Please Limited time Offer!


The figure below shows the behaviour of emissions by a radioactive isotope x. Use it to answer the question follow

(a) Explain why isotope X emits radiations.                 (1mk)

          -is unstable //has n/p ratio greater/less than one

(b) Name the radiation labeled T                         (1mk)

                             alpha particle                                        

(c) Arrange the radiations labeled P and T in the increasing order of ability to be deflected by an electric filed. (1mk)

                   T -> P

 a) Calculate the mass and atomic numbers of element B formed after 21280X has emitted three beta particles, one gamma ray and two alpha particles.

Mass number

           = 212 – (0  beta+ o gamma + (2 x 4 ) alpha = 204

Atomic number

          = 80 –  (-1 x3) beta + 0 gamma + (2 x 2 )) alpha =79

b)Write a balanced nuclear equations for the decay of  21280X to B using the information in (a) above.

            21280 X      ->     20479B   +    242He     +    3  0-1e    +   y

Identify the type of radiation emitted from the following nuclear equations.

 (i)       146C           ->     147N     + ………

          β  –  Beta

  • 11 H    +   10n       ->      21H       + ……

                   y -gamma

 (iii)     23592U                   ->       9542Mo      +  13957La  +  10n  +……

                   7 β – seven beta particles

  • 23892 U                    ->     23490Th           + … …


  • 146 C  +       11H         ->           157N         +     ……


X grams of a radioactive isotope takes 100 days to disintegrate to 20 grams. If the half-life period isotope is 25 days, calculate the initial mass X of the radio isotope.

  Number of half-lifes    =   (  100   /   25 )   =    4

   20g —–> 40g —-> 80g—–> 160g —–> 320g

                   Original mass X = 320g

Radium has a half-life of 1620 years.

(i)What is half-life?

The half-life period is the time taken for a radioactive nuclide to spontaneously decay/ disintegrate to half its original mass/ amount

b)If one milligram of radium contains 2.68 x 10 18 atoms ,how many atoms disintegrate during 3240 years.

Number of half-lifes    =   (  3240   /   1620  )   =    2

                   1 mg —1620—> 0.5mg —1620—-> 0.25mg

                      If 1mg      ->      2.68 x 1018  atoms

  Then   0.25 mg ->  ( 0.25  x   2.68  x  1018   ) =   6.7  x  1017

 Number of atoms remaining = 6.7  x  1017

  Number of atoms disintegrated  =

                              (2.68 x 1018     –    6.7  x  1017   )

                                                                = 2.01  x  1018

The graph below shows the mass of a radioactive isotope plotted against time

Using the graph, determine the half – life of the isotope

From graph 10 g to 5 g takes 8 days

From graph 5 g to 2.5 g takes 16 – 8 = 8 days

Calculate the mass of the isotope dacayed after 32 days

Number of half lifes= 32/8 = 4

Original mass = 10g

10g—1st  –>5g—2nd–>2.5—3rd –>1.25—4th –>0.625 g

Mass remaining = 0.625 g

Mass decayed after 32 days =  10g – 0.625 g = 9.375g

A radioactive isotope X2 decays by emitting two alpha (a) particles and one beta (β) to form 214 83Bi      

(a)Write the nuclear equation for the radioactive decay

  21286X      -> 214 83Bi   +    242He     +      0-1

   (b)What is the atomic number of X2?


(c) After 112 days, 1/16 of the mass of X2 remained. Determine the half life of X2

  1—x-> 1 /2 –x-> 1 /4 –x-> 1 /8–x-> 1 /16

    Number of t 1 /2 in 112 days   =  4

          t 1 /2       =           112      =      28 days


1.Study the nuclear reaction given below and answer the questions that follow.

126 C   –step 1–>127 N –step 2–> 1211Na

(a)126 C and 146 C are isotopes. What does the term isotope mean?

Atoms of the same element with different mass number /number of neutrons.          

(b)Write an equation for the nuclear reaction in step II

           127 N ->          1211Na             +      0 -1e

(c)Give one use of   146 C

          Dating rocks/fossils:

          Study of metabolic pathways/mechanisms on plants/animals

Study  the graph of a radioactive decay series for isotope H below.

  • Name the type of radiation emitted when  isotope 

(i) H changes to isotope J.

                   AlphaMass number decrease by 4 from 214 to 210(y-axis)

                             atomic number decrease by 2 from 83 to 81(x-axis)

   (ii) J changes to isotope K

          BetaMass number  remains 210(y-axis)

                            atomic number increase by  1 from 81 to 82(x-axis).

(b) Write an equation for the nuclear reaction that occur when  isotope 

(i)J changes to isotope L

21081 J          ->          21084L              +     3 0 -1e

(i)H changes to isotope  M

21483 H        ->          20682M             +     3 0 -1e +       2 4 2He

     Identify a pair of isotope of an element in the decay series

K and M

Have same atomic number 82 but different mass number K-210 and M-206

a)A radioactive substance emits three different particles.

Identify the particle:

          (i)with the highest mass.

                             Alpha/ α

 (ii) almost equal to an electron

                             Beta/ β

1.a)State two differences between chemical and nuclear reactions(2mks)

 (i) Nuclear reactions mainly involve protons and neutrons in the nucleus of an atom.Chemical  reactions mainly involve outer electrons in the energy levels an atom.

  (ii) Nuclear reactions  form a new element. Chemical   reactions do not form new elements

(iii) Nuclear reactions mainly involve evolution/production of large quantity of heat/energy.Chemical   reactions produce or absorb smaller quantity of heat/energy.

(iv)Nuclear reactions are accompanied by a loss in mass /mass defect.

Chemical   reactions are not accompanied by a loss in mass.

(v)Rate of decay/ disintegration of nuclide is independent of physical conditionsThe rate of a chemical reaction is dependent on physical conditions  of temperature/pressure/purity/particle size/ surface area

b)Below is a radioactive decay series starting from 21483 Bi and ending at 20682 Pb. Study it and answer the question that follows.

Identify the particles emitted in steps I and III   (2mks)

          I    –  α-particle

          III –  β-ray

ii)Write the nuclear equation for the reaction which takes place in (a) step  I

                                21483Bi           ->               21081Bi        + 4 2 He

(b) step  1 to 3

                                21483Bi           ->               21081Bi        + 4 2 He +  0 -1 e

(c) step  3 to 5

                                21082Pb           ->     20682Pb        + 4 2 He +  2  0 -1 e

(c) step  1 to 5

                                21483Bi ->     20682Pb        + 2 4 2 He +  3  0 -1 e

The table below give the percentages of a radioactive isotope of Bismuth that remains after decaying at different times.

Time (min)0612223862100
Percentage of Bismuth10081654629123

i)On the  grid below , plot a graph of the percentage of Bismuth remaining(Vertical axis) against time.

ii)Using the graph, determine the:

          I. Half – life of the Bismuth isotope

          II. Original mass of the Bismuth isotope given that the mass that remained after 70 minutes was 0.16g    (2mks)

d)      Give one use of radioactive isotopes in medicine (1mk)

14.a)Distinguish between nuclear fission and nuclear fusion.                  (2mks)

Describe how solid wastes containing radioactive substances should be disposed of.                              (1mk)

 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                Z2              0   

iii)What type of nuclear reaction is represented  in b (i) above?

A radioactive cobalt   6128Co  undergoes decay by emitting a beta particle and forming Nickel atom,

Write a balanced decay equation for the above change                                                                                          1 mark

If a sample of the cobalt has an activity of 1000 counts per minute, determine the time it would take for its activity to decrease to 62.50 if  the half-life of the element is 30years                                                                              2 marks

Define the term half-life. The diagram below shows the rays emitted by a radioactive sample

  1. Identify the rays  S,R and  Q

S- Beta ( β )particle/ray

          R- Alpha (α )particle/ray

          Q- Gamma (y )particle/ray

b) State what would happen if an aluminium plate is placed in the path of ray R,S and Q:

          R-is blocked/stopped/do not pass through

          Q-is not blocked/pass through

           S-is blocked/stopped/do not pass through

(c)The diagram bellow is the radioactive decay series of nuclide A which is 24194Pu.Use it to answer the questions that follow. The letters are not the actual symbols of the elements.

(a)Which letter represent the : Explain.

(i)shortest lived nuclide

L-has the shortest half life

          (ii)longest lived nuclide

          P-Is stable

(iii) nuclide with highest n/p ratio

          L-has the shortest half life thus most unstable thus          easily/quickly decay/disintegrate

(iv) nuclide with lowest n/p ratio

          P-is stable thus  do not decay/disintegrate

(b)How long would it take for the following:

(i)Nuclide A to change to B

          10 years (half life of A)

(ii) Nuclide D to change to H

               27days +162000years+70000years+16days

                             232000 years and 43 days

(iii) Nuclide A to change to P

               27days +162000years+70000years+16days

                             232000 years and 43 days


Scroll to Top