Matric Notes Physics 10th Chapter 18 Atomic And Nuclear Physics Exercise Questions

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Note: In the representation of element the first no is atomic number and second is atomic mass

Q18.1 What is the difference between atomic number and atomic mass number? Give a symbolical representation of a nuclide.

Ans. Atomic number

The number of protons present in a nucleus is known as atomic number. It is represented by Z.

Atomic Mass number

The number of protons and neutron present in the nucleus is called atomic mass number. It is represented by A.

Symbol for Nuclide:

Generally the nuclide of an atom is represented by the symbol ^A _Z X e.g. Nuclide of hydrogen atom having only one proton is ¹ ₁ H.

Q18.2 What do you mean by the term radioactivity? Why some elements are radioactive but some are not?

Ans. Radioactivity :

The spontaneous emission of radiation by unstable nuclei is called natural radioactivity and the elements which emit such radiation are called radioactive elements.

The SI unit of radioactivity is Becquerel (Bq).

Radioactive and non radioactive elements:

The elements whose atomic number is between 1 and 82 are stable elements they do not emit radiation naturally. Such elements are non radioactive elements. On the other hand, the elements whose atomic number is greater than 82 are unstable and these elements emit radiations naturally, Such elements are called radioactive elements.

Q18.3 How can we make radioactive elements artificially? Describe with a suitable example.

Ans. Artificial radioactive elements

The stable or non-radioactive elements can also be changed into unstable or radioactive elements by bombarding them with neutrons, protons or alpha particles such artificially produced radioactive elements are called radioactive isotopes or radioactive isotopes.

Example 1:

¹ ₀ n  + ²³ ₁₁ Na  ----> ²⁴ ₁₁ Na  + gamma (γ) rays

Example 2:

⁴ ₂ He  + ²⁷ ₁₃ Al  -----> ³⁰ ₁₅ P + ¹ ₀ n

 

Q 18.4 What are three basic radioactive decay processes and how do they differ from each other?

Ans. Three basic radioactive process are:

1. Alpha α Decay:
2. Beta β Decay:
3. Gamma γ Decay:

Alpha α Decay:

General Equation:

^A _Z X (Parent nuclide)------> ^A-4 _Z-2 X (daughter nuclide)+ ⁴ ₂ He (alpha α- particle) +  Energy

Example:

²²⁶ ₈₈ Ra (radium)------> ²²² ₈₆ Rn (radon)+ ⁴ ₂ He (alpha α- particle) +  Energy

It means in alpha decay, the proton number or atomic number Z of the parent nuclide reduces by 2 and its mass number or nucleon number A decrease by 4.

Beta β Decay:

General Equation:

^A _Z X (Parent nuclide)------> ^A _Z+1 X (daughter nuclide)+ ⁰ _-1 e (beta β- particle) +  Energy

Example:

¹⁴ ₆ C (carbon)------> ¹⁴ ₇ N (nitrogen)+ ⁰ _-1 e (beta β- particle)+  Energy

This shows that with the emission of beta-rays its mass number or nucleon number A remains unchanged and proton number increase by 1.

Gamma γ Decay:

General Equation:

^A _Z X (Parent nuclide)------> ^A _Z X (daughter nuclide)+ γ (gamma rays)

Example:

⁶⁰ ₂₇ Co^* (Cobalt)------> ⁶⁰ ₂₇ Co (cobalt)+ ⁰ ₀ γ (gamma rays) +  Energy

This shows that gamma rays are usually emitted at the same moment as either an alpha or a beta particle.

Alpha Particle

• Charge +2
• Least penetration
• Transmutes nucleus:
• A --> A - 4
• Z --> Z - 2
• N --> N - 2

Beta Particle

• Charge -1
• Moderate penetration
• Transmutes nucleus:
• A --> A
• Z --> Z + 1
• N --> N - 1

Gamma Ray

• No Charge
• Highest penetration
• Transmutes nucleus:
• A --> A
• Z --> Z
• N --> N

Q18.5 Write the alpha-decay process for ²³⁴ ₉₁ Pa .Identify the parent and daughter nuclei in this decay.

Ans. For ²³⁴ ₉₁ Pa  alpha decay process is given as:

²³⁴ ₉₁ Pa  ----> ²³⁰ ₈₉ Ac  + ⁴ ₂ He  + Energy

Protactinium    Actinium   α-particle

Parent nuclei is ²³⁴ ₉₁ Pa and daughter nuclei is ²³⁰ ₈₉ Ac.

Q 18.6 Explain whether the atomic number can increase during nuclear decay. Support your answer with an example.

Ans. Beta (β) decay:

Yes, in beta - decay, the parent nuclide has its proton number (Z) increased by one but its mass number (A) remain unchanged. An electron does not exist inside the nucleus so, it is created at the time of emission when one neutron transforms into a proton and an electron is emitted out of the nucleus as β-particle.

^A _Z X-----> ^A _Z+1 Y + ⁰ _-1 e + Energy

Example:

¹⁴ ₆ C ----> ¹⁴ ₇ N + ⁰ _-1 e + Energy

Q 18.7 What do you understand by half life of a radioactive element?

Ans. Half life: The time during which half of the unstable radioactive nuclei disintegrate is called the half life of the radioactive element.

Half life and its measurement

The rate of radioactive decay is proportional to the number of unstable nuclei present, a constant fraction of large number of unstable radioactive nuclei in a certain time. So that, the half life time of unstable nuclei is unlimited and is difficult to measure.

Every radioactive element has its own half-life. For example; radium 226 has a half life of 1620 years. That means after 1620 years the element will remain half.

If the half life of radioactive element is T_1/2 then at the end of this time, the number of atom will become half i.e. 1/2, and after a time 2 T_1/2 the number of remaining atom will become 1/2 x 1/2 = 1/2 ² = 1/4 and after 3 T_1/2 , the number of remaining atom will be 1/2 x 1/2 x 1/2 = 1/2 ³ = 1/8 and at the end of "t" half lives the number of atom will be 1/2 ^t .

The number of atoms in the sample of radioactive element left after "t" half lives can be calculated as:

Remaining atom = original atom x 1/2 ^t

Or N = N_O x 1/2 ^t

The process of radioactivity does not depend upon chemical combination or reactions and is also not affected by any change in physical conditions like temperature, pressure, electric or magnetic fields.

Half Lives of Isotopes

• Element: Hydrogen
• Isotope: ³ ₁ H
• Half-Life: 12.3 years
• Radiation Produced: β

• Element: Carbon
• Isotope: ¹⁴ ₆ C
• Half-Life: 5730 years
• Radiation Produced: β

• Element: Cobalt
• Isotope: ⁶⁰ ₂₇ Co
• Half-Life: 30 years
• Radiation Produced: β, γ

• Element: Iodine
• Isotope: ¹³¹ ₅₃ I
• Half-Life: 8.07 days
• Radiation Produced: β, γ

• Element: Lead
• Isotope: ²¹² ₈₂ Pb
• Half-Life: 10.6 hours
• Radiation Produced: β

• Element: Polonium
• Isotope: ¹⁹⁴ ₈₄ Po
• Half-Life: 0.7 seconds
• Radiation Produced: α

• Element: Polonium
• Isotope: ²¹⁰ ₈₄ Po
• Half-Life: 138 days
• Radiation Produced: α, γ

• Element: Uranium
• Isotope: ²³⁵ ₉₂ U
• Half-Life: 7.1 x 10⁸ years
• Radiation Produced:  α, γ

• Element: Uranium
• Isotope: ²³⁸ ₉₂ U
• Half-Life: 4.51 x 10⁹ years
• Radiation Produced:  α, γ

• Element: Plutonium
• Isotope: ²³⁶ ₉₄ Pu
• Half-Life: 2.85 years
• Radiation Produced:  α

• Element: Plutonium
• Isotope: ²⁴² ₉₄ Pu
• Half-Life: 3.97 x 10⁵years
• Radiation Produced:  α, γ

Q18.8 Is radioactivity a spontaneous process? Elaborate you answer with a simple experiment.

Ans. Yes, Radioactivity is the spontaneous process. Radioactive decay involves the spontaneous transformation of one element into another. Radioactive decay also known as nuclear decay of radioactivity. It is the process by which the nucleus of an unstable atom loses energy by spontaneously emits such radiation is considered radioactive.

Experiment

The Discovery of Radioactivity

In 1896, Henry Becquerel was using naturally fluorescent minerals to study the properties of X-ray, which had been discovered in 1895 by William Roentgen. He exposed the sample to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun's energy and then emitted it as x-rays. This hypothesis was disproved on the 26th - 27th February, when his experiment "failed" because it was overcast in Paris. For some reason, Becquerel  decided to develop the photographic plates. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity.

Becquerel show that the radiation he discovered could not be x-rays. X-rays are neutral and cannot be bent in a magnetic field. The new radiation was bent by the magnetic field so that the radiation must have charge and different than x-rays. When different radioactive substances were put in magnetic field, they deflected in different directions, showing that there were three classes of radiations: negative, positive, and electrically neutral.

The term radioactivity was actually coined by Marie Curie, who together with her husband Pierre, began investigating the phenomenon discovered by Becquerel. The Curies extracted uranium from ore and to their surprise, he found that the leftover ore showed more activity than the pure uranium.

They concluded that the ore contained other radioactive elements. This led to the discoveries of the elements polonium and radium. Radioactivity is a natural part of our environment.

Q18.9 Describe two uses of radioisotopes in medicine, industry or research.

Ans. Tracers in industry:

In industry tracers are used to locate, the wear and tear of the moving part of the machinery. They are used to locate the leakage in underground pipes.

Use in Medical Treatment:

Radio isotopes are used in nuclear medicine for curing various diseases. For example Cobalt-60 is used for curing cancerous cells. The radiation kills the cells of malignant tumors.

Tracers in Medical Field:

Iodine-131: Iodine-131 is used to monitor thyroid function.

Phosphorous-32: To diagnose brain tumors phosphrous-32 is used.

Q 18.10 What are two common radiation hazards? Briefly describe the precautions that are taken against them.

Ans. Radiation Hazards:

Although radiation are useful for us, but due to excessive used we have following Hazards.

• Radiation burns, mainly due to beta and gamma radiations, which may cause redness and sores on the skin.
• Sterility (i.e. inability to produce children).
• Genetic mutations in both human and plants. Some children are born with serious deformities.
• Leukemia (cancer of the blood cells)
• Blindness or formation of cataract in the eye.

Safety Measures against Radiation Hazards:

We should follow the following safety measures of radiation

• The source should be handled with tongs and forceps.
• The user should use gloves and hands should be washed carefully after experiment.
• All radioactive sources should be stored in thick lead containers.
• Never point a radioactive source towards a person.
• Frequent visits to the radiation sensitive areas should be avoided.

Q 18.11 Complete this nuclear reaction ²³⁵ ₉₂ U ------> ¹⁴⁰ ₅₄ X + ? + 2 ¹ ₀ n. Does this reaction involve fission or fusion? Justify your answer. 

Ans. ²³⁵ ₉₂ U ------> ¹⁴⁰ ₅₄ X + ⁹³ ₃₈ Y + 2 ¹ ₀ n

²³⁵ ₉₂ U ------> ¹⁴⁰ ₅₄ Xe + ⁹³ ₃₈ Sr + 2 ¹ ₀ n

Yes, this is a fission chain reaction in which the heavy nucleus breaks in two smaller nuclei with the release of energy.

Q 18.12 Nuclear fusion reaction is more reliable and sustainable source of energy than nuclear fission chain reaction. Justify this statement with plausible arguments.

Ans. Yes, nuclear fusion is more reliable and sustainable source of energy because it does not contain nuclear waste and there is less danger of radiation and large amount of heat is released with the combination of smaller nuclei.

Q 18.13 A nitrogen nuclide ¹⁶ ₇ N decay to become an oxygen nuclide by emitting an electron. Show this process with an equation.

Ans. ¹⁶ ₇ N ------> ¹⁶ ₈ O + ⁰ _-1 e

A nitrogen nuclide ¹⁶ ₇ N decays to become an oxygen nuclide by emitting one β-ray.

Q 18.14 Determine which of these radioactive decay processes are possible.

(a) ²¹⁴ ₈₄ Po ----> ²¹⁴ ₈₄ Po + ⁴ ₂ He 

(b) ²³⁰ ₉₀ Th ------> ²²⁶ ₈₈ Ra +  ⁴ ₂ He

(c)  ²³³ ₉₁ Pa -------> ²³³ ₉₂ U +  ⁰ _-1 β

(d) ¹² ₆ C -------> ¹⁴ ₇ N + ⁰ _-1 β

Ans. (a) ²¹⁴ ₈₄ Po ----> ²¹⁴ ₈₄ Po + ⁴ ₂ He 

This radioactive decay is not possible because atomic number and mass number does not change in this reaction after decaying alpha particles.

(b) ²³⁰ ₉₀ Th ------> ²²⁶ ₈₈ Ra +  ⁴ ₂ He

Th-230 is converted into Ra-226 after decaying alpha particles. So, decay process is possible.

(c)  ²³³ ₉₁ Pa -------> ²³³ ₉₂ U +  ⁰ _-1 β

Pa-223 is converted into U-233 so this radioactive decay is also possible.

(d) ¹² ₆ C -------> ¹⁴ ₇ N + ⁰ _-1 β (not possible)

¹² ₆ C + 2 ¹ ₀ n-------> ¹⁴ ₇ N + ⁰ _-1 β (possible)

This shows that decay is possible by bombarding C-as with neutrons, otherwise it is not possible.