Atoms And Nuclei(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

ATOMS AND NUCLEI MCQs

Atoms, Nuclei (12th Grade)

Total Questions : 60 | Page 6 of 6 pages

Question 51. The wave number of energy emitted when electron jumps from fourth orbit to second orbit in hydrogen in 20497

c m^{- 1}

. The wave number of energy for the same transition in

H e^{+}

5,099 cm−1
20,497 cm−1
40,994 cm−1
81,988 cm−1

Discuss Question

Answer: Option D. -> 81,988 cm−1
:
D

\to ν \propto z^{2}

\frac{{\to ν}_{2}}{{\to ν}_{1}} = {[\frac{z_{2}}{z_{1}}]}^{2} = {[\frac{2}{1}]}^{2} = \frac{4}{1}

{\to ν}_{2} = 4 {\to ν}_{1} = 4 \times 20497 = 81988 c m^{- 1}

Question 52. The radius of a nucleus of a mass number A is directly proportional to

Discuss Question

Answer: Option D. -> A13
:
D

R = R_{0} A^{\frac{1}{3}} \Rightarrow R \propto A^{\frac{1}{3}}

Question 53.

M_{n}

and

M_{p}

represent mass of neutron and proton respectively. If an element having nuclear mass M has N-neutron and Z-proton, then the correct relation will be

M
M>[NMn+ZMp]
M=[NMn+ZMp]
M=N[Mn+Mp]

Discuss Question

Answer: Option A. -> M
:
A
Actual mass of the nucleus is always less than total mass of nucleons so

M < [N M_{n} + Z M_{p}]

Question 54. If a Hydrogen nucleus is completely converted into energy, the energy produced will be around

1 MeV
931 MeV
9.38 MeV
238 MeV

Discuss Question

Answer: Option B. -> 931 MeV
:
B
Mass ofHydrogen nucleus =mass of proton =1 amu energy equivalent to 1 amu is 931 MeV so correct option is (b).

Question 55. The masses of neutron and proton are 1.0087 amu and 1.0073 amu respectively. If the neutrons and protons combine to form a helium nucleus (alpha particles) of mass 4.0015 amu. The binding energy of the helium nucleus will be [1 amu= 931 MeV]

28.4 MeV
20.8 MeV
27.3 MeV
14.2 MeV

Discuss Question

Answer: Option A. -> 28.4 MeV
:
A
Helium nucleus consist of two neutrons and two protons.
So binding energy

E = Δ m \times 931 M e V

\Rightarrow E = (2 \times m_{p} + 2 m_{n} - M) \times 931 M e V = (2 \times 1.0073 + 2 \times 1.0087 - 4.0015) \times 931 = 28.4 M e V

Question 56. A radioactive nucleus decays from

_{92} X^{235}

_{91} Y^{231}

. Which of the following particles are emitted

One alpha and one electron
Two deuterons and one positron
One alpha and one proton
One proton and four neutrons

Discuss Question

Answer: Option A. -> One alpha and one electron
:
A

_{92} X^{235} α ⟶_{90} X^{231} - 1^{e^{0}} ⟶_{91} Y^{231}

Question 57. 1 g of hydrogen is converted into 0.993 g of helium in a thermonuclear reaction. The energy released

63×107J
63×1010J
63×1014J
63×1020J

Discuss Question

Answer: Option B. -> 63×1010J
:
B

Δ m = 1 - 0.993 = 0.007 g m

∴

E = Δ m c^{2} = 0.007 \times 10^{- 3} \times (3 \times 10^{8})^{2} = 63 \times 10^{10} J

Question 58. The radioactive element which is used to find the age of the fossil is

C-14
U-234
U-238
Po-94

Discuss Question

Answer: Option A. -> C-14
:
A
C-14 is carbon dating substance.

Question 59. If three

^{4} H e

nuclei combine to form a

^{12} C

nucleus, energy is liberated. Why can't helium nuclei combine on their own and minimize the energy?

He gas is rare, so collisions between nuclei are also rare;
It happens and the 12C we see in nature was formed by this method only in Earth's atmosphere;
The strong nuclear force between nucleons is attractive but short range; the Coulomb force between protons is long range, but repulsive;
Helium nucleus is more stable than carbon.

Discuss Question

Answer: Option C. -> The strong nuclear force between nucleons is attractive but short range; the Coulomb force between protons is long range, but repulsive;
:
C
Nuclear interactions are interesting battle grounds of different forces - mainly the Coulomb force, which is electrostatic in nature, and the strong nuclear force.
The strong nuclear force is a strong attractive force, but only activates when you bring the nucleons closer to each other than a few femtometers. At those length scales, no force in the universe is stronger than the strong nuclear force.
The problem is,

^{4} H e

nuclei are positively charged, because of the two protons. To even bring them to a close distance, we will need to overcome the Coulomb repulsion between them, which increases as separation decreases. Only when they have reached a separation of few fermi the strong nuclear attraction becomes large enough to hold the nucleons together against the Coulomb repulsion.