Chapter 4: Structure of the Atom
Learning Notes (Set 1) for Class 9 Science

Question 1: Who discovered the electron?

Answer: J.J. Thomson discovered the electron through his cathode ray tube experiments.

Question 2: What did Ernest Rutherford's alpha-particle scattering experiment prove?

Answer: It proved that most of the space inside an atom is empty and that there is a small, dense, positively charged nucleus at its center.

Question 3: What was the main drawback of Rutherford's model of the atom?

Answer: The model could not explain the stability of the atom, as a revolving electron would lose energy and fall into the nucleus.

You are on Chapter 4: Structure of the Atom (Learning Notes for Class 9 Science) Set 1

Question 4: How did Niels Bohr's model address the problem of atomic stability?

Answer: Bohr proposed that electrons revolve in specific, discrete orbits or energy shells without radiating energy.

Question 5: What are protons and neutrons collectively known as?

Answer: They are collectively known as nucleons.

Question 6: What is the atomic number (Z) of an element?

Answer: The atomic number is the total number of protons in the nucleus of an atom.

Question 7: How is the mass number (A) of an atom calculated?

Answer: The mass number is the sum of the number of protons and neutrons in the nucleus.

Question 8: What is the relationship between the atomic number and the number of electrons in a neutral atom?

Answer: In a neutral atom, the number of electrons is equal to the atomic number (Z), which is the number of protons.

You are on Chapter 4: Structure of the Atom (Learning Notes for Class 9 Science) Set 1

Question 9: What are isotopes?

Answer: Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to a varying number of neutrons.

Question 10: What are isobars?

Answer: Isobars are atoms of different elements that have different atomic numbers but the same mass number.