Chapter 4: Structure of the Atom
Learning Notes (Set 3) for Class 9 Science

Question 1: What is the mass of an electron?

Answer: The mass of an electron is considered negligible compared to that of a proton or neutron.

Question 2: Who discovered the neutron and in which year?

Answer: J. Chadwick discovered the neutron in 1932.

Question 3: What is a key characteristic of the nucleus of an atom according to Rutherford's model?

Answer: The nucleus is a positively charged center that contains nearly all of the atom's mass.

You are on Chapter 4: Structure of the Atom (Learning Notes for Class 9 Science) Set 3

Question 4: What is the formula used to calculate the maximum number of electrons in a shell?

Answer: The maximum number of electrons in a shell is given by the formula 2n², where n is the orbit number.

Question 5: What is the mass number of an atom?

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

Question 6: How is the valency of an atom determined?

Answer: Valency is determined by the number of electrons an atom needs to gain, lose, or share to have a complete outermost shell.

Question 7: What do you call atoms of different elements with different atomic numbers but the same mass number?

Answer: Such atoms are called isobars.

Question 8: Give an example of an isotope of carbon mentioned in the document.

Answer: Carbon-12 and carbon-14 are isotopes of carbon.

You are on Chapter 4: Structure of the Atom (Learning Notes for Class 9 Science) Set 3

Question 9: Why is an atom electrically neutral according to Thomson's model?

Answer: An atom is electrically neutral because the positive and negative charges within it are equal in magnitude.

Question 10: What is the charge of a proton in relation to an electron?

Answer: A proton has a positive charge that is equal in magnitude but opposite in sign to the negative charge of an electron.