WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Synopsis

Outcomes of Rutherford’s Alpha-Scattering Experiment:

Based on his famous alpha scattering experiment, in 1911, Ernest Rutherford first proposed the idea of nuclear atomic model. It was the first satisfactorily proposed atomic model, even though it failed to explain the stability of an atom.

Rutherford’s Atomic Model: 

1. All atoms contain a tiny but massive positively charged nucleus at their centre.
2. The size of the nucleus is very small compared to that of the atom as most of the space inside any atom is found to be empty.
3. The entire positive charge and almost whole of the mass of an atom is concentrated at its nucleus.
4. Negatively charged electrons revolve around the nucleus in circular paths present at the extranuclear part.
5. Total number of protons present in the nucleus is equal to the total number of electrons present in the extranuclear part of an atom.
6. The electrostatic force of attraction between the electrons and the nucleus is balanced by the centrifugal force of the revolving electrons. This prevents the electrons from falling into the nucleus.

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Limitations Of Rutherford’s Atomic Model:

1. It is unable to explain the stability of an atom.
2. This model fails to explain the discontinuous nature of atomic spectra.

Discovery Of Neutron:

In 1932, scientist James Chadwick discovered neutrons, the neutral particle present in an atom, by bombarding beryllium (₄Be⁹) metal with fast-moving alpha (α) particles.

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Short And Long Answer Type Questions

Question 1. On the basis of Rutherford’s α – scattering experiment, answer the given questions

1. What is an α – particle?
2. Why was gold foil used in α scattering experiment?
3. What arrangement was made to detect the path of α -particles?

Answer:

1. An α -particle is a particle having a mass of 4 units and 2 units of positive charge. It consists of 2 protons and 2 neutrons. As these are identical with helium nuclei, α – particles are often represented as ₂He²⁺. During the disintegration of the nuclei of many radioisotopes, emission of α – particles take place.
2. Thinner the metal foil used, higher will be the probability of α-particles to collide with the atoms of the metal foil. As gold is a soft and highly malleable metal, it can be easily converted into a very thin foil. Due to this Rutherford used gold foil in his experiment.
3. A circular fluorescent screen coated with zinc sulphide (ZnS) was set up around the metal foil. After penetrating through the metal foil, the α-particles hit the screen generating a flash of light.
4. This helped in detecting the path of scattered α-particles.

 

 

Question 2. State the observations made by Rutherford in his a -scattering experiment.

Answer:

Observations Made By Rutherford In His A -Scattering Experiment:-

Rutherford made the following observations in his α – scattering experiment

1. Most of the α – particles passed straight through the gold foil and hit the zinc sulphide (ZnS) screen Without suffering any deviation.
2. A few α-particles passed through the foil after suffering small deviations.
3. Very few α-particles (1 out of 20000) suffered deviation through large angles or even deflected by an angle of 180°.

 

 

Question 3. What were the conclusions drawn by Rutherford from his α-scattering experiment?

Answer:

The Conclusions Drawn By Rutherford From His Α-Scattering Experiment:-

From the observations of his α-scattering experiment, Rutherford drew the following conclusions

1. As metal foil is made up of metal atoms and most of the α-particles pass through the gold foil without any deviation, major part of an atom is empty.
2. α-particles are positively charged. So, they can be deflected only by another positive charge having greater mass than themselves. Hence, it was concluded that all the positive charge and mass of an atom is concentrated within a very small region in the atom. This is the nucleus of an atom.
3. Only those α-particles which hit the nucleus suffer deviation by large angles (90° or more) or even deflected back at an angle of 180°.
4. Negatively charged electrons remain outside the nucleus. As α-particles are much heavier than electrons, their paths are not affected by the presence of electrons in an atom.

Question 4. Describe Rutherford’s atomic model.

Answer:

Rutherford’s Atomic Model

Based on the conclusions of his α-scattering experiment, in 1911, Rutherford proposed an atomic model known as Rutherford’s atomic model. The postulates of this model are as follows

1. An atom has two parts—a positively charged nucleus and the extra nuclear part where the negatively charged electrons are present.
2. Entire mass and positive charge of an atom is concentrated within a very small region in the atom. This is known as the nucleus. Most of the space in an atom is empty.
3. The nucleus is very small in size with respect to an atom. The diameter of an atom is around 10^-8 cm while that of a nucleus is 10^-13 cm.
4. The electrons revolve around the nucleus in different circular orbits.
5. Total number of electrons revolving around the nucleus is equal to the number of positively .charged protons present within the nucleus.
6. Two opposite forces act on an electron moving along an orbit—the electrostatic force of attraction between negatively charged electrons and positively charged nucleus and the centrifugal force acting on the electrons due to its rotational motion which acts away from the nucleus.
7. These two forces being equal in magnitude but opposite in direction, maintain the stability of the electrons in the orbit.

Question 5. What is radioactivity? How did the proposing his atomic model?

Answer:

Radioactivity:-

Radioactivity is a nuclear phenomenon in which the nuclei of heavier elements, like uranium, thorium etc., in elemental state or in compound form, disintegrate spontaneously to form nuclei with greater stability [i.e., new elements are formed) and invisible rays are emitted (radioactive radiation) during this process.

From the results of electric discharge through gases at low pressure and the discovery of atom it was evident that atoms can be divided into smaller particles. After the discovery of radioactivity, Rutherford performed his famous q -scattering experiment on the basis of which he proposed his atomic model.

Question 6. Mention the merits of Rutherford’s atomic model.

Answer:

Merits Of Rutherford’s Atomic Model Are:-

1. The existence of a nucleus is first observed in this model. The concept that almost the entire mass of an atom is concentrated at its nucleus is accepted in future days.
2. The concept of revolving of electrons in several orbits around the nucleus first known from this atomic model.
3. The total negative (-ve) charge of revolving electrons is numerically equal to the total positive (+ve) charge of the nucleus—from this concept neutrality of an atom can be justified.

Question 7. Discuss the limitations of Rutherford’s atomic model.

Answer:

Limitations Of Rutherford’s Atomic Model:-

There were some limitations of Rutherford’s atomic model. These are discussed below

According to Maxwell’s theory of electromagnetic radiation, if a negatively charged electron revolves around the positively charged nucleus, then it should continuously emit radiation. Due to this, energy of the electron would decrease in every turp and it would move closer to the nucleus in a spiral pathway.

Ultimately the electron will fall into the nucleus, making the atom unstable. However, atoms are found to be quite stable. Thus, Rutherford’s model failed to explain the stability of an atom.

 

 

If electrons lose energy continuously, then the respective atomic spectra should be continuous in nature. However, atomic spectra are found to be discontinuous with well-defined lines (line spectrum or discontinuous spectrum). Rutherford’s theory failed to explain the formation of line spectra by atoms.

 

 

It gives no information about the velocity and energy of electrons as well as the electronic configuration of atoms.

Question 8. Why is an atom electrically neutral?

Answer:

An Atom Electrically Neutral Because:-

An atom contains equal number of protons and electrons. Now, the charge of a proton is equal in magnitude but opposite in nature with respect to the charge of an electron.

Hence, the total positive charge in the nucleus of an atom is equal to the total negative charge of electrons revolving in orbits. Hence, an atom is electrically neutral.

Question 9. The electrons outside the nucleus are negatively charged and the protons in the nucleus are positively charged. Then why do the electrons not fall into the nucleus due to its attraction?

Answer:

According to Rutherford’s atomic model, the electrons revolve around the nucleus in different circular orbits with high velocity.

While rotating in an orbit, the centrifugal force acting on an electron is equal in magnitude to the electrostatic force of attraction acting between the negatively charged electron and the positively charged nucleus.

However, these two forces act in opposite directions. Hence, the resultant force acting on an electron is zero and the electron maintains its stability in the orbit and does not fall into the nucleus.

 

Question 10. How did Rutherford conclude that an atom contains equal number of protons and electrons?

Answer:

The presence of negatively charged electrons and positively charged protons in an atom had been proved experimentally. Also, a proton and an electron have equal but opposite charges. As an atom is electrically neutral, Rutherford concluded that an atom should contain equal number of protons and electrons.

Question 11. Which discovery became possible from the findings of α-particle scattering experiment?

Answer:

From α-particle scattering experiment it was proved that, all the positive charge and mass of an atom is concentrated in a very small region within the atom. This small region is known as nucleus.

Thus, discovery of nucleus became possible due to the α-particle scattering experiment.

Question 12. What would have happened if  Rutherford had used a foil of a tighter metal than gold foil in his α-scattering experiment?

Answer:

The relative mass of an α-particle is 4 units. In the α-scattering experiment, instead of gold foil, if a foil of any other lighter metal was used, then the α-particle might have passed through the foil without deflection. As A result, scattering of α-particles would not have taken place.

Question 13. Which observation led to the discovery of neutrons in an atom?

Answer:

Observation Led To The Discovery Of Neutrons In An Atom:-

Except ordinary hydrogen, the masses of all other atoms were found to be heavier than the total mass of electrons and protons in the atoms. This observation forced scientists to think about the presence of any other neutral particle having fixed mass in the atom. This ultimately led to the discovery of neutrons in an atom.

Question 14. Can an atom possess more number of protons than neutron?

Answer:

Except ordinary hydrogen, all other stable isotopes of elements have either equal or greater number of neutrons than protons in the nucleus. Neutrons present in the nucleus decrease the repulsion between protons.

If number of protons exceeds the number of neutrons in the nucleus, then repulsion between them will increase and the nucleus will become unstable. So, none of the stable isotopes have greater number of protons than neutrons. However there are certain exceptions.

For instance, in ordinary hydrogen (₁H¹), there is only 1 proton but no neutron. In an isotope of helium (₂He³), there are 2 protons but 1 neutron.

Question 15. Mention the similarities between a proton and a neutron.

Answer:

The similarities between a proton and a neutron are as follows

1. Both are stable fundamental particles of an atom.
2. Both are present in the nucleus of an atom.
3. The diameter of both the particles is about 2.4 x 10^-13 cm.
4. Both protons and neutrons are much heavier than electrons.
5. The mass of a nucleus depends on the number

Question 16. Write the differences between protons and neurons.

Answer:

The differences between protons and neutrons are as follows

Question 17. Write down the differences between electrons and neutrons.

Answer:

The differences between electrons and neutrons are as follows

Question 18. Mention the mass and charge of electron, proton and neutron.

Answer:

The mass and charge of the three fundamental particles of an atom, i.e., electron, proton and neutron is given below

Question 19. Why is the nucleus of an atom positively charged and the heaviest part of the atom?

Answer:

Nucleus contains positively charged protons and neutral particles neutrons. Due to this reason, nucleus of an atom is positively charged. On the other hand, as electron is the lightest particle of an atom, the total mass of protons and neutrons present in the nucleus is the main contributory factor to the entire mass of an atom.

Neutron is resides inside the nucleus. Presence of neutron inside the nucleus makes nucleus the heaviest part of the atom.

Question 20. Which problem regarding the structure Of an atom was solved by the discovery of neutrons?

Answer:

Rutherford observed that except ordinary hydrogen, the mass of all other atoms are greater than the combined mass of protons and electrons present in their atoms. On that basis, Rutherford, in 1920, suggested the presence of a neutral particle of unit mass within the nucleus of an atom.

He named the particle neutron. Later, in 1932, Chadwick with the help of a series of experiments proved the presence of neutrons in the nucleus. Thus, with the discovery of neutrons, the problem regarding the mass of an atom could be solved.

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Very Short Answer Type Questions Choose The Correct Answer

Question 1. Which metal plate was used in Rutherford’s α-scattering experiment?

1. Aluminium
2. Gold
3. Silver
4. Zinc

Answer: 2. Gold

Question 2. The existence of which of the following particles of an atom was proved by Rutherford’s α-scattering experiment?

1. Electron
2. Proton
3. Nucleus
4. Neutron

Answer: 3. Nucleus

Question 3. An alpha (α) particle is

1. Positively charged hydrogen ion
2. Unipositive helium ion
3. Dipositive helium ion
4. A chargeless particle

Answer: 3. Dipositive helium ion

Question 4. Which of the following elements is an α particle emitter?

1. Radium
2. Iron
3. Lead
4. Bismuth

Answer: 1. Radium

Question 5. In Rutherford’s experiment, most of the α – particles passed straight through the thin metallic foil without suffering any deflection. The reason is that

1. α-particles are very small in size with respect to electrons
2. α-particles are positively charged
3. Most of the part inside an atom is empty
4. α-particles are slow-moving particles

Answer: 3. Most of the part inside an atom is empty

Question 6. The nucleus of an atom is

1. Positively charged
2. Negatively charged
3. Neutral
4. Partly positive and partly negative

Answer: 1. Positively charged

Question 7. According to Rutherford’s atomic model, the entire mass of an atom,

1. Is uniformly distributed throughout the atom
2. Is concentrated within the nucleus
3. Is distributed outside the nucleus
4. Remains partly within the nucleus and partly outside the nucleus

Answer: 2. Is concentrated within the nucleus

Question 8. According to Rutherford’s atomic model, protons are

1. Present within the nucleus
2. Present outside the nucleus
3. Revolving around the nucleus
4. None of these

Answer: 1. Present within the nucleus

Question 9. Electrons present in an atom

1. Remain static within the nucleus
2. Remain static outside the nucleus
3. Revolve around the nucleus
4. Revolve within the nucleus

Answer: 3. Revolve around the nucleus

Question 10. The inference which cannot be drawn from Rutherford’s α-scattering experiment is that

1. The nucleus is small and heavy
2. The nucleus is always positively charged
3. The size of an atom is almost 10⁵ times that of the nucleus
4. The nucleus was hit by a large number of α-particles

Answer: 4. The nucleus was hit by a large number of α-particles

Question 11. An accelerated charged particle

1. Absorbs energy
2. Emits energy
3. Neither absorbs nor emits energy
4. Initially absorbs energy and then emits it

Answer: 2. Emits energy

Question 12. Which of the following atoms does not have any neutrons in it?

1. Protium
2. Deuterium
3. Tritium
4. Helium

Answer: 1. Protium

Question 13. Neutron is a

1. Positively charged particle
2. Negatively charged particle
3. Neutral particle
4. Either positively or negatively charged particle

Answer: 3. Neutral particle

Question 14. The mass of a neutron is almost equal to the mass of a/an

1. Electron
2. Proton
3. Nucleus
4. Helium atom

Answer: 2. Proton

Question 15. A neutron is heavier than an electron by

1. 1839 times
2. 1836 times
3. 1857 times
4. 1957 times

Answer: 1. 1839 times

Question 16. The heaviest subatomic particle is

1. Neutron
2. Proton
3. Electron
4. Positron

Answer: 1. Neutron

Question 17. \({ }_4^9 \mathrm{Be}+{ }_2^4 \mathrm{He} \rightarrow{ }_0^1 n+?\)

1. \({ }_6^{12} C\)
2. \({ }_6^{14} C\)
3. \({ }_7^{14} \mathrm{~N}\)
4. \({ }_8^{16} 0\)

Answer: 1. \({ }_6^{12} C\)

Question 18. Which one of the following is positively charged particle, emitted from nucleus?

1. α-ray
2. β-ray
3. Electron
4. γ-ray

Answer: 1. α-ray

Question 19. Neutrons are expressed as

1. \({ }_{-1}^0 n\)
2. \({ }0^1 n\)
3. \({ }_0^0 n\)
4. \({ }_1^1 n\)

Answer: 2. \({ }0^1 n\)

Question 20. Ratio of radius of an atom to the radius of the nucleus is

1. 10⁵
2. 10^-5
3. 10^-10
4. 10¹⁰

Answer: 1. 10⁵

Question 21. Which of the following can be detected by Rutherford’s α – scattering experiment?

1. Centre of the atom
2. Outer electronic part
3. Whole atom
4. Neutrons

Answer: 1. Centre of the atom

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Answer In Brief

Question 1. Which particles did Rutherford use for his scattering experiment?

Answer: Alpha (α) particles.

Question 2. Name the different rays that are emitted from radioactive elements.

Answer: Alpha (α) rays, beta (β) rays and gamma, (γ) rays

Question 3. What is the ratio of penetrating powers of α, β and γ-rays?

Answer: The ratio of penetrating powers of α, β and γ-rays is approximately 1:100:1000.

Question 4. What is the nature of γ-ray, obtained as a result of radioactive radiation?

Answer: γ-ray is electromagnetic radiation.

Question 5. Mention nature of the α-particle.

Answer: α-particle is just like helium nucleus bearing 2 units of positive charge and 4 units of mass.

Question 6. What is the approximate measure of the diameter of nucleus of an atom?

Answer: 10^-12 to 10^-13 cm.

Question 7. How is the mass arranged in an atom according to Rutherford’s atomic model?

Answer: According to Rutherford’s atomic model almost all of the mass are centralised at the nucleus.

Question 8. What is the name of the heaviest particle present in an atom?

Answer: Neutron.

Question 9. What is the diameter of an atom?

Answer: The diameter of an atom is approximately 10^-8 cm.

Question 10. Which atom contains only two fundamental subatomic particles?

Answer: It is protium (₁H¹) which contains only two fundamental subatomic particles named proton and electron.

Question 11. Give an example of electromagnetic radiation.

Answer: Visible light.

Question 12. Who coined the term neutron?

Answer: Scientist Rutherford.

Question 13. What is the mass of a neutron particle?

Answer: Mass of a neutron particle is 1.675 x 10^-27 kg.

Question 14. Who discovered neutrons?

Answer: Sir James Chadwick (In 1932).

Question 15. How many spectral lines are present in the visible region of hydrogen spectra?

Answer: The visible region of hydrogen spectra contains four spectral lines—red, bluish-green, blue and violet.

Question 16. What is the nature of atomic spectra?

Answer: Atomic spectra is discontinuous in nature and is basically a line spectra.

Question 17. Name the charge-less or neutral subatomic particle.

Answer: Neutral subatomic particle is neutron.

Question 18. What is the approximate diameter of the nucleus of an atom?

Answer: The diameter of the nucleus of an atom is in the range of 10^-12,-10^-13 cm.

Question 19. Do the electrons remain static within an atom?

Answer: According to Rutherford’s atomic model, electrons revolve around the nucleus in circular paths and thus, do not remain static within an atom.

Question 20. If a charged particle moves with accelerated motion, then what will be the change in its energy?

Answer: If a charged particle moves with accelerated motion, it will continuously emit radiation. Hence, the energy of the particle will gradually decrease.

Question 21. Which metal was bombarded by a- particles during the experiments conducted for the discovery of neutrons?

Answer: Beryllium (Be) metal.

Question 22. What is the approximate diameter of an atom?

Answer: Diameter of an atom is approximately 10⁸ cm.

Question 23. Which atom does not contain any neutron?

Answer: Protium (₁H¹) does not contain any neutron.

Question 24. Which atom contains two protons and two neutrons in the nucleus?

Answer: The helium atom (₂He⁴) contains two protons and two neutrons in the nucleus.

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Fill In The Blanks

Question 1. Most of the part inside an atom is ________

Answer: Empty

Question 2. The positively charged heavy part of an atom is known as the ________

Answer: Nucles

Question 3. Scientist who carried out the a-particle scattering experiment was ________

Answer: Rutherford

Question 4. An electron is attracted towards the nucleus by ________ force of attraction.

Answer: Electrostatic

Question 5. The number of ________ in the nucleus is equal to the number of __________ revolving around the nucleus in different orbits.

Answer: Protons, electrons

Question 6. Rutherford’s atomic model fails to explain the __________ of an atom.

Answer: Stability

Question 7. The mass of a neutron is nearly equal to the mass of a __________

Answer: Proton

Question 8. The charge of a neutron is __________

Answer: Zero

Question 9. For a neutron, e/m value is __________

Answer: Zero

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron State Whether True Or False

Question 1. Size of the nucleus is extremely small compared to the size of an atom.

Answer: True

Question 2. Radioactive elements emit alpha, beta and gamma rays.

Answer: True

Question 3. The chemical properties of an element depend on the number of neutrons present in the nucleus.

Answer: False

Question 4. Gold, being a soft and highly malleable metal, was used as a thin foil in Rutherford’s α-particle scattering experiment.

Answer: True

Question 5. Rutherford’s atomic model gives an idea about the velocity and energy of electrons.

Answer: False

Question 6. α-particle is bi positively charged helium ion.

Answer: True

Question 7. Neutron is absent in deuterium.

Answer: False

Question 8. The lightest fundamental particle in an atom is neutron.

Answer: False

Question 9. Atomic spectra is a discontinuous spectra.

Answer: True

Question 10. β-ray is the stream of positively charged particles.

Answer: False

Question 11. No idea about the radius of orbit of an electron can be obtained from Rutherford’s atomic model.

Answer: True