exist naturally or made artificially.
d)
Isotope with unstable nucleus
i.
is called radioisotopes
ii.
tend to decay
e)
Example:
5.2 ANALYSING RADIOACTIVITY
1.
Radioactivity is the spontaneous disintegration of unstable nucleus into
more stable nucleus with the emission of energetic particles
or photons.
2.
The decay occurs randomly
3.
Unstable nucleus has a big number of proton and number of nucleon
4.
Three types of nuclear radiation emitted during decay
a)
Alpha, α
b)
Beta, β
c)
Gamma, γ
5.
Due to their different charges and masses, they have different
a)
Ionising power (ions per mm in air)
Radiation which has a stronger ionizing power will have a lower
penetrating effect.
b)
The nuclear radiation can be stopped by:
c) Effect on electric field
7. Common detectors for radioactive emissions
RADIOACTIVE DECAY
1. Radioactive decay is the process
by which unstable atomic nuclei emit subatomic particles or radiation
2. During the decay, its nucleus
breaks up, emits an alpha particle or beta particle and energy, and forms a new
atom of a different element.
3. Parent nuclide X changes into a
daughter nuclide Y.
4. Three types of radioactive decay
:
- alpha decay,
- beta decay
- gamma decay
8. Radioactive decay series
a) Sometimes the daughter nuclide of
a radioactive is still unstable. It will eventually decay into another nuclide
which is also unstable.
b) This process continues as a
radioactive decay series until a stable nuclide is reached.
c) Each decay will emit either an
alpha particle or a beta particle and may be gamma rays.
HALF-LIFE
1. Half-Life
is the time taken for half the atoms in a given sample to decay
2. The
concept of Half-Life
a) The decay is
a random process.
b) As the time
increases,
- the number
of atoms remaining decreases
- the number
of atom disintegrating increases
3. After a certain
period of time, half of initial number of atoms have
disintegrated and the
other half remains.
4. A typical decay curve for radioisotope
5. Example:
a) 
b) A radioactive source gives a reading of 640
counts per second when
measured
using G-M tube. One day later, the reading drops to 40 counts
per
second. What is the half life of this source?
Exercise:
1. An
artifact recovered from an archeological site was found to have
1/64 of its
initial amount of radioactive carbon. If the half-life of the radioactive
carbon is 5600 years, determine the age of the artifact.
(33600 years)
2. A
radioactive material has half-life of 48 seconds. Its activity is 16
count/second. What was its activity 6 minutes 24 seconds ago?
(4096)
5.3 UNDERSTANDING THE USES OF
RADIOISOTOPE
1. Properties of radioisotopes which make them
suitable in many application
a) Emits radioactive radiation.
b) Radioactive radiation can kill cells.
c) Radioactive radiations have different penetrating ability
with materials
of different thickness and densities.
d) Can cause cell mutation
e) Can ionise molecules
f) Activity decreases with time
2. Applications of
radioisotopes are in the field of
a) Industries
b) Medicine
c) Agriculture
d) Archaeology
APPLICATION IN INDUSTRIES
1. Thickness control
a) To control the thickness of paper, plastic, clothes and
metal.
b) Place the radioactive source (gamma ray for metal sheets, beta particles for paper,
plastic and clothes) at one side of the material
c) The detector (G-M tube) at the other side
Food containers
control
a) To ensure that the
food containers (cans and food packages) fill to the specific amount.
b) Place the radioactive source (beta particles) at one side
of the material
c) The detector (G-M tube) at the other side
d) When the containers is inadequately filled, the detector
will register a higher reading
Detecting leaks in underground water pipes
a) adding sodium-24 into the water in the storage tank.
b) moving the detector (G-M tube) above the underground pipe
c) the leakage can be detected when the G-M tube detects a
higher reading.
Determine the degree
of wear and tear of engine parts
- added radioisotopes to engine oil
- added radioisotopes to engine oil
Smoke detectors
a) Contain a weak radioactive source such as Americium-241.
b) Alpha particles emitted from the source and ionized air
molecules.
c) The ionized air molecules conduct electricity and a small
current flows in the smoke detector.
d) When smoke enters the detector, the soot particles in the
smoke absorb some of the alpha particles.
e) the current that flows in the smoke detector decrease and
trigger the alarm.
f) Americium-241 has a long half-life, 460 years so that the
substance will last longer.
APPLICATION IN MEDICAL FIELD
Tests and treats overactive thyroid glands or
certain kinds of thyroid cancer.
- Intravenous injection of iodine-123
(half-life 13 hours) or Iodine-131
(half-life 8 days)
Detect the position of blood clots /
thrombosis
- Inject sodium-24 into the bloodstream
Detect and treats brain tumor
- Using phosphorous-32
Cobalt-60 (emits gamma ray) is used to
- Destroy cancer cell in radiotherapy
- Sterillise medical equipments
It is important
to administer the correct dosage because
- Excessive radiation will damage too many healthy
cells.
- Insufficient radiation will not stop the cancer
from spreading.
APPLICATION IN AGRCULTURE
Kill pests using radioactive rays
- Used gamma rays
Stop the pest from reproducing.
- Expose the pests to low dosage of gamma rays.
- Induce mutation in the pest.
Study the effectiveness of fertilisers
- Used
nitrogen-15 or phosphorus-32
- Added radioisotopes to the soil water.
- The radioisotopes can track uptake of fertiliser
from root to leaves in plant.
- Scientists can find out how much fertilizer has
been absorbed by the plant.
Induce genetic mutation in plant
- Use radioactive radiation to induce genetic
mutation.
- In order to produce a better strain (has a
higher resistance against diseases).
APPLICATION IN ARCHEOLOGY
Determine the age of artifacts (carbon
dating)
a) By comparing the activity of the dead sample with the
activity of same mass of living sample.
b) Carbon-14 is a radioisotope with a half-life of 5730 years
and decays
by emitting beta particles.
c) Living animals and plants have a known proportion of carbon-14
in
their tissues which remains constant.
d) When living things die, the amount of carbon- 14 in their
body decreases
at a known rate.
e) The amount of carbon-14 left in a decayed plant or animal
can be used
to tell its age
5.4
NUCLEAR ENERGY
Nuclear
fission
1. is the splitting of a heavy nucleus into two
lighter nuclei.
2. occurs when the nucleus of an atom is
bombarded with a neutron.
3. The energy of the neutron causes the target
nucleus to
- split into two (or more) nuclei that are
lighter than the parent nucleus,
- releasing a large amount of energy during the
process.
4. Examples:
- A neutron hitting a uranium-235 nucleus,
causing it to split producing
xenon-143, strontium-90 and three neutrons +
energy.
- Or a neutron hitting a uranium-235 nucleus,
causing it to split producing barium-141, krypton-92 and three neutrons +
energy
5. In the fission reactions,
a) One neutron starts the fission process,
b) The nucleus is divided into two large fragments of
roughly equal mass
c) Three neutrons are produced
d) There is a significant mass defect
e) If one of these neutrons bombards another uranium-235
nucleus
f) more fissions will occur and releasing more neutrons.
g) A chain reaction is produced.
h) Example:
i.
As uranium atoms
continue to split,
ii.
a significant amount
of energy is released during each reaction,
iii.
the heat released is
greatest
iv.
A controlled chain
reaction is used in nuclear power stations to
generate electrical energy.
v. An uncontrolled chain reaction is used in nuclear
bombs
Nuclear fusion
1. Nuclear fusion is believed to be the process by which energy is
released by the Sun.
3. Hydrogen bomb uses the principle of nuclear fusion for its design.
Atomic
mass unit ,
a.m.u
a) is the unit of mass for atoms and subatomic particles
such as the proton,
b) neutron and electron
c) 1 atomic mass unit or 1 u is 1
of the mass of the carbon-12 atom
12
d) The mass of one carbon-12 atom 
e) Nuclear fission and fusion release a large amount of
energy.
f) The source of this energy is from the loss of mass in
nuclear reactions.
g) The sum of the masses of the particles before the
reaction is more than
the sum of the masses
of the particles after the reaction. 
i) A loss of mass in a nuclear reaction means that the mass
had
changed
to energy.
j) The relationship between the mass and the energy:
Example
1
Below is an equation for the decay of radium-226
(Ra-226)
(a) Find the mass defect in
(i) a.m.u
(ii) kg
(b) Calculate the amount of energy released
in
(i) J
(ii) eV
Answer:
(a) mass defect
(i) In a.m.u
= 226.02536 – (222.01753
+ 4.00260)
= 226.02536 – 226.02013
= 0.00523 a.m.u
(ii) In Kg
= 0.00523 x 1.66 x 10-27 Kg
= 0.00868 x 10-27 Kg
= 8.68 x 10-30 Kg
(b) energy released in
(i) Joule
E = mc2
= 8.68 x 10-30 x (3.0 x 10 8)2
= 78.1362 x 10-14
= 7.81 x 10-13 J
(ii) eV
= 7.81 x 10-13 J
1.66 x 10-19
J
= 4.70 x 106 eV
No comments:
Post a Comment