WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Concept Of Atom

Chapter 4 Matter Concept Of Atom Synopsis

Subatomic Particles Of An Atom: Electron, proton, and neutron are the three main subatomic particles of an atom.

Cathode Rays: A strong potential difference (10000 V) is applied between two electrodes present inside an electric discharge tube containing a gas at a very low pressure (0.01 mm Hg).

As a result, the cathode emits certain invisible ray, known as cathode ray which produces a faint greenish fluorescence on the glass wall opposite to that of the cathode.

Discovery Of Electrons: Considering various properties of cathode rays, J. J. Thomson concluded (in 1897) that cathode rays are composed of negatively charged material particles and named them negatrons. Later these particles were named electrons.

Anode Rays: When perforated cathode is used inside the discharge tube, a positively charged ray is emitted from the anode which moves towards the cathode. This is known as anode ray.

Discovery Of Protons: When hydrogen gas was taken in the discharge tube, a positively charged particle was obtained during the anode ray experiment. This particle is known as proton.

Mass Of Fundamental Particles Of An Atom:

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The mass of an electron is 9.11 x 10-13kg.

A proton is 1836 times heavier than an electron while a neutron is 1839 times heavier than an electron.

Charge of fundamental particles of an atom: The charge of an electron is -1.6 x 10-19C.

The magnitude of charge of a proton is equal to that of an electron but is opposite in nature. Neutrons do not have any charge. Electrons are negatively charged whereas protons are positively charged particles.

Chapter 4 Matter Concept Of Atom Short And Long Answer Type Questions

Question 1. How is cathode ray produced in an electric discharge tube?

Answer:

The Cathode Ray Produced In An Electric Discharge Tube As Follows:-

When gas inside the discharge tube is kept at a very low pressure (about 0.01 mm Hg pressure) and a very high potential difference (10000 V) is applied between two metallic electrodes of the discharge tube, an invisible ray is produced at the cathode which then moves towards the anode. This ray is called the cathode ray.

It creates fluorescence on the glass wall opposite to that of the cathode in the discharge tube.

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Concept Of Atom Formation Of Anode Rays

Question 2. Why is the charge of an electron considered to be the smallest unit of electricity?

Answer:

The Charge Of An Electron Considered To Be The Smallest Unit Of Electricity Because:-

Scientist Millikan was the first to determine the charge of an electron as -1.602 x 10-19 coulomb or -4.8 x 10-10 esu. However, no negatively charged particle with a lower charge value than an electron has been discovered till date. Thus, the charge of an electron is considered to be the smallest unit of electricity.

The magnitude of charge of any positively or negatively charged particle is thus considered to be equal to or an integral multiple of the charge of an electron.

Question 3. How is anode ray produced in an electric discharge tube?

Answer:

An Anode Ray Produced In An Electric Discharge Tube:-

When gas inside the discharge tube is kept at a very low pressure (about 0.01 mm Hg pressure) and a very high potential difference (10000 V) is applied between two metallic electrodes and if during this process a perforated cathode is used, an invisible ray is produced at the anode which then moves towards the cathode.

This ray is called the anode ray. In this condition, if the anode rays are allowed to pass through the perforated cathode, they produce a reddish glow on the glass wall behind the cathode.

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Concept Of Atom Formation Of Cathode Rays

Question 4. What Is canal ray? Why is it named so?

Answer:

Canal Ray:-

Inside the discharge tube, if a high potential difference is applied between the two electrodes at a very low pressure (about 0.01 mm Hg pressure), an invisible ray consisting of positively charged particles is produced at the anode, moves towards the cathode and passes through the perforated cathode in a straight line. This ray is called the canal ray or anode ray.

The ray is so named because it passes through the perforated cathode.

Question 5. Which observation of Thomson’s discharge tube experiment led to the conclusion that cathode ray is not electromagnetic radiation?

Answer:

In Thomson’s experiment, it was observed that the cathode rays were deflected in the positive direction when an electric field was applied. This proved that cathode ray is not electromagnetic radiation but a stream of negatively charged particles.

If a light paddle wheel is placed in the path of cathode rays, it is seen that the wheel starts rotating. This too proves that cathode ray is a stream of particles having fixed mass.

Question 6. Write four applications of electric discharge through a gas at low pressure.

Answer:

Applications Of Electric Discharge Through A Gas At Low Pressure:-

Electric discharge through a gas at low pressure has the following applications in

  1. Neon-sign advertising boards,
  2. Television tubes,
  3. Fluorescent tube,
  4. Sodium vapour lamp.

Question 7. State three important characteristics of cathode rays.

Answer:

Important Characteristics Of Cathode Rays:-

Three important characteristics of cathode rays are as follows

  1. Cathode rays move in a straight line. If an opaque substance is placed in the path of cathode rays, its shadow is formed. The path of cathode rays remains unaffected by the position of the anode.
  2. Cathode ray is a stream of particles, each having a definite but small mass. So, if a light paddle wheel is placed in the path of the cathode rays, it rotates due to collision with the particles.
  3. Cathode rays consist of negatively charged particles. It gets deflected in presence of magnetic and electric field. In an electric field, cathode rays deflect towards the positive plate while in a magnetic field, cathode rays deflect towards the north pole.

Question 8. The e/m (charge/mass) ratio of cathode ray is the same for all gases taken in an electric discharge tube, but the e/m ratio of anode ray is different for different gases. Why?

Answer:

Cathode ray is a stream of electrons. These electrons are emitted from the metallic cathode when high potential difference is applied between the electrodes in an electric discharge tube containing gases at a very low pressure.

Electron is a fundamental particle present in the atoms of all elements. Hence, the e/m ratio of cathode ray is constant irrespective of the gas being used in the discharge tube.

On the other hand, anode rays are produced when high potential difference is applied in a discharge tube at a very low pressure. Under these circumstances, the gaseous atoms lose electrons and get converted into cations.

Anode ray is the stream of these cations. As the atomic mass is different for different gases, the mass of anode ray particles varies when different gases are used in the discharge tube. Hence, the e/m ratio is different for different gases.

Question 9. How was the existence of protons detected in anode rays?

Answer:

Existence Of Protons Detected In Anode Rays:-

The nature of deflection of anode rays in presence of electric and magnetic fields indicate that it is a collection of positively charged particles (it deviates towards the negatively charged plate).

Again, it is observed by using different gases in the electric discharge tube that e/m ratio of the anode ray is different for different gases. When hydrogen gas is used, the mass of the positively charged particle obtained in the electric discharge tube is found to be the minimum and the charge of each particle is equal to that of an electron.

From this, it was concluded that the particles present in anode ray in the presence of hydrogen gas are similar to H+ ion. This led to the discovery of proton in anode rays by using naturally occurring hydrogen gas.

Question 10. Which incidents led to the following conclusions about anode rays

  1. It moves in a straight line,
  2. It is a stream of particles having definite mass,
  3. It is a stream of positively charged particles.

Answer:

Conclusions About Anode Rays:-

  1. If an opaque substance is placed in the path of anode rays, its shadow is formed. This proves that anode rays move in a straight line.
  2. If a light paddle wheel is placed in the path of anode rays, it rotates due to the collision with the particles. It indicates that anode ray is a stream of particles with a definite mass.
  3. Anode ray is deflected by electric or magnetic field. In an electric field it is deflected towards the negative plate. Thus, it can be said that, it is a stream of positively charged particles.

Question 11. Mention two differences between cathode ray and anode ray.

Answer:

The Differences Between Cathode Ray And Anode Ray Are As Follows:-

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Concept Of Atom Two Differences Between Cathode Ray And Anode Ray

Question 12. Describe the plum pudding model or watermelon model of an atom.

Answer:

Watermelon Model Of An Atom:-

  1. In 1898, Sir J. J. Thomson proposed the plumpudding model or watermelon model for describing the structure of an atom. According to this model—
  2. An atom is a positively charged uniform sphere.
  3. The electrons are evenly embedded into it to neutralise the positive charge. These are just like the seeds embedded in a watermelon or the plums placed inside a pudding.
  4. The electrons are distributed throughout the sphere in such a manner that the total positive and negative charges are neutralised by each other properly. Hence, the structure is neutral.
  5. The mass of the atom is uniformly distributed throughout the atom.

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Concept Of Atom Watermelon Model Of An Atom

Question 13. Mention the similarities between a proton and an electron.

Answer:

Similarities Between A Proton And An Electron:-

The similarities between protons and electrons are as follows

  1. Both are stable fundamental particles of an atom.
  2. Both are charged particles and the magnitude of charge for both the particles is same, 1.602 x 10-19 C or 4.8 x 10-10 esu.
  3. The number of protons and electrons are equal in an atom.
  4. Both protons and electrons are deflected by electric and magnetic fields.

Question 14. Between proton and electron, which particle has a higher e/m value and why?

Answer:

e/m Value Of An Electron Is Greater Than That Of A Proton:-

The e/m (charge/mass) value of a proton is 9.58 x 104 C • g-1 and that of an electron is 1.76 x 108 C • g-1. Thus, e/m value of an electron is greater than that of a proton.

Although the magnitude of charge of both electron and proton are equal but opposite in nature, mass of an electron (9.11 x 10-28g) is 1/1836 times that of a proton. So, e/m value of an electron is greater than that of a proton.

Question 15. Write the differences between electrons and protons.

Answer:

The Differences Between Electrons And Protons Are As Follows:-

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Concept Of Atom Two Differences Between Electrons And Protons

Question 16. Mass of a hydrogen atom is equal to the mass of a proton—explain.

Answer:

Mass Of A Hydrogen Atom Is Equal To The Mass Of A Proton:-

Ordinary hydrogen contains one proton at the nucleus and one electron in the extra-nuclear orbit. Mass of an electron is 1/1836 part to the mass of a proton. That’s why only the mass of proton is taken into account while expressing the mass of a hydrogen atom.

As the mass of an electron is not taken into account, an error takes place during this calculation but the extent of error is negligible. Hence the statement ‘Mass of a hydrogen atom is equal to the mass of a proton’ is justified.

Chapter 4 Matter Concept Of Atom Very Short Answer Type Questions Choose The Correct Answer

Question 1. The constituent particle of cathode rays is

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

Answer: 1. Electron

Question 2. Which of the following is not a fundamental particle of matter?

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

Answer: 3. Positron

Question 3. Negatron is also known as

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

Answer: 1. Electron

Question 4. e/m ratio of cathode rays is

  1. 2.276 x 1011 C • kg-1
  2. 3.76 x 1011 C • kg-1
  3. 1.76 x 1011 C • kg-1
  4. 4.276 x 1011 C • kg-1

Answer: 3. 1.76 x 1011 C • kg-1

Question 5. The heaviest particle of an atom is

  1. Neutron
  2. Proton
  3. Electron
  4. None of these

Answer: 1. Neutron

Question 6. Speed of anode rays

  1. Is higher than that of cathode rays
  2. Is much less than that of cathode rays
  3. Is equal to that of cathode rays
  4. Cannot be determined

Answer: 2. Is much less than that of cathode rays

Question 7. Canal ray is commonly known as

  1. Cathode ray
  2. Anode ray
  3. X-ray
  4. UV-ray

Answer: 2. Anode ray

Question 8. Cathode ray consists of

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

Answer: 1. Electron

Question 9. Which one is not electromagnetic radiation?

  1. Cathode ray
  2. X-ray
  3. Radio wave
  4. UV-ray

Answer: 1. Cathode ray

Question 10. Thomson’s atomic model is known as

  1. Plum pudding model
  2. Proton-electron model
  3. Plum cake model
  4. Pomegranate model

Answer: 1. Plum pudding model

Question 11. Cathode ray is deflected by electric field as well as by magnetic field as it is

  1. Electromagnetic radiation
  2. Stream of positively charged particles
  3. Stream of neutral particles
  4. Stream of negatively charged particles

Answer: 4. Stream of negatively charged particles

Question 12. Nature of charge of an electron is

  1. Neutral
  2. Negative
  3. Positive
  4. None of the above

Answer: 2. Negative

Question 13. A proton is

  1. 1836 times heavier than an electron
  2. 1746 times heavier than an electron
  3. Lighter than an electron
  4. 1236 times heavier than an electron

Answer: 1. 1836 times heavier than an electron

Question 14. The particle formed when an electron is removed from a hydrogen atom

  1. Proton
  2. Neutron
  3. α – particle
  4. β – particle

Answer: 1. Proton

Chapter 4 Matter Concept Of Atom Answer In Brief

Question 1. At what pressure are cathode rays produced in an electric discharge tube?

Answer: In an electric discharge tube, cathode rays are produced at a very low pressure of about 0.01 mm of Hg.

Question 2. In which direction will the cathode rays deflect in the presence of an electric field?

Answer: In presence of an electric field, the cathode rays will be deflected towards the positive plate.

Question 3. What Is the magnitude of e/m (charge/mass) of a neutron?

Answer: The magnitude of e/m of a neutron is zero.

Question 4. Why is cathode ray formed at very low pressure only?

Answer: At a very low pressure the resistance inside the discharge tube is less, hence the formation of cathode ray becomes feasible.

Question 5. What happens when cathode rays strike the surface of various hard metals?

Answer: Cathode rays produce X-rays when they strike the surface of various hard metals like tungsten, molybdenum etc.

Question 6. Write down the similarity between cathode rays and ordinary light.

Answer: Just like ordinary light, cathode rays also affect photographic plates.

Question 7. Who discovered electrons?

Answer: J. J. Thomson, in 1897, discovered electrons.

Question 8. Who named the negatively charged particles, present in cathode rays, electrons?

Answer: Scientist G.J. Stoney.

Question 9. Which scientist is considered as the ‘father of atomic physics’?

Answer: John Dalton.

Question 10. Who determined the charge of an electron by oil drop experiment?

Answer: Scientist Robert A. Millikan.

Question 11. Who performed the discharge tube experiment using a perforated cathode?

Answer: Scientist E. Goldstein.

Question 12. Who discovered protons?

Answer: Scientist Ernest Rutherford (in 1911).

Question 13. Who named the positively charged particles, found in anode rays, protons?

Answer: Scientist E. Rutherford .

Question 14. Who found a method to determine the number of protons in an atom?

Answer: Scientist Moseley (in 1913).

Question 15. Who proposed the watermelon model or plum pudding model for an atom?

Answer: Scientist J. J. Thomson.

Question 16. What is the mass of an electron in SI unit?

Answer: The mass of an electron is 9.1 x 10-31 kg.

Question 17. What is the mass of a proton?

Answer: The mass of a proton is 1.6725 x 10-27 kg.

Question 18. What is the charge of a proton?

Answer: In SI unit, the charge of a proton is + 1.602 x 10-16 C and in CGS unit it is +4.8 x 10-10 esu.

Question 19. What is the charge of an electron?

Answer: In SI unit, the charge of an electron is -1.602 x 10-19 C and in CGS unit it is -4.8 x 10-10 esu.

Question 20. For which ray e/m ratio is not constant?

Answer: e/m ratio is not constant for anode ray.

Chapter 4 Matter Concept Of Atom Fill In The Blanks

Question 1. Cathode ray is a stream of _____

Answer: Electrons

Question 2. Positive ray is commonly known as _______ ray.

Answer: Anode

Question 3. The nucleus of an atom does not consist of _______

Answer: Electrons

Question 4. Cathode rays are emitted _______ from the cathode surface and travel towards the _______

Answer: Perpendicularly, anode

Question 5. The watermelon model for the structure of an atom was proposed by _________

Answer: J.J Thomson

Question 6. ________ is produced when cathode rays hit the walls of a discharge tube or fall on the surface of substances like ______

Answer: Fluorescence, Zinc sulphide

Question 7. When cathode rays hit the surface of metals like tungsten or molybdenum, ______ rays are produced.

Answer: X

Question 8. During production of cathode rays, the pressure in the discharge tube is equal to the pressure of ________ mercury column.

Answer: 0.01

Question 9. Speed of anode rays is much ________ than the speed of cathode rays.

Answer: Less

Question 10. A proton is ________ times than an electron.

Answer: 1836, Heavier

Chapter 4 Matter Concept Of Atom State Whether True Or False

Question 1. Mass of a hydrogen atom is almost same to that of a proton.

Answer: True

Question 2. Cathode ray is streamed from anode to cathode.

Answer: False

Question 3. First discovered subatomic particle is proton.

Answer: False

Question 4. Mass of a hydrogen atom is part to the mass of an electron.

Answer: False

Question 5. Watermelon model of an atom is also known as the plum pudding model.

Answer: True

Question 6. An electron is 1836 times heavier than a proton.

Answer: False

Question 7. The e/m ratio of cathode ray depends upon the gas used in the discharge tube.

Answer: False

Question 8. The number of protons present influences the mass of an atom.

Answer: True

Question 9. Thomson named anode ray as positive ray.

Answer: True

Question 10. The plum pudding model of Thomson gives the concept of nucleus in an atom.

Answer: False

Question 11. Cathode ray is emitted perpendicularly from the cathode plane.

Answer: True

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