# Physics Questions and Answers – Atomic Masses and Composition of Nucleus

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Atomic Masses and Composition of Nucleus”.

1. The neutron was discovered by whom?
a) Marie Curie
b) Pierre Curie
c) Rutherford

Explanation: Neutrons are chargeless subatomic particles. It was discovered by James Chadwick in 1932. They are present in all-atom nuclei except those of ordinary hydrogen. The neutron was initially very difficult to detect and therefore, the last to be discovered.

2. The magnetic moment of a revolving electron around the nucleus varies with the principal quantum number as which of the following?
a) μ ∝ n
b) μ ∝ $$\frac {1}{n}$$
c) μ ∝ n2
d) μ ∝ $$\frac {1}{n^2}$$

Explanation: L = $$\frac {nh}{2\pi }$$ and μ = $$\frac {eL}{2m}$$.
μ = $$\frac {enh}{2m.2\pi }$$
Therefore, μ ∝ n.

3. Which of the following has the highest neutron ratio?
a) 8O16
b) 2He4
c) 26Fe56
d) 92U235

Explanation: The nucleus of Uranium 235(92U235) has 234 – 92 = 142 neutrons.
Therefore, It has the highest proton to neutron ratio, i.e.
= The number of protons: the number of neutrons
= 142:92
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4. When a radioactive substance emits an α-particle, its position in the periodic table is lowered by which of the following?
a) One place
b) Two places
c) Three places
d) Four places

Explanation: When a radioactive substance emits an α-particle, its atomic number decreases by a factor of 2. Therefore, its position in the periodic table is also lowered by two places.

5. If alpha, beta, and gamma rays carry the same momentum, which has the longest wavelength?
a) Alpha rays
b) Beta rays
c) Gamma rays
d) All have the same wavelength

Explanation: Wavelength of de Broglie waves is given as:
λ = $$\frac {h}{p}$$.
As momentum is the same for all rays, all have the same wavelength λ.

a) True
b) False

Explanation: No, this is a false statement. β-rays are charged particles emitted by radioactive nuclei. Therefore, these rays are highly energetic electrons that are released from the inside of a nucleus. Thus, these are not electromagnetic radiations.

7. If elements with principal quantum number n > 4 were not allowed in nature, then what will be the number of possible elements?
a) 60
b) 20
c) 4
d) 64

Explanation: Maximum number of electrons in an orbit = 2n2.
As n > 4 is not allowed, the number of possible elements = 2(1)2 + 2(2)2 + 2(3)2
Number of possible elements = 2(4)2
Number of possible elements = 60.

8. For uranium nucleus how does its mass vary with volume?
a) m ∝ V
b) m ∝ $$\frac {1}{V}$$
c) m ∝ √V
d) m ∝ V2

Explanation: Mass of nucleus = Nuclear density × Volume.
The nuclear density of an atom is constant (10-17 kgm-3) for all nuclei, therefore, the mass of the nucleus is directly proportional to the volume.

9. Which of the following is a correct statement?
a) Beta rays are the same as for cathode rays
b) Gamma rays are high energy neutrons
c) Alpha particles are singly ionized helium atoms
d) Protons and neutrons have the same mass

Explanation: β-rays are electrons of nuclear origin. Therefore, they have the same nature as the cathode rays. The other statements are not valid.

10. Which is a non-central force?
a) Electrostatic force
b) Nuclear force
c) Gravitational force
d) Spring force

Explanation: Nuclear force is a non-central force. They are dependent on the spin of the nuclei, and not on whether the nucleons are protons or neutrons. The tensor force exists between two nucleons, and therefore, it has a tensor component.

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