[5 Mark Question Answer]

Question 15 Marks

Complete the following fission reaction:
$
{ }_{92} U ^{235}{ }_0 n ^1 \longrightarrow{ }_a Sb ^{133}+{ }_{41} Co ^b+4{ }_0 n _1+\text { energy }
$
Write down the values of $a$ and $b$.

Answer

$
{ }_{92} U ^{235}{ }_0 n ^1 \longrightarrow{ }_a Sb ^{133}+{ }_{41} Co ^b+4{ }_0 n _1
$
Hence, $235+1=133+b+4$
or $236=b+137$
$
\therefore b=236-137=99
$
Similarly $92=a-41$
$
\therefore a=92-41=51
$
Hence,
$
{ }_{92} U ^{235}{ }_0 n ^1 \longrightarrow{ }_a Sb ^{133}+{ }_{41} Co ^b+4{ }_0 n _1
$

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Question 25 Marks

An element P disintegrates by beta-emission and the new element suffers two further disintegration, both by beta-emission, to form an element Q. Show that P and Q are isotopes.

Answer

Let mass number of $P=A$
Let atomic number of $P = Z$
Nuclear reaction for $\alpha$-emission
$
{ }_z P ^{ A } \longrightarrow{ }_{ Z -1} X ^{ A -4}+{ }_2 He ^4
$
$(\alpha-$ particles)
Nuclear reactions foe two further $\beta$-emission
$
Z -2 X ^{ A -4} \longrightarrow{ }_{ Z -4} Y ^{ A -4}+{ }_{-1} e ^0
$
( $\beta$-emission)
$
{ }_{ Z -1} Y ^{ A -4} \longrightarrow{ }_{ Z } Q ^{ A -4}+{ }_{-1} e ^0
$
( $\beta$-emission)
The atomic numbers ( $Z$ ) of $P$ and $Q$ are the same but their mass numbers $[A$ and $(A-4)]$ are different. So $P$ and $Q$ are isotopes.

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Question 35 Marks

What is nuclear energy? Name the process used for producing electricity using the nuclear energy.

Answer

When a heavy nucleus is bombarded with slow neutrons, it splits into two nearly equal light nuclei with a release of tremendous amount of energy. In this process of nuclear fission, the total sum of masses of products is less than the total sum of masses of reactants. This lost mass gets converted into energy. The energy so released is called nuclear energy.

Principle: The heat energy released due to the controlled chain reaction of nuclear fission of uranium-235 in a nuclear reactor is absorbed by the coolant which then passes through the coils of a heat exchanger containing water. The water in heat exchanger gets heated and converts into steam. The steam is used to rotate the turbine which in turn rotates the armature of a generator in a magnetic field and thus produces electricity.

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Question 45 Marks

An imaginary radioactive element ${ }_{92} X^{235}$ decays to form $X_1 ; X_2 ; X_3 ; X_4 ; X_5$ and $X_6$ nuclides, by ejecting two $\beta$-particles, followed by $\alpha$-particles and again two $\beta$-particles, followed by $\alpha$-particle. Represent the various nuclear changes in the form of an equation. State the mass number and atomic number of $X_6$. List the isotopes and isobars in the above nuclear equation.

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Question 55 Marks

State the kind of nuclear reaction taking place in a nucleus during the emission of a β-particle.

Answer

In the process of emission of a β-particle from the nucleus, a neutron changes into a proton by emitting a β-particle (electron).
$\underset{\text { (neutron) }}{0^{ n ^1}} \longrightarrow \underset{\text { (proton) }}{+1} p ^1+\underset{(\beta \text {-particles) }}{-1_e{ }^0}$
Hence in the nucleus, the number of neutrons decreases by 1 with the emission of a β-particle and the number of protons increases by one. So the sum of protons and neutrons, i.e., atomic mass remains the same but the atomic number increases by one.

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Question 65 Marks

A certain nucleus X has a mass number 15 and atomic number 7. Find the number of neutrons. The nucleus X losses one α-particle and Express the change and write the symbol of the new nucleus.

Answer

Mass number $=$ No. of protons + No. of neutrons
Atomic number $=$ No. of protons
$\therefore$ No. of neutrons $=$ Mass number - Atomic number
$
=15-7=8
$
Element $X$ can be written as
$
{ }_7^{15} X
$
After the loss of one $\alpha$-particle, the mass number is decreased by 4 and atomic number isdecreased by 2 . The element $X$ changes to $S$.
$
{ }_7^{15} X \longrightarrow{ }_5^{11} Z +{ }_2^4 He
$

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Question 75 Marks

A certain nucleus X has a mass number 15 and atomic number 7. Find the number of neutrons. The nucleus X losses one β-particle and Express the change and write the symbol of the new nucleus.

Answer

Mass number $=$ No. of protons + No. of neutrons
Atomic number $=$ No. of protons
$\therefore$ No. of neutrons $=$ Mass number - Atomic number
$
=15-7=8
$
Element $X$ can be written as
${ }_7^{15} X$
After the loss of one $\beta$-particle, the mass number will remain the same, the atomicnumber will increase by 1 . The element $X$ changes to $Z$.
$
{ }_7^{15} X \longrightarrow{ }_8^{15} Z +{ }_{-1}^0 e +\gamma-\text { ray }
$

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Question 85 Marks

A certain nucleus X has a mass number 15 and atomic number 7. Find the number of neutrons. The nucleus X losses one proton and Express the change and write the symbol of the new nucleus.

Answer

Mass number $=$ No. of protons + No. of neutrons
Atomic number $=$ No. of protons
$\therefore$ No. of neutrons $=$ Mass number - Atomic number
$
=15-7=8
$
Element $X$ can be written as
$
{ }_7^{15} X
$
After the loss of 1 proton, the mass number and atomic number will decrease by 1 . The change can be written as:
$
{ }_5^{15} X \longrightarrow{ }_6^{14} Y +{ }_1^1 P
$

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Question 95 Marks

How many alpha and beta particles are emitted when Uranium nucleus $9 U ^{238}$ decays to Lead If ${ }_{82} Pb^{206}$ ?

Answer

On emission of one alpha particle, the mass number decreases by 4 . With the emission of an alpha particle, the atomic number decreases by 2 but when a $\beta$-particle is emitted, the atomic number increases by 1.
${ }_{92} U ^{238} \xrightarrow{-\alpha}{ }_{90} X ^{234}$
, where $X$ represents the new nucleus
${ }_{90} X ^{234} \xrightarrow{-\beta}{ }_{91} X ^{234} \xrightarrow{-\alpha}{ }_{89}{ }^{230} \xrightarrow{-\beta}{ }_{90}{ }^{230} \xrightarrow{-\alpha} 88^{226} \xrightarrow{-\alpha}{ }^{226} \xrightarrow{-\alpha}{ }^{22}{ }^{222} \xrightarrow{-\alpha}{ }^{222} \xrightarrow{-\alpha}{ }^{218} \xrightarrow{-\beta}{ }^{218}{ }^{218}$
$\xrightarrow{-a} 85$
No. of $\alpha$-particles emitted $=8$.
No. of $\beta$-particles emitted $=6$.

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Question 105 Marks

What precautions must be taken for the safe disposal of nuclear waste?

Answer

The nuclear waste obtained from laboratories, hospitals, scientific establishments or power plants must be first kept in thick casks and then they should buried in the specially designed deep underground stores. The stores should be make quite far from the populated area and the casks should also be buried and scaled in useless mines after storing if.

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Question 115 Marks

Mention four properties of Becquerel rays.

Answer

The four properties of Becqueral rays are:
(i) They have penetrating power.
(ii) They affect a photographic plate.
(iii) They can ionize a gas or air through which they pass.
(iv) They are deflected by electric and magnetic fields.

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