Nuclei

Question 1

What is the overall structure of a nucleus?

Answer 1

The nucleus is the central part of an atom where the positive charge and mass are densely concentrated. It is much smaller in size compared to the atom and contains most of the atom’s mass.

Question 2

What experiments demonstrated the small size of the nucleus compared to the atom?

Answer 2

Experiments on the scattering of alpha particles showed that the radius of a nucleus was smaller than the radius of an atom by a factor of about 10^4.

Question 3

What is the ratio of the volume of a nucleus to the volume of an atom?

Answer 3

The volume of a nucleus is about 10^-12 times the volume of an atom.

Question 4

If an atom were enlarged to the size of a classroom, what would be the relative size of the nucleus?

Answer 4

The nucleus would be approximately the size of a pinhead.

Question 5

What percentage of the mass of an atom is contained within the nucleus?

Answer 5

More than 99.9% of the mass of an atom is concentrated within its nucleus.

Question 6

Does the nucleus have a structured composition? If so, what are its constituents?

Answer 6

Yes, the nucleus has a structured composition. It consists of protons and neutrons, collectively known as nucleons.

Question 7

How are the constituents of the nucleus held together?

Answer 7

The constituents of the nucleus, protons, and neutrons, are held together by the strong nuclear force.

Question 8

What is the mass of a carbon atom, and why is the atomic mass unit (u) used instead of kilograms?

Answer 8

The mass of a carbon atom is 1.992647 × 10^−26 kg. The atomic mass unit (u) is used because the mass of an atom is very small, making kilograms inconvenient for measurement.

Question 9

Define the atomic mass unit (u) and explain its relationship with the mass of a carbon-12 atom.

Answer 9

The atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom. Therefore, the mass of one carbon-12 atom is equivalent to 12 u.

Question 10

What are isotopes, and how do they differ from one another?

Answer 10

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They exhibit identical chemical properties but have different masses.

Question 11

How are atomic masses accurately measured, and what instrument is used?

Answer 11

Atomic masses are measured accurately using a mass spectrometer.

Question 12

Explain the concept of isotones and provide an example.

Answer 12

Isotones are nuclides with the same neutron number but different atomic numbers. An example is 198^80Hg and 197^79Au.

Question 13

Who discovered the neutron, and how was it verified?

Answer 13

The neutron was discovered by James Chadwick in 1932. Its existence was verified through the emission of neutral radiation observed when beryllium nuclei were bombarded with alpha particles.

Question 14

What is the composition of a nucleus, and what terms and symbols are used to describe it?

Answer 14

The composition of a nucleus includes protons and neutrons. Terms and symbols used to describe it include:

Z (atomic number) = number of protons
N (neutron number) = number of neutrons
A (mass number) = total number of protons and neutrons

Question 15

Describe the properties of a proton and its significance in atomic structure.

Answer 15

A proton carries one unit of fundamental charge and is stable. It determines the atomic number of an element and is located in the nucleus.

Question 16

Why are neutrons necessary in atomic nuclei, and who discovered them?

Answer 16

Neutrons are necessary in atomic nuclei to account for the observed mass of isotopes. James Chadwick discovered neutrons in 1932.

Question 17

What is the stability of a free neutron, and how does it decay?

Answer 17

A free neutron is unstable and decays into a proton, an electron, and an antineutrino with a mean life of about 1000 seconds. However, it is stable inside the nucleus.

Question 18

What is the mass of a carbon atom expressed in kilograms and atomic mass units (u)?

Answer 18

The mass of a carbon atom is approximately 1.992647×10^−26kg and 12 u.

Question 19

Define the atomic mass unit (u) and its relationship with the mass of a carbon-12 atom.

Answer 19

The atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom. It is used to express atomic masses conveniently.

Question 20

What is the significance of the atomic mass of chlorine being 35.46 u?

Answer 20

The atomic mass of chlorine being 35.46 u illustrates an exception to the general trend where atomic masses of various elements are close to integral multiples of the mass of a hydrogen atom.

Question 21

What are isotopes, and how are they identified?

Answer 21

Isotopes are atomic species of the same element that have the same chemical properties but differ in mass. They are identified through the measurement of atomic masses using a mass spectrometer.

Question 22

Explain the concept of average atomic mass using chlorine as an example.

Answer 22

The average atomic mass of chlorine is calculated as a weighted average of the masses of its isotopes, considering their relative abundances. For chlorine, it is approximately 35.47 u.

Question 23

What are the three isotopes of hydrogen, and what are their respective masses?

Answer 23

The three isotopes of hydrogen are protium (1.0078 u), deuterium (2.0141 u), and tritium (3.0160 u).

Question 24

Describe the composition of the nucleus in terms of protons, neutrons, and atomic number (Z).

Answer 24

The nucleus contains protons and neutrons, where the number of protons is equal to the atomic number (Z), and the mass number (A) is the sum of protons and neutrons.

Question 25

Explain the discovery of the neutron and its significance.

Answer 25

The neutron was discovered by James Chadwick in 1932. Its existence was confirmed through experiments involving the bombardment of beryllium nuclei with alpha particles, leading to the emission of neutral radiation. Chadwick’s discovery was significant as it provided insight into the composition of atomic nuclei.

Question 26

What is the mass of a neutron, and why is it considered stable inside the nucleus?

Answer 26

The mass of a neutron is approximately 1.00866 u or 1.6749×10 −27kg. Neutrons are considered stable inside the nucleus due to the strong nuclear force that binds them with protons.

Question 27

Define atomic number (Z), neutron number (N), and mass number (A), and explain their significance in describing nuclear species.

Answer 27

Atomic number (Z) represents the number of protons, neutron number (N) represents the number of neutrons, and mass number (A) is the sum of protons and neutrons. These parameters help in identifying different isotopes and nuclides of an element.

Question 28

Who was the pioneer credited with postulating the existence of the atomic nucleus?

Answer 28

Rutherford.

Question 29

What experiment did Geiger and Marsden perform at Rutherford’s suggestion?

Answer 29

The scattering of alpha particles from thin gold foils.

Question 30

What was the distance of closest approach to a gold nucleus for an alpha particle with a kinetic energy of 5.5 MeV?

Answer 30

Approximately 4.0 × 10^(-14) m.

Question 31

How did Rutherford explain the scattering of alpha particles by the gold sheet?

Answer 31

By assuming that the coulomb repulsive force was solely responsible for scattering.

Question 32

Why did Rutherford conclude that the actual size of the nucleus had to be less than 4.0 × 10^(-14) m?

Answer 32

Because the positive charge is confined to the nucleus.

Question 33

What effect did higher energy alpha particles have on the distance of closest approach to the gold nucleus?

Answer 33

It decreased the distance of closest approach.

Question 34

At what point did scattering begin to be affected by the short-range nuclear forces?

Answer 34

At some point beyond using alpha particles with 5.5 MeV kinetic energy.

Question 35

How were the sizes of nuclei of various elements accurately measured?

Answer 35

By performing scattering experiments with fast electrons as projectiles.

Question 36

What is the formula for calculating the radius R of a nucleus of mass number A?

Answer 36

R = RoA^1/3, where Ro = 1.2 x 10^-15m.

Question 37

What does the formula for the radius of a nucleus suggest about the volume of the nucleus?

Answer 37

It suggests that the volume of the nucleus is proportional to A.

Question 38

What is the density of nuclear matter, and how does it compare to the density of ordinary matter like water?

Answer 38

The density of nuclear matter is approximately 2.3 x 10^17 kg/m^3, much larger than the density of water (about 10^3kg/m^3).

Question 39

Why is the density of nuclear matter so much greater than that of ordinary matter?

Answer 39

Because most of the atom is empty space, but the nucleus is extremely dense.

Question 40

What did Einstein’s theory of special relativity propose regarding mass and energy?

Answer 40

Einstein’s theory of special relativity proposed that mass is another form of energy, and both are interchangeable.

Question 41

What is the mass-energy equivalence relation given by Einstein’s famous equation?

Answer 41

The mass-energy equivalence relation is given by the equation E = mc^2, where E represents energy, m represents mass, and c represents the velocity of light in a vacuum.

Question 42

How is the concept of mass-energy equivalence experimentally verified?

Answer 42

Experimental verification of mass-energy equivalence has been achieved in the study of nuclear reactions among nucleons, nuclei, electrons, and other particles.

Question 43

What is the significance of the conservation law of energy in understanding nuclear reactions?

Answer 43

The conservation law of energy states that the initial energy and the final energy are equal in a reaction, considering the energy associated with mass. This concept is crucial in understanding nuclear masses and interactions between nuclei.

Question 44

What is the definition of mass defect?

Answer 44

Mass defect is the difference between the mass of a nucleus and the total mass of its individual protons, neutrons, and electrons.

Question 45

How is the mass defect related to Einstein’s equivalence of mass and energy?

Answer 45

The mass defect arises because the mass of a nucleus is less than the sum of the masses of its constituents, which corresponds to a decrease in energy according to Einstein’s equivalence of mass and energy.

Question 46

Define binding energy in the context of nuclear physics.

Answer 46

Binding energy is the energy released when nucleons (neutrons and protons) come together to form a nucleus, or the energy required to break a nucleus into its constituent nucleons.

Question 47

What is the significance of binding energy per nucleon?

Answer 47

Binding energy per nucleon is a measure of the stability of a nucleus. It indicates the average energy required to separate a nucleus into its individual nucleons.

Question 48

Describe the relationship between binding energy per nucleon and mass number.

Answer 48

The binding energy per nucleon is practically constant for nuclei of middle mass numbers (30 < A < 170), indicating a strong attractive force between nucleons.

Question 49

What conclusions can be drawn from the constancy of binding energy per nucleon within the range of 30 < A < 170?

Answer 49

The constancy of binding energy per nucleon suggests a strong, short-ranged nuclear force, indicating that nucleons interact mainly with their nearest neighbors within the range of the nuclear force.

Question 50

What force determines the motion of atomic electrons?

Answer 50

The coulomb force

Question 51

What is the approximate binding energy per nucleon for average mass nuclei?

Answer 51

Approximately 8MeV

Question 52

What must be present to bind a nucleus together?

Answer 52

A strong attractive force, distinct from the Coulomb force, capable of overcoming the repulsion between protons and binding both protons and neutrons into the nuclear volume.

Question 53

What is significant about the constancy of binding energy per nucleon?

Answer 53

It can be understood in terms of its short-range, indicating the presence of a strong nuclear binding force.

Question 54

How does the strength of the nuclear force compare to the Coulomb force and gravitational force?

Answer 54

The nuclear force is much stronger than the Coulomb force and significantly stronger than the gravitational force.

Question 55

What happens to the nuclear force between two nucleons as their distance increases beyond a few femtometres?

Answer 55

It rapidly falls to zero.

Question 56

What is the significance of the saturation of forces in a medium or large-sized nucleus?

Answer 56

It leads to the constancy of binding energy per nucleon.

Question 57

At what distance does the potential energy between two nucleons reach a minimum?

Answer 57

At a distance of approximately 0.8 femtometres (0.8 fm).

Question 58

What is the nature of the nuclear force between nucleons of different types?

Answer 58

The nuclear force between neutron-neutron, proton-neutron, and proton-proton is approximately the same, and it does not depend on electric charge.

Question 59

Is there a simple mathematical form of the nuclear force?

Answer 59

No, unlike Coulomb’s law or Newton’s law of gravitation, there is no simple mathematical form of the nuclear force.

Question 60

How did A. H. Becquerel discover radioactivity in 1896?

Answer 60

A. H. Becquerel discovered radioactivity while studying the fluorescence and phosphorescence of compounds irradiated with visible light. He observed that pieces of uranium-potassium sulphate, illuminated with visible light and then wrapped in black paper, emitted something that could penetrate both the paper and a piece of silver, leading to blackening of a photographic plate after several hours of exposure.

Question 61

What phenomenon did Becquerel observe when he exposed pieces of uranium-potassium sulphate to visible light and then wrapped them in black paper?

Answer 61

Becquerel observed that the compound emitted something that could penetrate both black paper and a piece of silver, leading to blackening of a photographic plate after several hours of exposure.

Question 62

How did experiments subsequent to Becquerel’s discovery reveal the nature of radioactivity?

Answer 62

Experiments subsequent to Becquerel’s discovery showed that radioactivity was a nuclear phenomenon involving the decay of an unstable nucleus. This phenomenon is known as radioactive decay.

Question 63

What are the three types of radioactive decay that occur in nature?

Answer 63

The three types of radioactive decay that occur in nature are:
(i) α-decay, in which a helium nucleus (4
2He) is emitted;
(ii) β-decay, in which electrons or positrons are emitted;
(iii) γ-decay, in which high-energy photons are emitted.

Question 64

What is α-decay in radioactivity?

Answer 64

α-decay is a type of radioactive decay in which a helium nucleus (4
2He) is emitted.

Question 65

Describe β-decay in radioactivity.

Answer 65

β-decay is a type of radioactive decay in which electrons or positrons (particles with the same mass as electrons but with a charge exactly opposite to that of electrons) are emitted.

Question 66

What is γ-decay in radioactivity?

Answer 66

γ-decay is a type of radioactive decay in which high-energy photons are emitted, typically with energies of hundreds of keV or more.

Question 67

What is the significance of the flat region in the curve of binding energy per nucleon (Ebn) between A = 30 and A = 170, as shown in Fig. 13.1?

Answer 67

The binding energy per nucleon remains nearly constant (around 8.0 MeV) in this region, indicating greater stability for nuclei within this mass range.

Question 68

Describe the relationship between binding energy and total mass in a bound system like a nucleus.

Answer 68

Higher binding energy results in less total mass for a bound system like a nucleus.

Question 69

What is the significance of the energy release in nuclear reactions compared to exothermic chemical reactions?

Answer 69

Nuclear reactions release energy in the order of MeV, whereas exothermic chemical reactions release energy in the range of electron volts. Nuclear sources produce significantly more energy for the same quantity of matter compared to chemical sources.

Question 70

Explain the process and energy release in nuclear fission.

Answer 70

Nuclear fission occurs when a heavy nucleus, such as uranium-235, is bombarded with a neutron, splitting into intermediate mass fragments and releasing energy. The energy released per fissioning nucleus is approximately 200 MeV.

Question 71

What are the key features of nuclear fusion reactions in stars?

Answer 71

Nuclear fusion reactions in stars involve the fusion of light nuclei to form larger, more tightly bound nuclei, releasing energy. Fusion reactions are the source of energy output in stars’ interiors.

Question 72

How does the temperature affect nuclear fusion in stars?

Answer 72

Thermonuclear fusion in stars occurs when particles have enough kinetic energy to overcome the coulomb repulsion barrier. The estimated temperature required for fusion is around 3 × 10^9 K.

Question 73

Describe the proton-proton cycle in the sun and its significance.

Answer 73

The proton-proton cycle in the sun involves hydrogen burning into helium through a series of reactions, releasing energy. This cycle sustains the sun’s energy output.

Question 74

What is the potential outcome when the hydrogen in a star’s core gets depleted?

Answer 74

As the hydrogen in a star’s core depletes, the core temperature increases, leading to the fusion of helium nuclei into heavier elements like carbon. This process generates higher mass number elements through fusion.

Question 75

Explain the concept and aim of controlled thermonuclear fusion.

Answer 75

Controlled thermonuclear fusion aims to replicate the natural fusion process in stars to generate steady power. By heating nuclear fuel to temperatures around 10^8 K and confining the resulting plasma, fusion reactors aim to supply almost unlimited power to humanity.