WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model

Chapter 4 Matter Bohr Rutherfords Atomic Model Synopsis

Rectification Of Rutherford’s Atomic Model:

In 1913, scientist Niels Bohr partly modified the drawbacks of Rutherford’s atomic model. This modified version is known as Bohr’s atomic model or Bohr-Rutherford’s atomic / model, which explains the idea of stationary orbits of electrons.

Basic Postulates of Bohr’s Atomic Model:

  1. The electron in an atom revolves around the nucleus only in certain selected circular paths, called orbits, which are associated with definite energies.
  2. When an electron revolves in such selected orbits, termed as stationary orbits, it neither emits nor absorbs energy.
  3. When an electron jumps from a higher stationary energy level to a lower stationary energy level, it emits a fixed amount of energy in the form of radiations. If an electron absorbs a certain amount of energy, it moves to a higher energy level.

Limitations Of Bohr’s Atomic Model:

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  1. It fails to explain the spectra of atoms or ions having two or more electrons.
  2. This is a two dimensional model, hence it is unable to provide any idea about the actual three dimensional electronic model of an atom.

Atomic Number: Total number of protons present in the nucleus of the atom of a particular element is called atomic number of that element. It is denoted by ‘T’.

Mass Number: Sum of the number of protons and the number of neutrons present in the nucleus of an atom is called the mass number of that element. It is denoted by ‘A’.

Relation Between Atomic Number And Mass Number Of An Atom: A = Z + N

where Z = atomic number, A = mass number, N = number of neutrons

Nuclide: An atom or a nucleus characterised by a definite atomic number and a definite mass number is called a nuclide. For example, \({ }_{12}^{24} \mathrm{Mg},{ }_6^{12} \mathrm{C},{ }_8^{16} \mathrm{O}\) etc., are different nuclides.

Chapter 4 Matter Bohr Rutherfords Atomic Model Short And Long Answer Type Questions

Question 1. State the major postulates of Bohr- Rutherford atomic model.

Answer:

Postulates Of Bohr- Rutherford Atomic Model :-

The major postulates of the Bohr-Rutherford atomic model are as follows

  1. Electrons do not revolve around the nucleus in any circular orbit. Instead, they revolve only in some selected orbits of definite radii.
  2. While revolving along such an orbit, an electron neither emits nor absorbs any form of energy, i.e., the energy of the electron remains constant. So, these orbits are known as stationary orbits.
  3. An electron absorbs or emits energy only when it jumps from one orbit to another. When an electron jumps from a higher energy orbit to a lower energy orbit, it emits energy. On the other hand, an electron jumps from a lower energy orbit to a higher energy orbit by absorbing energy.

Question 2. What are stationary orbits? Why are they calied so?

Answer:

Stationary Orbits:-

  1. According to Bohr’s atomic model, the circular paths along which electrons revolve around the nucleus are called stationary orbits.
  2. When an electron moves along such an orbit, it neither emits nor absorbs energy and hence the energy of the electron remains fixed. So these orbits are called stationary orbits.

Question 3. State the significance Rutherford’s atomic model.

Answer:

Significance Of Rutherford’s Atomic Model:-

According to Bohr-Rutherford’s atomic model, electrons moving in stationary orbits around the nucleus neither absorb nor emit energy continuously. Hence, the energy of electrons in those orbits remains constant. Thus, the theory successfully explains the stability of an atom which Rutherford’s theory failed to explain.

Question 4. What is meant by the ground state and excited state of an atom?

Answer:

Ground State And Excited State Of An Atom:-

Ground state: At normal condition, the revolving electrons in an atom occupy the lowest energy orbits. This state of an atom is called its ground state.

Excited state: When the electrons absorb energy from the surroundings in the form of heat or light, the electrons jump from lower energy orbits to higher energy orbits. This state of an atom is called its excited state.

Question 5. What is a nuclide? Give example.

Answer:

Nuclide:-

A nuclide is an atom or a nucleus, specified by its atomic number and mass number.

Example: \({ }_{12}^{24} \mathrm{Mg}\) or, \({ }_{12} \mathrm{Mg}^{24}\) is a nuclide of magnesium, \({ }_{17}^{35} \mathrm{Cl}\) is a nuclide of chlorine.

Question 6. Write the differences between mass number and atomic mass of an element.

Answer:

Differences Between Mass Number And Atomic Mass Of An Element:-

Mass number and atomic mass of an element have very close values. However, atomic mass differs from mass number in the following aspects

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model Differences Between Mass Number And Atomic Mass Of Number

Question 7. What is the relation between atomic number and mass number of an atom?

Answer:

Relation Between Atomic Number And Mass Number Of An Atom:-

Mass number (A) of an atom = number of protons (p) + number of neutrons (n).

So, it can be said that, A = p + n

Now, the number of protons in the nucleus of an atom is the atomic number (Z) of the element.

Therefore, Z = p. Hence, A = Z+ n.

Thus, mass number = atomic number + number of neutrons.

or, atomic number = mass number – number of neutrons.

Question 8. State the similarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model.

Answer:

Similarities Between Rutherford’s Atomic Model And Bohr-Rutherford’s Atomic Model:-

Eminent scientist Niels Bohr in 1913 rectified the drawbacks of Rutherford’s atomic model. This corrected model was known as Bohr-Rutherford’s model. The similarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model are as follows

  1. According to both models, the total positive charge and the entire mass of the atom is concentrated in the nucleus present at the center of the atom.
  2. As per both the atomic models, negatively charged electrons revolve around the nucleus in different circular orbits.
  3. Both the models suggested that the major part of an atom is vacant.
  4. Both the atomic models concluded that the electrostatic force of attraction between the negatively charged electrons and positively charged nucleus and the centrifugal force acting upon the electrons due to their rotational motion are equal in magnitude.

Question 9. State the dissimilarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model.

Answer:

Dissimilarities Between Rutherford’s Atomic Model And Bohr-Rutherford’s Atomic Model:-

The dissimilarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model are as follows

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model Differences Rutherford's And Bohr-Rutherford's Atomic Model

Question 10. Write down three differences between mass number and atomic number.

Answer:

Differences Between Mass Number And Atomic Number:-

Three differences between mass number and atomic number are

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model Differences Between Mass And Atomic Number

Question 11. Mention the limitations of Bohr- Rutherford’s atomic model.

Answer:

Limitations Of Bohr-Rutherford’s Atomic Model:-

  1. Spectra of atoms or ions having two or more electrons cannot be explained with the help of Bohr’s theory. This atomic model is applicable only for single electron system like H, He+, Li2+, Be3+, etc.
  2. Bohr-Rutherford’s atomic model fails to give any idea about the actual three-dimensional electronic model of an atom.
  3. Bohr-Rutherford’s atomic model ignored the dual character (i.e., particle and wave nature) of electron while according to de-Broglie (1924) electrons have dual character (wave as well as particle nature).
  4. Bohr-Rutherford’s theory cannot give any explanation for the splitting of spectral lines under the influence of magnetic field or electric field.

Question 12. Why is atomic number of an element considered as its intrinsic property?

Answer:

Atomic number of an element is considered as its intrinsic property for the following reasons

  1. Two elements can never have the same atomic number.
  2. Atomic number of an element = Number of protons; as proton is a fundamental particle, the fundamental characteristics of an element depend on its atomic number.
  3. With the change in atomic number of an element, the nature of the element changes i.e., a new element is formed.
  4. Existence of isotopes has proved that atoms of different mass numbers but same atomic number belong to the same element while existence of isobars has proved that atoms of same mass number but different atomic numbers do not belong to the same element.
  5. Moseley’s experiments on X-rays have also proved that atomic number of an element is its intrinsic property.

Question 13. Complete the following table.

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model

Answer:

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model

Question 14. X2- ion contains 10 electrons and 8 neutrons. Find the atomic number and mass number of the element. What is the name of the element?

Answer:

The number of electrons in the X2- ion is 10.

Here the atom X forms a negative ion. So, the number of electrons accepted by the atom is 2.

Hence, the number of electrons in X-atom = 10 – 2 = 8.

Now in an atom, number of protons = number of electrons.

Therefore, number of protons in X-atom = 8

So, the atomic number of the element X = 8

The number of neutrons in X-atom = 8

Therefore, mass number of the atom = number of protons + number of neutrons = 8 + 8 = 16.

As the atomic number of the element is 8, the element is oxygen.

Question 15. The number of protons and neutrons in the nuclei of two atoms A and B are given below

  1. A: number of protons = 6, number of neutrons = 6
    B: number of protons s 6, number of neutrons = 8.

Find the mass number of A and B. Is there any relation between 4 and S?

Answer:

Mass number of A = number of protons + number of neutrons = 6 + 6 = 12

Mass number of B = number of protons + number of neutrons = 6 + 8 = 14

Number of protons in both the atoms is the same. Hence, the atoms A and B are isotopes of an element.

Question 16. The atomic number of chlorine is 17. What will be the atomic number of Cl ion?

Answer:

The Atomic Number Of Both Cl -atom And CI ion Will Be The Same, i.e., 17

The atomic number of an element is the number of protons present in the nucleus of the atom of that element. When Cl -atom converts to Cl ion, it accepts an electron but the number of protons in the nucleus remains the same. Hence, the atomic number of both Cl -atom and CI ion will be the same, i.e., 17.

Question 17. Can an electron absorb or emit unlimited energy? Explain.

Answer:

No, an electron cannot absorb or emit unlimited energy. It only absorbs that amount of energy which is required to jump to a higher orbit. Similarly, when an electron jumps from a higher orbit to a lower orbit, it releases energy equal to the difference in energies of the two orbits.

WBBSE Solutions for Class 9 Physical Science Chapter 4 Matter Bohr Rutherfords Atomic Model Electron Absorb or Emit Unlimited Energy

Question 18. How does the energy vary when an electron moves from one orbit to another?

Answer:

The Energy Vary When An Electron Moves From One Orbit To Another Is As Follows:-

According to Bohr-Rutherford’s atomic model, when an electron jumps from a higher stationary energy level to a lower one, it emits fixed amount of energy in the form of radiation. On the contrary, when an electron moves to a higher energy level from a lower one, it absorbs certain amount of energy.

Question 19. Mass number and atomic number of a nuclide of an element are 35 and 17 respectively. Calculate the number of charged and neutral particles in its nucleus.

Answer:

Mass number of the nuclide = 35

Atomic number of the nuclide = 17

∴ Number of charged particles (protons) in the nucleus= 17

∴ Number of neutral particles (neutrons) in the nucleus = 35 – 17 = 18

Question 20. Calculate the number of electrons and protons in H+ and H”.

Answer:

Atomic number of H = 1

Number of electrons in H = 1 – 1 = 0

And number of protons in H = 1

Again number of electrons in HΘ = 1 + 1 = 2

And number of protons in HΘ = 1

Chapter 4 Matter Bohr Rutherfords Atomic Model Very Short Answer Type Questions Choose The Correct Answer

Question 1. In an atom, there are

  1. Equal number of neutrons and protons
  2. Equal number of protons and electrons
  3. Equal number of neutrons and electrons
  4. Equal number of protons and positrons

Answer: 2. Equal number of protons and electrons

Question 2. Mass number of an atom is usually expressed by the alphabet

  1. X
  2. M
  3. Z
  4. A

Answer: 4. A

Question 3. Atomic number of an atom is usually expressed by the alphabet

  1. X
  2. M
  3. Z
  4. A

Answer: 3. Z

Question 4. The circular paths along which electrons revolve around the nucleus are known as

  1. Ground state
  2. Stationary orbits
  3. Excited state
  4. Stationary state

Answer: 2. Stationary orbits

Question 5. The lowest energy state of an atom is known as its

  1. Ground state
  2. Equilibrium state
  3. Excited state
  4. Stationary state

Answer: 1. Ground state

Question 6. By absorbing energy, an electron

  1. Jumps to an outer orbit
  2. Jumps to an inner orbit
  3. Does not change its orbit
  4. Revolves faster
  5. Answer: 1. Jumps to an outer orbit

Question 7. Mass number of an atom indicates the total number of

  1. Protons and electrons
  2. Neutrons
  3. Protons and neutrons
  4. Protons, neutrons and electrons

Answer: 3. Protons and neutrons

Question 8. Number of neutrons present in deuterium

  1. 0
  2. 1
  3. 2
  4. 3

Answer: 2. 1

Question 9. According to Bohr’s atomic model number of stationary orbits is

  1. 9
  2. 5
  3. 7
  4. 11

Answer: 3. 7

Question 10. Mass number and number of neutrons of an atom X are 27 and 14 respectively. What is the total number of electrons in X3+ ion?

  1. 13
  2. 10
  3. 14
  4. 27

Answer: 2. 10

Question 11. Total number of charged particles present in 6C

  1. 6
  2. 8
  3. 12
  4. 14

Answer: 3. 12

Question 12. If the total number of electrons present in a trivalent cation is 10, its atomic number is

  1. 10
  2. 7
  3. 13
  4. 14

Answer: 3. 13

Chapter 4 Matter Bohr Rutherfords Atomic Model Answer In Brief

Question 1. Bohr’s theory is not applicable for which of the given species— H, H+, He+, Li2+?

Answer:

Application Of Bohr’s Theory:-

Bohr’s atomic model is applicable only for single electron system. As H+ does not have any electron, Bohr’s theory is not applicable for this ion.

Question 2. What is an orbit?

Answer:

Orbit:-

According to Bohr-Rutherford’s atomic model, an orbit is the circular path around the nucleus along which the electrons revolve around the nucleus in an atom.

Question 3. What is atomic number?

Answer:

Atomic Number:-

Atomic number of an atom is defined as the number of protons present in the nucleus of an atom.

Question 4. What is mass number?

Answer:

Mass Number:-

Mass number of an atom is the sum of the number of protons and neutrons present in the nucleus of an atom.

Question 5. What are the names of the Bohr orbits in increasing order of their distance from the nucleus?

Answer:

Bohr Orbits In Increasing Order:-

According to increasing order of their distance from the nucleus, the seven Bohr orbits are named as K, L, M, N, O, P, and Q orbits.

Question 6. How does the energy of the orbit change on moving away from the nucleus?

Answer: On moving away from the nucleus, the energy of the orbits gradually increases.

Question 7. How does an atom absorb or emit energy?

Answer:

An Atom Absorb Or Emit Energy As Follows:-

The absorption or emission of energy by an atom always takes place discontinuously. (Absorption or emission of energy always takes place in an integral multiple of the smallest unit of energy, called quantum)

Question 8. What happens when an electron absorbs excess energy?

Answer: When an electron absorbs excess energy, it jumps from a lower energy orbit to a higher energy orbit. (If sufficient energy is supplied, then the electron may be removed from the atom.)

Question 9. Which theory forms the basis of Bohr- Rutherford’s atomic model?

Answer: Quantum theory.

Question 10. What is the minimum number of orbits that an atom can have?

Answer: An atom may have a minimum 1 orbit.

Question 11. Write down the relation between atomic number and mass number.

Answer: Mass number = Atomic Number + Number of neutrons.

Question 12. There are 11 protons in an atom. Find the number of electrons in the uni-positive ion of that atom.

Answer: The atom contains 11 protons. So the number of electrons in the uni-positive ion of that atom will be 10.

Question 13. If an atom contains 8 protons, then find the number of electrons in the di-negative ion of that atom.

Answer: There are 8 electrons in the said atom. Hence, the di-negative ion of that atom will contain 10 electrons.

Question 14. What do 13 and 27 signify in the symbol \({ }_{13}^{27} \mathrm{Al}\)?

Answer: Here, 13 stands for the atomic number and 27 standsforthe mass number of aluminium.

Question 15. State whether the outermost electron in \({ }_3 \mathrm{Li}\) atom will absorb or release energy when it jumps to the third orbit.

Answer: The third electron in \({ }_3 \mathrm{Li}\) atom is present in the second orbit. So, when it jumps to the third orbit, it will absorb energy.

Question 16. What is expressed by the symbol, \({ }_8^{16} 0\)?

Answer: \({ }_8^{16} 0\) represents a nuclide of oxygen whose atomic number is 8 and mass number is 16.

Question 17. Number of electrons and neutrons in a bivalent anion are 18 and 20 respectively. What is the mass number of the corresponding atom?

Answer: Number of electrons in the corresponding atom = 18 – 2 = 16

∴ Number of protons in the atom = 16

∴ Mass no. of the atom = 16 + 20 = 36

Question 18. How many charged particles are present in an \({ }_9^{19} F\)-atom?

Answer: 18 (9 protons and 9 electrons).

Question 19. \({ }_6^{12} C\) and \({ }_6^{14} C\) differ in number of which sub atomic particle?

Answer: Neutrons.

Number of neutrons in \({ }_6^{12} C\) and \({ }_6^{14} C\) are 6 and 8 respectively.

Question 20. Number of which particle is same in \({ }_{15} \mathbf{p}^{31}\) and \({ }_{16} S^{32}\)?

Answer: It is neutron whose number is same (16) in \({ }_{15} \mathbf{p}^{31}\) and \({ }_{16} S^{32}\).

Question 21. What is the total number of charged particles present in Ca2+ ion?

Answer: 38 (number of protons and electrons are 20 and 18 respectively).

Question 22. Write down the nuclide consists of 8 neutrons and 6 electrons.

Answer: Atomic no. = no. of protons = no. of electrons = 6,

Mass no. = (n + p) = (8 + 6) = 14

∴ The corresponding nuclide is \({ }_6^{14} C\).

Question 23. Write down the number of neutrons and electrons present in \({ }_{92}^{235} \mathrm{U}^{3+}\).

Answer: Number of neutrons =235 – 92 = 143

Number of electrons = 92 – 3 = 89

Question 24. What is meant by \({ }_{11} \mathrm{Na}^{23}\)?

Answer: It stands for the nuclide of sodium atom.

Question 25. Calculate the number of electrons in a binegative ion of an atom containing 8 protons.

Answer: Number of electrons in the atom = Number of protons in it = 8. [As the atom is neutral]

∴ Number of electrons in the bi-negative ion = 8 + 2 = 10

Question 26. Mention the charge of the sample containing 6 protons, 8 electrons, and 6 neutrons.

Answer: Charge of the sample will be (6-8) = -2.

Question 27. Give example of an atom whose mass number and atomic number are same.

Answer: It is protium (\({ }_1 H^1\)) where atomic number = mass number.

Chapter 4 Matter Bohr Rutherfords Atomic Model Fill In The Blanks

Question 1. The number of neutrons in 11Na23 is _______

Answer: 12

Question 2. Maximum number of electrons that can be present in a principal energy level is ________

Answer: 2n2

Question 3. Bohr-Rutherford atomic model is ________ in nature.

Answer: Two dimensional

Question 4. Atomic number is always a _______ number.

Answer: Whole

Question 5. The circular path along which an electron revolves around the nucleus is known as _______

Answer: Orbit

Question 6. The orbits whose energy remain fixed are called ________ orbits.

Answer: Stationary

Question 7. When an electron jumps to a higher energy level by absorbing energy, it is said that the atom is in its _________ state.

Answer: Excited

Question 8. If an electron revolves in a selected orbit, it neither ________ nor _______ energy.

Answer: Emits, absorbs

Question 9. Atomic number indicates the number of _________ in an atom.

Answer: Protons

Question 10. Mass number = Number of ________ + Number of ________

Answer: Protons, neutrons

Chapter 4 Matter Bohr Rutherfords Atomic Model State Whether True Or False

Question 1. Atomic number of the element M is 13. Thus, number of electrons in M3- ion will be 10.

Answer: False

Question 2. Bohr’s atomic model is applicable for multi-electron systems.

Answer: False

Question 3. Bohr’s atomic model is unable to give an idea about the actual three-dimensional electronic model of an atom.

Answer: True

Question 4. Mass number = Number of protons + Number of electrons.

Answer: False

Question 5. An electron neither emits nor absorbs energy while moving along a stationary orbit.

Answer: True

Question 6. In \({ }_{15} p^{31}\) the atomic number of the respective element is 31.

Answer: False

Question 7. Atomic number of an element is the fundamental property of that element.

Answer: True

Question 8. \({ }_8^{16} 0\) is an example of nuclide.

Answer: True

Question 9. According to Bohr’s theory electrons can revolve in any circular orbit around the nucleus.

Answer: False

Question 10. Electrons do not emit energy during revolving in stationary orbit.

Answer: True

Question 11. Atomic number and mass number of protium is equal.

Answer: True

Question 12. Niels Bohr rectified the limitations of Rutherford’s model.

Answer: True

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