24. Radioactivity Questions and Answers

Radioactivity Questions

1.  Complete the following equation by determining the values of U and V.

Th Pa + e

U……… V……………..  

2.  (a) Distinguish between nuclear fusion and fission

 (b) Compete the nuclear equation below:-  

3.  Uranium -238 disintegrates by emitting an alpha particle to form substance Y.

  Nuclide Y emits a beta particle to form substance Z. Write down nuclear equations to show how

  substance Y and Z are formed  (U=At No. 92)

4.  (a) What is a nuclide?  

 (b) The graph below shows the radioactive decay of a certain nuclide. Determine the

half-life of the nuclide

(e) What effect do excessful exposures of radiation have on metals?

5.  (a) State one way in which nuclear reactions differ from ordinary chemical reactions  

 (b) The following is a part of Uranium decay series

(i) Which particles are emitted in step I and II

 (ii) If a beta particle is emitted in step III, find Z and A

 (iii) If the activity of Thorium -234 is reduced to 25% in 48hours, find its half life

6.  Substances A and B are represented by the formulae ROH and RCOOH respectively.

They belong to two different homologous series of organic compounds. If both A and B

react with potassium metal:

(a) Name the common product produced by both

 (b) State the observation made when each of the samples A and B are reacted with sodium

hydrogen carbonate

  (i) A

  (ii) B

7.  Some two elements are represented as:

  (a) How many protons does X have?

  (b) How many neutrons does Y have?  

 (c) Draw the structure of the compound formed between X and Y  

8.  Y grams of a radioactive isotope take 120days to decay to 3.5grams. The half-life period

of the isotope is 20days

 (a) Find the initial mass of the isotope  

 (b) Give one application of radioactivity in agriculture

9. Study the nuclear reactions given and answer the questions that follow:

X Y Z

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

(b) Give one use of Y (lmk)

10. Give two uses of radioactive isotopes in medicine.

11.  Study the information in the following table and answer the questions that follow. The letters

 do not represent the actual chemical symbols of the elements.

 Which of the above elements are:

 (i) Likely to be radioactive?

 (ii) Able to form a compound with the highest ionic character?  

12.  The isotope decays by Beta,  -emission to a stable nuclide. The half-life of the

isotope is 15hours 2.0g of is allowed to decay. Determine the mass of left after 90hours  

13.  (a) Complete the following nuclear equation

Cr Mn + ____________

 (b) 100g of a radioactive substance was reduced to 12.5g within 15.6 years. Determine

the half-life of the substance  

Radioactivity Answers

1.  u= 234 √ V = 91√

2.  (a) Nuclear fusion is a process whereby smaller nuclei combine to form a larger one at high

  temperatures;  

  Nuclear fission is whereby a large nuclide splits to form smaller one when hit by a neutron

  (b)

3.  238 234 4

  U Y + H_e √1  (1 mk)

 92 90 2

2

 234 234 0

  Y Z   +   e  √1  (1 mk)

  90     91 -1

4.   (a) Is an atom or atomic nucleus characterized by its atomic number and mass number

(b) 14 = 7 from the graph

  2

half –life is 10days

(c) Destroys physical properties of metals e.g. lower tensile strength

5. a) nuclear reactions involve the nucleus of an atom but chemical reactions involved valence elections

• Nuclear reactions are independent of external factors but chemical reactions depend on external factors
  
• In nuclear reactions new elements are formed but no new elements are formed in chemical
  

reactions (any one of them

b) i)step I-Alpha√ ½

II- Beta√ ½

ii) Z = 234√ ½

A= 92√ ½

1^st t^1/2 2^nd t^1/2

II. 100% 50% 25%  

2t ½ = 48hours

t^1/2=?

t^1/2=48 = 24hours

2

6.  a) Hydrogen. ^√1

 b) (i) A No effervescence takes place. ^√½

(ii)B There is effervescence ^√½ and the gas produced turns lime water into white precipitate.^√½

7.  (a) 8 (protons number same as atomic number)

  (b) 27 -13 = 14

8.   (a) No. of half –lifes (n) = 120 = 6

  20

Y x (½)⁶ = 3.5

Y = 3.5 x 2⁶

Y = 224g

 (all steps for equation )

OR:

(b)   – To study the rate of absorption of fertilizer by plants using radioactive phosphorous

– Tracing chemical and physiological processes such as photosynthesis

– Sterilizing equipment (1ny one )

9.  a) 14 Y ________ 14 Z + 0

6 7 -1

 b) carbon dating

10.  – Gramma rays are used to sterilize surgical equipment

 – Detection and treatment of goiter

11.  i) U,V,Y,Z All the 4 or nay 3 exclusively correct penalize ½ mk if wrong answer

  ii) YZ is/are included any 2 correct ½ mk  

12. No. of t ½ = 90  = 6

  15

Remaining Fraction = (½ )⁶ = ¹/₆₄

Mass left   = ^1/₆₄ X 2 = 0.03125g

13.  a) -1 C

 b) 100-50 -25 – 12.5

3t ½ = 15.6

T ½ = 15.6

3

= 5.2 years