KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom

Karnataka Board Class 8 Science Chapter 3 Structure of Atom

KSEEB Class 8 Science Chapter 3 Textual Questions & Answers

I. Four alternatives are given to each of the following incomplete statements, question 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 of 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 change in.
(a) Atomic number
(b) Electron number
(c) Neutron number
(d) Proton number.
Answer:
(d) Proton number.

II. Fill in the blanks:

Question 1.
The total number of protons and neutrons in the nucleus of an atom in known as ……….
Answer:
Atomic mass

Question 2.
Almost all the mass of an atom is concentrated in a small region of space called the ………..
Answer:
nucleus

Question 3.
The particle of an atom that has no charge is ………….
Answer:
neutrons

Question 4.
The fundamental particle not present in a normal hydrogen atom is …………..
Answer:
neutron

Question 5.
The atomic mass of an atom is 23 and its atomic number is 11. The atom has ………….. neutrons.
Answer:
12

III. Answer the following:

Question 1.
State the postulates of Dalton’s atomic theory.
Answer:
In 1803 English chemist Dalton put forward his atomic theory. The main postulates of the theory are as follows.

  1. Elements are made up of very small indivisible particles called atoms.
  2. All atoms of a particular element are identical in shape, mass and other properties, but are different from the atom of other elements.
  3. Atom of one element can not be converted into that of another element.
  4. Atoms of an element combine in integral ratio with those of other elements to form a compound
  5. An atom can neither be created nor destroyed.

Question 2.
Describe J.J Thomson’s experiment on the discovery of cathode rays.
Answer:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 2

J.J Thomson’s and others conducted an experiment with a vacuum pump. Metal electrodes are fitted to the ends of the glass tube, as shown in the figure. 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.

The rays which are emitted from the cathode hit the anode and cause the greenish glow. The streams of rays emitted from the cathode are called cathode rays. Which are made of small particles? He called electrons.

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

  1. They consist of positively charged particles.
  2. They are deflected by electrical and magnetic fields. The deflection is very little when compared to the deflection of cathode rays because it consists of heavy particles
  3. The deflection of the anode rays depends on the nature of the rays in the discharge tube. Heavier the gas lowers the depletion.

Question 4.
Describe Rutherford’s model of an atom?
Answer:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 4

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 Neil’s Bohr’s atomic model?
Answer:

  1. Electrons revolve around the nucleus only in certain permissible circular orbits, (orbit is some times called a shell. The first
  2. shell nearest to the nucleus is called ‘K.’ shell. The subsequent shells as we move away from the nucleus are L, M, N, O respectively).
  3. Electrons in each shell have been associated with a definite amount of energy. Electrons in higher shell have more energy than those nearer to the nucleus.
  4. The energy of an electron remains constant so long as it revolves in its own shell. The shells also represent energy levels.

Question 6.
Mention the properties of cathode rays.
Answer:
The properties of cathode rays are,

  1. cathode rays are negatively charged particles
  2. cathode rays are a stream of particles which travels in a straight line.
  3. Cathode rays can affect a photographic plate

Question 7.
Who proposed 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:
Positively charged particles of an atom are held together in the central region of the atom. It is called the nucleus. It is surrounded by electrons.

  1. The volume of the nucleus is small when compared to the volume of an atom.
  2. The mass of an atom is concentrated in the nucleus.
  3. The nucleus is positively charged which is different in magnitude for different elements.
  4. In a neutral atom, the number of electrons outside the nucleus are 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 particles (protons) in the nucleus is equal to the number of negatively charged particles (electrons) outside the nucleus. Therefore an atom is neutral.

Question 10.
List the three fundamental particles and their properties.
Answer:
The fundamental particles present in an atom are.

  1. Positively charged particles called protons (p)
  2. The negatively charged particles called electrons (e) and.
  3. neutral particles called neutrons (n)

Question 11.
Define the term atomic number and atomic mass number of an element.
Answer:

  1. Atomic number: The number of protons present in the nucleus of an atom is known as the atomic number.
    It is denoted by ‘Z’
  2. Atomic mass number: The total number of protons and neutrons present in the nucleus of an atom is known as mass number. It is denoted by A.

Generally, the atomic number and mass number of the atom of an element is represented by a symbol ZXA.
‘X’ is an element.

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.
Ex: Hydrogen has three types of atoms,

  1. 1H1 (protium)
  2. 1H2 (deuterium)
  3. 1H3 (tritium)

Question 13.
From the symbol 19k40 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 number of potassium (A)= 40
  2. The atomic number of potassium (Z)=19
  3. The electronic configuration of potassium = 1S2, 2S22Ps 3S23P6, 4S1

IV. Match the following:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 1
Answer:

  1. (f)
  2. (d)
  3. (a)
  4. (e)

KSEEB Class 8 Science Chapter 3 Additional Questions & Answers

Question 1.
What is an element?
Answer:
Any Substance made up of same kind of atom is called an element. 118 elements are known to us.

Question 2.
Write an experiment to show that the cathode rays are made up of high-velocity material particles
Answer:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 2add

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 high velocity.

Question 3.
Write an Experiment to demonstrate the properties of cathode rays.
Answer:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 3add

J.J. Thomson applied an electric field parallel to the path of the rays. The cathode rays deflected towards the anode. He concluded that cathode rays negatively charged.

Question 4.
List out the characteristics of cathode rays.
Answer:
The characteristics of cathode rays are:

  1. They consist of negatively charged particles.
  2. cathode rays are also called as electrons.
  3. electrons are lighter than the Hydrogen atom.
  4. electrons mass is very small or less.

Question 5.
Describe the gold stein experiment on the discovery of anode rays.
Answer:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 5add

In 1886 Gold stein used a modified type of discharged tube with a perforated cathode. He observed that certain rays are traveling in the direction opposite to that of cathode rays. He applied an electric field parallel to the path of rays and observed that the path deflected towards the cathode. Since these rays contain positively charged particles he called positive rays or anode rays.

Question 6.
When the hydrogen gas is taken inside the discharge tube the deflection will be maximum give season.
Answer:
The positive particles of hydrogen atoms are the lightest hence the deflection will be maximum.

Question 7.
Describe the alpha-particle scattering experiment conducted by rutherford.
Answer:
KSEEB Solutions for Class 8 Science Chapter 3 Structure of Atom 7add
Rutherford directed high speed positively charged particles called alpha particles towards a thin gold foil. He observed that most of the alpha particles passed through the foil without deflection. A few of them were deflected at very large angles from their original direction and a few were even turned back on their path.

Question 8.
Why did most of the alpha particles pass through the foil?
Answer:
Most of the alpha particles passed through the foil because most of the space in the atom was empty.

Question 9.
Why did a Few of them alpha particles deflect their original path through large angles?
Answer:
The deflection was due to the enormous repulsive force.

Question 10.
State the conclusions drawn by Scattering experiment of rutherford.
Answer:

  1. Positively charged particles of an atom are held together in the central region of the atom. It is” called the nucleus. It is surrounded by electrons.
  2. The volume of nucleus is 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 are equal to the number of positively charged particles in the nucleus.

Question 11.
Ruther ford atomic model could of explain the stability of the atom. Explain the above statement.
Answer:
An electron revolving around the nucleus continuously should lose energy. As 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 ‘Rutherford’s atomic model could not explain the stability of the atom.

Question 12.
State the postulates of the Bohr model.
Answer:
In 1913 Neil’s Bohr presented a model of an atom which is now called the model. The postulates of this model are.

  1. Electron revolves around the nucleus only in certain permissible circular orbits called shells.
  2. The shells are designated by the letters K.L.M.N. O.P. etc.
  3. Electron in each shell has been associated with a definite amount of energy. Electrons in higher shell have more energy than those nearer to the nucleus.
  4. The energy of an electron remains constant as long as it revolves in its own shell.

Question 13.
From the symbol 17 CI35 state
(1) mass number chlorine
(2) atomic number of chlorine
(3) write the distribution of electron in an atom of chlorine.
Answer:

  1. The mass number of chlorine (A) = 35
  2. The atomic number of chlorine (z) = 17
    The number of electron = number of protons = 17
  3. The electronic configuration of chlorine = 1S2, 2S22P6, 3S23P5

Question 14.
Mention the uses of Isotopes?
Answer:

  1.  Medical Science
    a) Treatment of cancer
    b) Treatment of thyroid
  2.  In industries to check defects
    a) To check faults in machinery.

KSEEB Solutions for Class 8 Science

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