KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom

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Karnataka State Syllabus Class 8 Science Chapter 3 Structure of Atom

KSEEB Class 8 Science Structure of Atom Textbook Exercise Questions and Answers

I. Four alternatives are given to each of the following incomplete statements/questions. Choose the right answer.

Question 1.
The heaviest fundamental particle is
(a) proton
(b) electron
(c) neutron
(d) positron
Answer:
(c) neutron

Question 2.
In the scattering experiment, the deflection of alpha particle is due to
(a) increase in kinetic energy
(b) force of repulsion
(c) force of attraction
(d) decrease in kinetic energy.
Answer:
(b) force of repulsion

Question 3.
The simplest atom that does not contain neutron is
(a) Deuterium
(b) Tritium
(c) Hydrogen
(d) Helium
Answer:
(c) Hydrogen

Question 4.
The reason for the existence of Isotopes is a change in
(a) atomic number
(b) electron number
(c) neutron number
(d) proton number
Answer:
(c) neutron number

II. Fill in the blanks with suitable words:

1. The total number of protons and neutrons in the nucleus of an atom is known as
2. Almost all the mass of an atom is concentrated on a small region of space called
3. The particle of an atom that has no charge is
4. The fundamental particle not present in a normal hydrogen atom is
5. The atomic mass of an atom is 23 and its
atomic number is 11, the atom has neutrons.
Answers:
1. Atomic mass,
2. Nucleus,
3. Neutron,
4. neutron,
5. 12

III. Answer the following:

Question 1.
State the postulates of Dalton’s atomic theory.
Answer:
The postulates of Dalton’s atomic theory are:
(a) Elements are made up of very small indivisible particles called atoms.
(b) All atoms of a particular element are identical in shape, mass, and other prope ties, but are different from the atom of other elements.
(c) The atom of one element cannot be converted into that of another element.
(d) Atoms of an element combine in integral ration with those of other elements to form compounds.
(e) An atom can neither be created nor destroyed.

Question 2.
Describe J. J. Thomson’s experiment on the discovery of Cathode rays.
Answer:
J. J. Thomson conducted an experiment using a discharge tube of about 15 cm long and 3 cm in diameter which was connected to a vacuum pump. Metal electrodes were fitted to the ends of the glass tube as shown in the diagram. He created very low pressure inside the discharge tube and applied high voltage. He observed a greenish glow near the anode of the glass tube. According to him, the rays which were emitted from the cathode hit the anode and caused the greenish glow and those stream of cathode rays which are made of small particles, he called as electrons.

Question 3.
List out the characteristics of anode rays.
Answer:

1. They consist of positively charged particles
2. They are deflected by electrical and magnetic fields.
3. The deflection is little when compared to the deflection of cathode rays as they consist of heavy particles.
4. The deflection of anode rays depends on the nature of gas in the discharge tube. Heavier the gas, lower will be the deflection.

Question 4.
Describe Rother ford’s model of an atom
Answer:
Ruther ford’s atomic model was also called the nuclear atom or planetary model of the atom. The model described the atom as a tiny dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light negative constituents called electrons, revolve at some distance, much like planets revolving around the sun.

Question 5.
Describe the Neils Bohr atomic model.
Answer:
The postulates of Bohr’s model are:

1. Electrons revolve around the nucleus only in certain permissible circular orbits called shells. The shells are designated by the letters K, L, M, N, O, P, etc.
2. The electrons in each shell have been associated with a definite amount of energy. The electrons in the higher shell have more energy than those nearer to the nucleus.
3. The energy of an electron remains constant as long as it revolves in its own shell.
4. When an electron jumps from a higher energy level to a lower one, some energy is emitted while in the reverse case, some energy is absorbed.

Question 6.
Mention the properties of Cathode rays.
Answer:
(a) Cathode rays are a stream of particles that travel in a straight line.
(b) Cathode rays are negatively charged particles.
(c) Cathode rays can affect a photographic plate.
(d) The nature of Cathode rays does not depend upon the nature of the gas in the discharge tube and the nature of the Cathode material.

Question 7.
Who proposed that the atomic nucleus is positively charged?
Answer:
Rutherford proposed that the atomic nucleus is positively charged.

Question 8.
State the conclusions drawn by the scattering experiment of Rutherford?
Answer:
Based on the scattering experiment the conclusions drawn from the following are;

1. Positively charged particles of an atom held together in the central region of the atom. It is called the nucleus. It is surrounded by electrons.
2. The volume of the nucleus is very small when compared to the volume of an atom.
3. The mass of an atom is concentrated in the nucleus.
4. The nucleus is positively charged which is different in magnitude for different elements.
5. In a neutral atom, the number of electrons outside the nucleus is equal to the number of positively charged particles in the nucleus.

Question 9.
Why is an atom neutral in spite of the presence of charged particles in it?
Answer:
The number of positively charged (protons) particles in the nucleus is equal to the number of negatively charged particles (electrons) outside the nucleus. Therefore an atom is neutral.

Question 10.
Name the fundamental particles present in an atom.
Answer:
The three fundamental particles present in an atom are positively charged particles called protons. The negatively charged particles called electrons and neutral particles called neutrons.

Question 11.
Define the terms atomic number and atomic mass number of an element.
Answer:
The number of protons present in the nucleus of an atom is known as the atomic number and denoted by Z. The total number of protons and neutrons present in the nucleus of an atom is known as the mass number and it is denoted by Answer:

Question 12.
Define the term Isotope of an atom with a suitable example.
Answer:
Atoms of an element with different mass numbers are called Isotopes. Hydrogen has three types of atoms tH1 (protium/ hydrogen), (deuterium), 3 (tritium).

Question 13.
From the symbol 1gK40 state (1) mass number of Potassium, (2) atomic number of potassium (3) Write the distribution of electrons in an atom of Potassium.
Answer:
1. The mass No. of Potassium (A) = 40
2. The atomic No. of Potassium (Z) = 19 The No. of electrons = No. protons = 19
3. The electronic configuration of potassium = 1s² , 2s², 2p⁶, 3s², 3p⁶, 4s¹

IV. Match the following:
A – B
1. Proton – (a) negligible mass
2. L shell – (b) 32 electrons
3. Electrons – (c) Dalton’s atomic theory
4. Neutrons – (d) 8 electrons, (e) Electrically neutral, (f) positively charged, (g) 18 electrons
Answers:
1. f
2. d
3. a
4. e

KSEEB Class 8 Science Structure of Atom Additional Questions and Answers

Question 1.
Write an experiment to show that the cathode rays are made up of high-velocity material particles.
Answer:
When J. J. Thomson placed a light paddle wheel in the path of cathode rays. The paddle wheel started to rotate. This can happen only if the material particles have a high velocity.

Question 2.
Write an experiment to show how Thomson found that Cathode rays are negatively charged particles.
Answer:
J. J. Thomson applied an electric field parallel o the path of the rays. The cathode rays deflected towards the anode. He concluded that cathode rays are negatively charged.

Question 3.
When the hydrogen gas is taken inside the discharge tube the deflection will be maximum. Why?
Answer:
The positive particles of hydrogen atoms are the lightest hence the deflection will be maximum,

Question 4.
Why did most of the alpha particles pass through the foil?
Answer:
Most of the alpha particles passed through the foil as most of the space in the atom was empty.

Question 5.
Why did a few of them deflect from their original path through large angles?
Answer:
The deflection was due to the enormous repulsive force.

Question 6.
Rutherford’s atomic model could not explain the stability of the atom. Explain the above statement.
Answer:
An electron revolving around the nucleus continuously should lose energy. Ac a result, it should be gradually pulled towards the nucleus and end up colliding with it. if this should happen then the atom would collapse. Therefore, the Rutherford atomic model could not explain the stability of the atom.

II. Fill in the blanks:

Question 1.

1. The Indian sage was the first to suggest that ail the matter is made up of very small particles called para Manu.
2. Kannada was the first who propounded that the material universe is made up of ……………..
3. The Greek philosopher who gave the name atom to the ultimate particle was ……………..
4. Any substance made up of the same kind of atom is called ……………..
5. The number of elements known to us is……………..
6. There are …………….. natural elements.
7. The two elements which are not added to the natural elements are …………….. and ……………..
8. The mass of one proton is about the mass of …………….. electrons.
9. The shells are also called as ……………..
10. An element is represented by the symbol ……………..
11. The mass of a proton is …………….. amu
12. α – particles are …………….. charged.
13. Rutherford’s nuclear model was modified by ……………..

Answers:

1. Kanada,
2. Kana
3. Democritus
4. element,
5. 118
6. 90
7. Astatin & Technetium,
8. 1840
9. energy levels,
10. _zX^A
11. 1 amu,
12. positively
13. Bohr

III. Match the following

A – B
1. electron – (a) Rutherford
2. proton – (b) Neils Bohr
3. Neutron – (c) Goldstein
4. Nucleus – (d) J. J. Thomson
5. Energy levels – (e) James Chadwick
Answers:
1. d,
2. c,
3. e,
4. a,
5. b

Four alternatives are given to each of the following. Choose the right answer:

Question 1.
The particles present in the nucleus of an atom are
(a) proton and electron
(b) electron and neutron
(c) proton and neutron
(d) positron and neutron
Answer:
(c) proton and neutron

Question 2.
When a gas at reduced pressure is subjected to an electric discharge, the rays originating from the negative electrode are
(a) anode rays
(b) cathode rays
(c) neutron rays
(d) radioactive rays
Answer:
(b) cathode rays

Question 3.
In Rutherford’s gold foil experiment, most of the cc – particles pass across the gold foil without any deviation from their paths. This indicated that
(a) the atom is spherical
(b) there is a positively charged nucleus at the centre of the atom
(c) the entire mass of the atom is concentrated at the nucleus of the atom
(d) most portion of the atom is an empty space.
Answer:
(d) most portion of the atom is an empty space.

Question 4.
Isotopes of an element are the atoms which have
(a) the same number of neutrons in their nuclei.
(b) same number of protons in their nuclei.
(c) the same mass number.
(d) different number of electrons in their orbits.
Answer:
(b) same number of protons in their nuclei.

Question 5.
An atom can be represented as
(a) \(z^{X^{A}}\)
(b) \(A^{X^{z}}\)
(c) \(z^{A^{x}}\)
(d) \(x^{A^{z}}\)
Answer:
(a) \(z^{X^{A}}\)

Question 6.
The relation between atomic mass and atomic number can be represented as.
(a) A + N = Z
(b) A + Z = N
(c) N – Z = A
(d) A – Z = N
Answer:
(d) A – Z = N

Question 7.
The fundamental particles which revolve round the nucleus are
(a) Protons
(b) Electrons
(c) Neutrons
(d) Nucleons
Answer:
(b) Electrons

Question 8.
The fundamental particles which take part in chemical reactions are
(a) Protons
(b) Neutrons
(c) Nucleons
(d) Electrons
Answer:
(d) Electrons

Question 9.
The maximum number of electrons that can be accommodated in any shell is
(a) 2n²,
(b) 2n
(c) 2²n
(d) (2n)²
Answer:
(a) 2n²,

Question 10.
The atomic number of an element is 11 and its atomic mass is 23. The number of neutrons are
(a) 1.1
(b) 23
(c) 12
(d) 34
Answer:
(c) 12