Class 9

9.2.1 Introduction to Atoms

This is the first lecture from Chapter 2: ‘Atomic Structure’ of the new book for Class 9 Chemistry (Punjab Board – PCTB). It discusses the basic introduction and brief history of the atom. This lecture includes MCQs based quiz, short questions and long question notes.

MCQs Based Quiz

9.2.1 Introduction to Atoms

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1. Which of the following elements has a lustrous appearance?

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2. In discharge tube, the existence of negative particles was confirmed due to their deflection towards:

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3. In discharge tube, the existence of positive particles was confirmed due to their deflection towards:

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4. A proton is ___ times heavier than electron.

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5. Which experiments discovered that atoms can be further sub-divided?

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6. Electrons were discovered as a result of which experiments?

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7. Who was the first scientist who provided the experimental evidence for the existence of the atoms?

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8. According to Democritus, all the matter is made up of:

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9. The idea of atom was first proposed by:

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10. Among iron, aluminium, zinc and carbon which appears dull?

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11. Among iron, aluminium, zinc and carbon, which is heavier?

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12. What is the most common state of elements on the earth?

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Short Questions

Q1. Give two examples that prove that elements are different from each other.

  1. Iron is heavier compared to zinc or aluminium.
  2. Metals like gold are shinier than non-metals like carbon.

Q2. Why do elements differ from each other?

Elements differ from one another because of the differences in the properties of the atoms that make them. For example, atomic number, mass number and electronic arrangement.

Q3. Who first proposed the idea of the Atom, and what did they call it?

Democritus, a Greek philosopher, was the first to propose the idea of atom. He said that matter is made of tiny particles called “atom”.

Q4. What is the contribution of Democritus in the development of ideas about the atom?

Democritus, a Greek philosopher, was the first to propose the idea of atom. He said that matter is made of tiny particles called “atom”.

Q5. What is the contribution of John Dalton in the development of ideas about the atom?

John Dalton was an English chemist. He conducted many experiments with various different compounds, which led him to propose that matter is made of smaller particles (atoms).

Q6. What is the importance of discharge tube experiments?

Discharge tube experiments played a crucial role in atomic research. These experiments led scientists to discover that atoms can be further divided into smaller particles such as electrons and protons.

Q7. What are the sub-atomic particles?

An atom is made up of three types of smaller particles called sub-atomic particles:

  1. Electrons (Negative)
  2. Protons (Positive)
  3. Neutrons (Neutral)

Descriptive Question

Q1. Write a historical note on the development of modern concept of the atom.

The modern understanding of the atom has developed over centuries through several key discoveries:

  1. Democritus (Ancient Greece): Proposed that matter is composed of tiny indivisible particles called “atoms.”
  2. John Dalton (1800s): Provided experimental evidence for the existence of atoms.
  3. Discharge Tube Experiments: Provided insights into the existence of particles within atoms (cathode and anode rays).
  4. J. J. Thomson (1897): Discovered electrons by studying cathode rays.
  5. Goldstein (1886): Observed anode rays (later identified as protons).
  6. Ernest Rutherford (1911): Discovered the nucleus and protons. He also proposed the nuclear model of the atom.
  7. James Chadwick (1932): Discovered the neutron, completing the set of sub-atomic particles.
  8. Niels Bohr (1913): Proposed a refined atomic model where electrons revolve around the nucleus in discrete energy levels.
  9. Quantum Mechanical Model: The modern understanding of atom, in which electrons are considered as clouds of negative charge.

Q2. Write an introductory note on the basic structure of an atom.

Atoms are the basic units of all elements and possess all the properties of those elements.

Sub-Atomic Particles:

Atoms consist of three sub-atomic particles:

  1. Neutrons (no charge)
  2. Protons (positive charge)
  3. Electrons (negative charge)

Structure of Atom:

An atom has two portions:

  1. Nuclear portion (centre)
  2. Extra-nuclear portion

Nuclear Portion (Nucleus):

  1. The nucleus is located at the centre of the atom and contains protons and neutrons.
  2. These particles are held together by a strong nuclear force.
  3. Since protons and neutrons are approximately 1836 times heavier than electrons, most of the atom’s mass is concentrated in the nucleus.
  4. However, the nucleus is extremely small, occupying only about 1/100,000th of the atom’s total volume.
  5. It is positively charged due to the presence of protons.

Extra-Nuclear Portion:

  1. The area surrounding the nucleus is called the extra-nuclear portion.
  2. It contains electrons, which revolve around the nucleus.
  3. The paths or regions where electrons are likely to be found are called shells.
  4. Shells are divided into sub-shells, which are further divided into orbitals.
  5. According to the modern quantum mechanical model, electrons are viewed as charged clouds, not fixed particles in orbits.