Q: When a boron nucleus (510 B) is bombarded by a neutron, a α-particle is emitted. Which nucleus will be formed as a result?

It is given that when a boron nucleus (510 B) is bombarded by a neutron, an α-particle is emitted.Let X nucleus be formed as a result of the bombardment.According to the charge and mass conservation, Charge : Mass : 510B+01n→510X+24He013724 The mass number of X should be 7 and its atomic number should be 3.∴X=37Li

Question 1

If neutrons exert only attractive force, why don't we have a nucleus containing neutrons alone?

Accepted Answer

Nuclear forces are short range strong attractive forces that act between two proton-proton, neutron-proton and neutron-neutron pairs. Two protons have strong nuclear forces between them and also exert electrostatic repulsion on each other. However, electrostatic forces are long ranged and have very less effect as compared to the strong nuclear forces.So, in a nucleus (that is very small in dimension), there's no such significance of repulsive force as compared to the strong attractive nuclear force. On the other hand, an atom contains electrons revolving around its nucleus. These electrons are kept in their orbit by the strong electrostatic force that is exerted on them by the protons present inside the nucleus. Hence, a nucleus contains protons as well as neutrons.

Question 2

Consider two pairs of neutrons. In each pair, the separation between the neutrons is the same. Can the force between the neutrons have different magnitudes for the two pairs?

Accepted Answer

Neutrons are chargeless particles and they exert only short range nuclear forces on each other. If we have two pairs of neutrons and the separation between them is same in both the pairs. The force between the neutrons will be of same magnitude for the two pairs until there is some other influence on any of them.

Question 3

A molecule of hydrogen contains two protons and two electrons. The nuclear force between these two protons is always neglected while discussing the behaviour of a hydrogen molecule. Why?

Accepted Answer

Inside the nucleus, two protons exert nuclear force on each other. These forces are short-ranged (a few fm), strong and attractive forces. They also exert electrostatic repulsive force (long-ranged). While discussing the behaviour of a hydrogen molecule, the nuclear force between the two protons is always neglected. This is because the separation between the two protons in the molecule is  ∼ 70 pm which is much greater than the range of the nuclear force.

Question 4

Is it easier to take out a nucleon (a) from carbon or from iron (b) from iron or from lead?

Accepted Answer

Binding energy per nucleon of a nucleus is defined as the energy required to break-off a nucleon from it.(a) As the binding energy per nucleon of iron is more than that of carbon, it is easier to take out a nucleon from carbon than iron.(b) As the binding energy per nucleon of iron is more than that of lead. Therefore, it is easier to take out a nucleon from lead as compared to iron.

Question 5

Suppose we have 12 protons and 12 neutrons. We can assemble them to form either a24Mg  nucleus or two 12C  nuclei. In which of the two cases more energy will be liberated?

Accepted Answer

If we assemble 6 protons and 6 neutrons to form 12C  nucleus, 92.15 MeV (product of mass number and binding energy per nucleon of carbon-12) of energy is released. Therefore, the energy released in the formation of two carbon nuclei is 184.3 MeV. On the other hand, when 12 protons and 12 neutrons are combined to form a24Mg  atom, 198.25 MeV of energy (binding energy) is released. Hence, in case of ​ 24Mg  nucleus, more energy is liberated.

Question 6

What is the difference between cathode rays and beta rays? When the two are travelling in space, can you make out which is the cathode ray and which is the beta ray?

Accepted Answer

Cathode rays consist of electrons that are accelerated using electrodes. They do not carry high energy and do not harm human body. On the other hand, beta rays consist of highly energetic electrons that can even penetrate and damage human cells. Beta rays are produced by the decay of radioactive nuclei. If the two are travelling in space, they can be distinguished by the phenomenon named production of Bremsstrahlung radiation, which is produced by the deceleration of a high energy particle when deflected by another charged particle, leading to the emission of blue light. Only beta rays are capable of producing it.

Question 7

If the nucleons of a nucleus are separated from each other, the total mass is increased. Where does this mass come from?

Accepted Answer

When the nucleons of a nucleus are separated, a certain amount of energy is to be given to the nucleus, which is known as the binding energy.Binding energy = [(Number of nucleons)  x (Mass of a nucleon) - (Mass of the nucleus)] When the nucleons of a nucleus are separated, the increase in the total mass comes from the binding energy, which is given to the nucleus to break-off its constituent nucleons as energy is related to mass by the relation given below.E = Δmc2

Question 8

In beta decay, an electron (or a positron) is emitted by a nucleus. Does the remaining atom get oppositely charged?

Accepted Answer

In beta decay, a neutron from the nucleus is converted to a proton releasing an electron and an antineutrino or a proton is converted to a neutron releasing a positron and a neutrino﻿i.e.β−decay:n→p+e+νˉβ+decay : p→n+e++ν﻿   Since the number of valence electrons present in the parent atom do not change, the remaining atom does not get oppositely charged. Instead, due to a change in the atomic number, there's a formation of a new element.

Question 9

When a boron nucleus  (510​ B)  is bombarded by a neutron, a α-particle is emitted. Which nucleus will be formed as a result?

Accepted Answer

It is given that when a boron nucleus  (510​ B)  is bombarded by a neutron, an α-particle is emitted.Let X nucleus be formed as a result of the bombardment.According to the charge and mass conservation, Charge :  Mass : ​510​B+01​n→510​X+24​He01​3​7​​24​  The mass number of X should be 7 and its atomic number should be 3.∴X=37​Li

Question 10

Does a nucleus lose mass when it suffers gamma decay?

Accepted Answer

Gamma rays consist of photons that are produced when a nucleus from its excited state comes to its ground state releasing energy. Since gamma rays are chargeless and massless particles, the nucleus does not suffer any loss in mass during the gamma decay.

Question 11

In a typical fission reaction, the nucleus is split into two middle-weight nuclei of unequal masses. Which of the two (heavier or lighter) has greater kinetic energy? Which one has greater liner momentum?

Accepted Answer

Two photons having equal liner momentum have equal wavelengths as here for both the photons the direction and magnitude of linear momentum will be same. For the rest of the options, magnitude will be same but nothing can be said about the direction of the photons.

Question 12

If three helium nuclei combine to form a carbon nucleus, energy is liberated. Why can't helium nuclei combine on their own and minimise the energy?

Accepted Answer

When three helium nuclei combine to form a carbon nucleus, energy is liberated. This energy is greater than the that liberated when these nuclei combine on their own. Hence, formation of carbon nucleus leads to much more stability as compared to the combination of three helium nuclei.

Question 13

The mass of a neutral carbon atom in ground state is:

Accepted Answer

exact 12 uIn nuclear physics, a unit used for measurement of mass is unified atomic mass unit, which is denoted by u.It is defined such that 1u=121​×( Mass of neutral carbon atom in its ground state)  Mass of neutral carbon atom in its ground state = 12 × 1 u = 12 uThus, the mass of neutral carbon atom in its ground state is exactly 12 u.

Question 14

The mass number of a nucleus is equal to:

Accepted Answer

the number of nucleons in the nucleusMass number of a nucleus is defined as the sum of the number of neutron and protons present in the nucleus, i.e. the number of nucleons in the nucleus.

Question 15

As compared to12C atom, 14C  atom has:

Accepted Answer

two extra neutrons and no extra electron 12C and 14C  are the two isotopes of carbon atom that have same atomic number, but different mass numbers. This means that they have same number of protons and electrons, but different number of neutrons. Therefore,  12C  has 6 protons, 6 electrons and 6 neutrons, whereas ​ 14C  has 6 electrons, 6 protons and 8 neutrons.

Question 16

The mass number of a nucleus is:

Accepted Answer

sometimes more than and sometimes equal to its atomic numberMass number of a nucleus is defined as the sum of the number of neutron and protons present in the nucleus, i.e. the number of nucleons in the nucleus, whereas atomic number is equal to the number of protons present. Therefore, the atomic number is smaller than the mass number. But in the nucleus (like that of hydrogen  1H1​ ), only protons are present. Due to this, the mass number is equal to the atomic number.

Question 17

The graph of ln(R/R0​ ) versus ln A(R = radius of a nucleus and A = its mass number) is:

Accepted Answer

(a) a straight lineThe average nuclear radius (R) and the mass number of the element (A) has the following relation ​R=Ro​A1/3R/Ro​=A1/3ln(R/Ro​)=31​lnA​ Therefore, the graph of ln(R/R0​ ) versus ln A is a straight line passing through the origin with slope 1/3.

Question 18

Let Fpp​ , Fpn​ and  Fnn​  denote the magnitudes of the net force by a proton on a proton, by a proton on a neutron and by a neutron on a neutron respectively.  neglect gravitational force. When the separation is 1 fm.

Accepted Answer

Fpp​  Fpn​  = Fnn​ Protons and neutrons are present inside the nucleus and they exert strong attractive nuclear forces on each other, which are equal in magnitude. Due to their positive charge, protons repel each other. Hence the net attractive force between two protons gets reduced, but the nuclear force is stronger than the electrostatic force at a separation of 1 fm.∴ Fpp​  Fpn​ =Fnn​  Here,Fpp​ , Fpn​ and Fnn​ denote the magnitudes of the net force by a proton on a proton, by a proton on a neutron and by a neutron on a neutron, respectively.

Question 19

Let Fpp​ , Fpn​ and Fnn​ denote the magnitudes of the net force by a proton on a proton, by a proton on a neutron and by a neutron on a neutron respectively.  neglect gravitational force. When the separation is 1 fm.

Accepted Answer

Fpp​=Fpn​=Fnn​ Protons and neutrons are present inside the nucleus and they exert strong attractive nuclear force on each other. These forces are equal in magnitude, irrespective of the charge present on the nucleons.∴Fpp​=Fpn​=Fnn​ Here, Fpp​,Fpn​andFnn​  denote the magnitudes of the nuclear force by a proton on a proton, by a proton on a neutron and by a neutron on a neutron, respectively.

Question 20

Two protons are kept at a separation of 10 nm. Let Fn​  and Fe​  be the nuclear force and the electromagnetic force between them.

Accepted Answer

Fe​  >> Fn​ Two protons exert strong attractive nuclear force and repulsive electrostatic force on each other. Nuclear forces are short range forces existing in the range of a few fms. Therefore, at a separation of 10 nm, the electromagnetic force is greater than the nuclear force, i.e. Fe​  >> Fn​.

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Solution for Class 12, Physics, Nuclei

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