Chapter 5 Atomic Structure Textbook Solutions

NIOS Class 10 Science Chapter 5 Question Answers

INTEXT QUESTIONS 5.1

1. Name two charged particles which constitute all matter.

Ans: Electrons and protons

2. Describe a cathode ray tube.

Ans: A cathode ray tube consists of two metal electrodes in a partially evacuated glass tube. The negatively charged electrode is called the cathode, while the positively charged electrode is called the anode. These electrodes are connected to a high-voltage source.

3. Name the negatively charged particles emitted from the cathode in the cathode ray tube.

Ans: Electron

4. Why do the canal rays obtained by using different gases have different e/m values?

Ans: When the electrons emitted from the cathode collide with the neutral atoms of the gas present in the tube, these remove one or more electrons present in them. This leaves behind positively charged particles, which travel towards the cathode. As the atoms of different gases have different numbers of protons present in them, these give positively charged ions with different e/m values.

INTEXT QUESTIONS 5.2

1. Describe Thomson’s model of atom. What is it called?

Ans: According to Thomson’s model, atoms can be considered as a large sphere of uniform positive charge with several small negatively charged electrons scattered throughout it. This model was called the plum pudding model.

2. What would have been observed in the α-ray scattering experiment if Thomson’s model were correct?

Ans: If the Thomson’s model was correct, then most of the α-particles in the α-ray scattering experiment would have passed straight through the atom

3. Who performed the α-ray scattering experiment, and what were the observations?

Ans: The α-ray scattering experiment was performed by Geiger and Marsden. When a stream of α-particles from a radioactive source was directed on a thin piece of gold foil, most of the α-particles passed straight through the gold foil, some α-particles were deflected by small angles, a few particles by large angles, and very few experienced a rebound.

4. Describe the model of atom proposed by Rutherford.

Ans: According to Rutherford’s model, the atom contains a dense and positively charged region called the nucleus at its centre, and the negatively charged electrons move around it. All the positive charge and most of the mass of the atom are contained in the nucleus.

INTEXT QUESTIONS 5.3

1. Give any two drawbacks of Rutherford’s model of atom.

Ans: The Rutherford model could not explain the stability of the atom, the distribution of electrons and the relationship between the atomic mass and atomic number (the number of protons).

2. State the postulates of Bohr’s model.

Ans: The two postulates of Bohr’s model are :

I. The electrons move in definite circular paths of fixed energy around a central nucleus.

II. The electron can change its orbit or energy level by absorbing or releasing energy.

3. How does Bohr model of an atom explain the stability of the atom?

Ans: The Bohr’s model explains the stability of atom by proposing that the electron does not lose energy when present in a given energy level.

INTEXT QUESTIONS 5.4

1. What is a neutron, and where is it located in the atom?

Ans: It is a neutral subatomic particle present in the nucleus of the atom.

2. How many neutrons are present in the α-particle?

Ans: An α-particle contains two neutrons.

3. How will you distinguish between an electron and a proton?

Ans: The electron and proton can be distinguished in terms of their charge and mass. While the electron is negatively charged, the proton is positively charged. Secondly, the proton is much heavier than the electron; it is about 1840 times heavier.

INTEXT QUESTIONS 5.5

1. A sodium atom has an atomic number of 11 and a mass number of 23. Calculate the number of protons, electrons and neutrons in a sodium atom.

Ans:

No of protons = 11

No. of electrons = 11

No. of neutrons = 12

2. What is the mass number of an atom which has 7 protons and 8 neutrons?

Ans: Mass number = number of protons + number of neutrons. Therefore, mass number = 7 + 8 = 15

3. Calculate the number of electrons, protons and neutrons in 40Ar18 and 49K19

Ans: ⁴⁰₁₈Ar : Number of protons = atomic number = 18

Number of electrons = number of protons = 18

Number of neutrons = mass number – number of protons = 40 – 18 = 22

⁴⁰₁₉K Number of protons = atomic number = 19

Number of electrons = number of protons = 19

Number of neutrons = mass number – number of protons = 40 – 19 = 21

INTEXT QUESTIONS 5.6

1. How many shells are occupied in the nitrogen (atomic number =7) atom?

Ans: The electronic configuration of nitrogen is 2, 5. Thus, two shells are occupied. The first shell (capacity = 2) is filled, while the second shell (capacity = 8) is partially filled.

2. Name the element which has completely filled first shell.

Ans: Helium

3. Write the electronic configuration of an element having atomic number equal to 11.

Ans: The electronic configuration of an element having atomic number 11 is 2, 8, 1.

TERMINAL EXERCISE

1. How did J.J.Thomson discover the electron? Explain his “plum pudding” model of the atom.

Ans: J J Thomson experimented with the presence of an external electric field.

He found that cathode rays travel in a straight line and that the particles that constitute cathode rays carry mass and kinetic energy. He also observed that these particles travel fast, carry a negative charge and were attracted towards the positively charged plate.

These particles, constituting the cathode ray,s were later called electrons. Since it was observed that the nature of cathode rays was the same irrespective of the metal used for the cathode or the gas filled in the cathode ray tube. This led Thomson to conclude that all atoms must contain electrons.

Thomson concluded that there must be an equal amount of positive charge present in an atom. He proposed a model for the structure of an atom. According to his model, atoms can be considered as a large sphere of uniform positive charge with several small negatively charged electrons scattered throughout it. This model was called as plum pudding model, as the electrons represent the plums in the pudding made of negative charge.

2. What made Thomson conclude that all atoms must contain electrons?

Ans: The particles constituting the cathode rays were later called electrons. Since it was observed that the nature of cathode rays was the same irrespective of the metal used for the cathode or the gas filled in the cathode ray tube. This led Thomson to conclude that all atoms must contain electrons. This led Thomson to conclude that there must be an equal amount of positive charge present in an atom.

3. Identify the following subatomic particles:

(a) The number of these in the nucleus is equal to the atomic number

(b) The particle that is not found in the nucleus

(c) The particle that has no electrical charge

(d) The particle that has a much lower mass than the other subatomic particles

Ans:

(a) The number of these in the nucleus is equal to the atomic number: Protons
(b) The particle that is not found in the nucleus: Electrons
(c) The particle that has no electrical charge: Neutrons
(d) The particle that has a much lower mass than the other subatomic particles: Electrons

4. Which of the following are usually found in the nucleus of an atom?

(a) Protons and neutrons only

(b) Protons, neutrons, and electrons

(d) Electrons and neutrons only

Ans: (a) Protons and neutrons only

5. Describe Ernest Rutherford’s experiment with alpha particles and gold foil. How did this lead to the discovery of the nucleus?

Ans: Ernest Rutherford’s experiment with alpha particles and gold foil.

In this experiment, a stream of α particles from a radioactive source was directed on a thin (about 0.00004 cm thick) piece of gold foil. Based on Thomson’s model, it was expected that the alpha particles would just pass straight through the gold foil and could be detected by a photographic plate placed behind the foil. It was observed that:

(i) Most of the α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by small angles.
(iii) A few particles were deflected by large angles.
(iv) About 1 in every 12000 particles experienced a rebound.

According to Rutherford’s model

An atom contains a dense and positively charged region located at its centre; it is called as nucleus
All the positive charge of an atom and most of its mass were contained in the nucleus
The rest of an atom must be empty space, which contains the much smaller and negatively charged electrons.

6. What does the atomic number tell us about an atom?

Ans: The atomic number represents the number of protons in the nucleus of this atom. The number of protons is also equal to the number of electrons. Thus, the atomic number of an atom represents the number of both protons and electrons.

7. What is the relationship between the number of electrons and protons in an atom?

Ans: The number of protons is also equal to the number of electrons.

8. How did Niels Bohr revise Rutherford’s atomic model?

Ans: Niels Bohr proposed that the electrons move in definite circular paths of fixed energy around

a central nucleus. These paths were called orbits or energy levels. He also proposed that the electron can change its shells or energy level by absorbing or releasing energy. In other words, when an electron absorbs energy, it could move to a higher level or come down to a lower one by losing some energy.

9. What is understood by a stationary state?

Ans: An electron moving in an orbit does not radiate or lose energy, and this is called a stationary state.

10. What is a shell? How many electrons can be accommodated in L shell?

Ans: A shell is a three-dimensional region surrounding the nucleus, having a definite energy. Shells of definite energy are represented by letters K, L, M, N, etc. The L shell can accommodate a maximum of 8 electrons.

11. State the rules for writing the electronic configuration of elements.

Ans: The electron distribution is governed by the following rules:

I. These orbits or shells in an atom are represented by the letters K, L, M, N,…
II. The orbits are arranged in the order of increasing energy. The energy of the M shell is more than that of the L shell, which in turn is more than that of the K shell.
III. The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the number of the orbit or the shell. Thus, the maximum number of electrons that can be accommodated in different shells is as follows:
- Maximum number of electrons in K shell (or n = 1 level) = 2n² = 2 × (1)² = 2
- Maximum number of electrons in L shell (or n = 2 level) = 2n² = 2 × (2)² = 8
- Maximum number of electrons in M shell (or n = 3 level) = 2n² = 2 × (3)² = 18, and so on.
IV. The shells are occupied in the increasing order of their energies.
V. Electrons are not accommodated in a given shell unless the inner shells are filled.