RADIOCTIVITY
INTRODUCTION
Radioactivity is a process where an unstable nuclide breaks up to yield another nuclide of different composition with emission of particles and energy
Radioactive decay is the spontaneous disintegration/decay of a radioactive nuclide.
Radioisotopes are isotopes which are radioactive
.
Radioactivity is a nuclear reaction and not a chemical reaction
similarities:between Nuclear and chemical reaction
(i)-both involve the subatomic particles; electrons, protons and neutrons in an atom
(ii)-both involve the subatomic particles trying to make the atom more stable.
(iii)-Some form of energy transfer to the environment take place.
Differences between chemical reactions and nuclear reactions
| Nuclear reaction | Chemical reaction |
| Takes place within the nucleus and involves neutrons and protons | Takes place on the outer energy elevel and only involves valency electrons |
| Release large amounts of heat energy | Much less energy released |
| Not affected by environmental factors such as temperature | Are affected by environmental factors such as temperature and pressure |
| New element formed | No new element formed |
CHARACTERISTICS OF RADIOACTIVITY
All atoms with atomic number above 82 are radioactive
Radioactivity reactions are spontaneous and produce a lot of energy
Radioactivity is not affected by external factors like temperature and pressure
Types of radiation
There are three types of radiations emitted when radioactive nuclides disintegrate
| (i)alpha(α) particle decay
|
(ii)Beta (β) particle decay
|
iii)Gamma (y) particle decay |
| I. Is positively charged and are attracted to the negative plate of electric field
|
is negatively charged hence attracted to the positive plate of electric field. | No charge |
| II. Has mass number 4 and atomic number 2 therefore equal to a charged helium atom ( 42he2+)
|
no mass number and atomic number negative one(-1) therefore equal to a fast moving electron (0 -1e)
|
has no mass number and atomic number therefore equal to electromagnetic waves. |
| they show a lesser deflection by electric filed ,due to their large mass | Show are greater deflection due to the lesser mass | Not deflected |
| have very low penetrating power and thus can be stopped a thin sheet of | Have medium penetrating power and thus can be stopped thin sheet of aluminum foil.
|
very high penetrating power and thus can be stopped by a thick block of lead.. |
| I. have high ionizing power thus cause a lot of damage to living cells.
|
Have medium ionizing power thus cause less damage to living cells than α particle. | have very low ionizing power thus cause less damage to living cells unless on prolonged exposure |
Alpha decay;
a nuclide undergoing α-decay has its mass number reduced by 4 and its atomic number reduced by 2
Examples of alpha decay
210 84 Pb 206 82 Pb + 42He 2+
226 88 Ra 222 88 Rn + 42He 2+
complete the equations below
266 106 Sg mn RF + 42He 2+
251 98 Cf 238 92U + …………………
285 112 Cn pq Hs + 2 42He 2+
z a Es 235 93 Np + 3 42He 2+
288 114 Uuq 278 104 Rf + ………………
226 88 Ra 222 88 Rn + 42He 2+
beta (β) decay
- v) a nuclide undergoing β -decay has its mass number remain the same and its atomic number increase by 1
Examples of beta (β) decay
22888Ra 22889Ac + 0-1e
22888Ra 22892Th +
. 23290Th 23291Pb +
lkTh 21293Np + 3+ 0-1e
Gamma y -decay
- v) a nuclide undergoing y -decay has its mass number and its atomic number remain the same.
The sketch diagram below shows the penetrating power of the radiations from a radioactive nuclide.
radioactive nuclide sheet of paper aluminium foil thick block of lead
(radiation source) (block α-rays) (block β-rays) block y-rays)
α-rays β-rays y-rays
The sketch diagram below illustrates the effect of electric /magnetic field on the three radiations from a radioactive nuclide
Radioactive disintegration/decay naturally produces the stable 20682Pb nuclide /isotope of lead.Below is the 238 92 U natural decay series. Identify the particle emitted in each case
B:NUCLEAR FISSION AND NUCLEAR FUSION
Radioactive disintegration/decay can be initiated in an industrial laboratory through two chemical methods:
- a) nuclear fission
- b) nuclear
a)Nuclear fission
Nuclear fission is the splitting process of a a heavy unstable nuclide releasing lighter nuclides, and a large quantity of energy when bombarded /hit by a fast moving neutron
Nuclear fission is the basic chemistry behind nuclear bombs made in the nuclear reactors.
Examples of nuclear equations showing nuclear fission
Supply the missing information to te equations below
10 n + 235 b U 9038 Sr + a54Xe + 310 n + energy
10 n + 2713 Al 2813 Al + y + energy
23592 U + 10 n 14757 La + 8735 Br + —- + energy
10 n + 235 b U 10 n + ………….. energy
24796Cm + 10 n ……….+ 10 n + energy
23595U + 10 n ……….+ 14256 Ba +310 n + energy
NUCLEAR FUSION.
Nuclear fusion is the process which smaller nuclides join together to form larger / heavier nuclides releasing a large quantity of energy..
Nuclear fusion is the basic chemistry behind solar/sun radiation.
Two daughter atoms/nuclides of Hydrogen fuse/join to form Helium nuclide on the surface of the sun releasing large quantity of energy in form of heat and light
21H + 21H abHe + 10 n + energy
21H + 21H ……….. + 11 H+ energy
4 11H 42He + ………….+ energy
147N + …………. 178O + 11 H+ energy
5324N + 42He . 10n + …………….+ energy
Similarities between nuclear fusion and nuclear fission
In both a large quantity of energy
Both processes results in chain reactions
In both cases sub-atomic particles such as neutrons accompany the peocess
Differences between nuclear fusion and nuclear fission
| Nuclear nuclear fission | nuclear fusion |
| Heavy nucleus is split to smaller nuclei | Smaller nuclei combine to form heavy nucleus |
| Have a lower activation energy | Have a higher activation energy |
| Produces larger amount of energy than nuclear fusion | Produces relatively lower amount of energy |
: HALF LIFE PERIOD (t1/2)
The half-life period is the time taken for a radioactive nuclide to spontaneously decay/ disintegrate to half its original mass/ amount.
It is usually denoted t 1/2.
The rate of radioactive nuclide disintegration/decay is constant for each nuclide.
The table below shows the half-life period of some elements.
| Element/Nuclide | Half-life period(t 1/2 ) |
| 238 92 U | 4.5 x 109 years |
| 14 6 C | 5600 years |
| 229 88 Ra | 1620 years |
The less the half life the more unstable the nuclide /element.
The half-life period is determined by using a Geiger-Muller counter (GM tube)
.A GM tube is connected to ratemeter that records the count-rates per unit time.
This is the rate of decay/ disintegration of the nuclide.
If the count-rates per unit time fall by half, then the time taken for this fall is the half-life period.
APPLICATIONS OF HALF LIFE
- Carbon dating
- Detecting leakages
- Monitoring plant growth
- In medicine to monitor plant growth.
Examples
a)A radioactive substance gave a count of 240 counts per minute but after 6 hours the count rate were 30 counts per minute. Calculate the half-life period of the substance.
If t 1/2 = x
then 240 120 60 30
From 240 to 30 =3x =6 hours
=>x = t 1/2 = ( 6 / 3 )
= 2 hours
- b) The count rate of a nuclide fell from 200 counts per second to 12.5 counts per second in 120 minutes.
Calculate the half-life period of the nuclide.
- c) After 6 hours the count rate of a nuclide fell from 240 counts per second to 15 counts per second on the GM tube. Calculate the half-life period of the nuclide.
- d) Calculate the mass of nitrogen-13 that remain from 2 grams after 6 half-lifes if the half-life period of nitrogen-13 is 10 minutes.
- e) What fraction of a gas remains after 1hour if its half-life period is 20 minutes?
- f) 348 grams of a nuclide A was reduced to 43.5 grams after 270days.Determine the half-life period of the nuclide.
- g) How old is an Egyptian Pharaoh in a tomb with 2grams of 14C if the normal 14C in a present tomb is 16grams.The half-life period of 14C is 5600years.
- h) 100 grams of a radioactive isotope was reduced 12.5 grams after 81days.Determine the half-life period of the isotope.
A graph of activity against time is called decay curve.
A decay curve can be used to determine the half-life period of an isotope since activity decrease at equal time interval to half the original
The graph below shows the rate of decay of carbon-14
(i)From the graph show and determine the half-life period of the isotope.
From the graph t 1/2 changes in activity from:
( 100 – 50 ) => ( 5700 – 0 ) = 5700 years
( 50 – 25 ) => ( 11400 – 5700 ) = 5700 years
Thus t ½ = 5700 years
(ii)Why does the graph tend to ‘O’?
Smaller particle/s will disintegrate /decay to half its original.
There can never be ‘O’/zero particles
The table below shows the change in mass of a radioactive isotope with time
| Time (days) | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 |
| Mass (g) | 10.0 | 8.7 | 7.5 | 6.2 | 5.0 | 4.1 | 3.4 | 2.9 | 2.5 | 2.3 |
On the grid provided ,plot a graph of the percentage of bismuth remaining against time. (3mks)
- From the graph determine
- half life of the radioisotope
- the mass after the 7th day
- the mass after the 20th day
- The table below shows the measurements of radioactivity in counts per minute from a radioisotope iodine-128
| Counts per minute | 240 | 204 | 176 | 156 | 138 | 122 | 112 |
| Time in days | 0 | 5 | 10 | 15 | 20 | 25 | 30 |
- Plot a graph of counts per minute against time
- Use your gaph to determine the half life of iodine-128
- From youethe graph determinecount rate after;
- 12 minutes
- 22minutes
- After how many minutes was the count rate ;
- 160 counts per minute
- 197 counts per minute
A quantity of 44Y was monitored with a GM tube and the folllowinf results were obtained over a period of 70 minutes.
| Counts per minute | 800 | 580 | 427 | 305 | 225 | 165 | 122 | 85 |
| Time | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 |
- the grid provided plot a graph of counts per minute against time.
- Determne the half life of Y
- On Starting with 32g of 44Y,how much of the isotope would remain after 110 minutes.
- Give two applications of half life
E: APPLICATION AND USES OF RADIOCTIVITY.
The following are some of the fields that apply and use radioisotopes;
a)Medicine: -\
- Treatment of cancer to kill malignant tumors through radiotherapy e,g colbalt-60 and caesium-137
- –Sterilizing hospital /surgical instruments by exposing them to gamma radiation.
- -to monitor growth in bones and healing of fractures
- For providing power in heart pacesetters
- b) Agriculture:
- monitor plant growth by tracing the route of the radioisotope.
- Radioactive phosphorus is used to determine rate of absorption of phosphate fertilizers
- c) Food preservation:
X-rays are used to kill bacteria in tinned food to last for a long time.
- d) Chemistry:
To study mechanisms of a chemical reaction, one reactant is replaced in its structure by a radioisotope e.g.
During esterification the ‘O’ joining the ester was discovered comes from the alkanol and not alkanoic acid.
During photosynthesis the ‘O’ released was discovered comes from water.
- e) Dating rocks/fossils:
Comparing the mass of 14C in living and dead cells, to determine their age,
F: DANGERS OF RADIOCTIVITY.
- Exposure to theses radiations causes chromosomal and /or genetic mutation in living cells.
- Living things should therefore not be exposed for a long time to radioactive substances.
- One of the main uses of radioactive isotopes is in generation of large cheap electricity in nuclear reactors.
- Those who work in these reactors must wear protective devises made of thick glass or lead
- Accidental leakages of radiations usually occur
- In 1986 the Nuclear reactor at Chernobyl in Russia had a major explosion that emitted poisonous nuclear material that caused immediate environmental disaster
- In 2011, an earthquake in Japan caused a nuclear reactor to leak and release poisonous radioactive waste into the Indian Ocean.
- The immediate and long term effects of exposure to these poisonous radioactive waste on human being is of major concern to all environmentalists.
Control
Proper use,storage and disposal of radioactive materials
Regular checks of equipment which emit radiations
Revision quiz RADIOACTIVITY
- 1993 Q P1A 7
The Table below gives the rate of decay for radioactive element Y.
| Number of days | Mass (g) |
| 0 | 384 |
| 270 | 48 |
Calculate the half-life of the radioactive element Y.
- 1995 P1A Q30
(a) 100g of radioactive 23391 Pa was reduced to 12.5g after 81 days.
Determine the half-life of Pa. (2 marks).
- b) 23391 Pa decays by Beta emission. What is the mass number and the atomic
number of the element formed? (1 mark)
- 1996 P1A Q 20
Complete the diagram below to show how α and β particles from radioactive can be
distinguished from each other. Label your diagram clearly. (3 marks)
Source of radiation Paper Metal foil
- 1997 P1A Q 7
M grammes of a radioactive isotope decayed to 5 grammes in 100 days.
The half –life of the isotope is 25 days.
(a) What is meant by half-life? (1 mark)
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(b) Calculate the initial mass M of the radioactive isotope. (2 marks)
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- 1998 P1A Q1
An isotope of Uranium 234 94U decays by emission of an alpha particle to thorium. Th.
(a). Write the equation for the nuclear reaction undergone by the isotope. (1 mark)
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(b). Explain why it is not safe to store radioactive substances in containers made from
Aluminum sheets. (1 mark)
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- 2000 Q 13
A radioactive isotope X2 decays by emitting two alpha (a) particles and one
beta (β) to from 214
Bi
83
(a) What is the atomic number of X2?
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(b) After 112 days, 1/16 of the mass of X2 remained. Determine the half life of X2
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- 2002 Q 10
The graph below represents a radioactive decay series for isotope H.
Study it and answer the questions that follow
(a) Name the type of radiation emitted when isotope H changes to isotope J.
……………………………………………………………………………………………………………….
(b) Write an equation for the nuclear reaction that occur when isotope J changes to isotope K
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- c) Identify a pair of isotope of an element in the decay series
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100 g of a radioactive substance was reduced to 12.5 g in 15.6 years.
Calculate the half – life of the substance. (2 marks)
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9.
(a) Complete the nuclear equation below. (1 mark)
37 18A….. 3719B +………..
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(b) State one:
(i) Use of radioisotopes in agriculture (1mark)
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(ii) Danger associated with exposure of human beings to radioisotopes (1 mark)
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- 2007 Q 14
- a) Distinguish between nuclear fission and nuclear fusion. (2 marks)
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Describe how solid wastes containing radioactive substances should be disposed of. (1 mark)
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11,. 2008 Q 24
- a) A radioactive substance emits three different particles. Give the symbol of the particle
with the highest mass. (1 mark)
…………………………………………………………………………………………………………………..
- b) (i) Find the values of Z1 and Z2 in the nuclear equation below
Z1 1 94 140 1
U + n Sr + Xe +2 n
92 0 38 Z0 0
- ii) What type of nuclear reaction is represented in represented in b (i) above?
(1mark)
Time (minutes)
Give the name of the:
- a) Process taking place between t0 and t (1mark)
……………………………………………………………………………………………………………………
- b) Energy change that occurs between t3 and t4
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- 2009 Q 6d P2
(d) Naturally occurring uranium consist of three isotopes which are radioactive.
Isotopes 234 u 235u 238u
Abundance 0.01% 0.72% 99.27%
(i) Which of these isotopes has the longest half-life? Give reasons. (1 mark)
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(ii) Calculate the relative atomic mass of uranium. (2 marks)
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(iii) 235 92U is an alpha emitter .If the product of the decay of this nuclide
is thorium (Th) .Write a nuclear equation for the process. (1 mark)
……………………………………………………………………………………………………………….
- iv) State one use of radioactive isotopes in the paper industry (2 marks)
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- 2011 Q 2
Complete the nuclear equation below:
131 131
I Xe +
53 54
The half life of 13153 I is 8 days.
Determine the mass of 13153I remaining if 50 grammes decayed for 40 days.
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Give one harmful effect of radioisotopes. (1 mark)
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- 2012 Q9 P1
120g of iodine – 131 has a half life of 8 days decays for 32 days. On the grid provided,
plot a graph of the mass of iodine – 131 against time. (3 marks)
