Class Nine Science Chapter 4 Structure of Atoms

Extra Questions-Answers Class Nine Science Chapter 4 Structure of Atoms


Q. 1. What is the ground state of an atom?

Ans. The ground state of an atom is the state in which all the electrons in the atom are in their lowest energy levels.

Q. 2. Why is perforated cathode used in the discharge tube used in the production and

detection of the anode rays?

Ans. The perforated cathode permitted the positive rays to pass through them to produce a glow on the glass wall of the discharge tube.

Q. 3. What are the canal rays?

Ans. The stream of positively charged particles produced in the sample of gas during electrical discharge is called the canal rays.

Q. 4. What kind of electric charge is acquired:

(a) by a glass, rod rubbed with silk cloth?

(b) by a plastic comb rubbed with dry hair?

Ans. (a) Positive charge, (b) Negative charge.

Q. 5. What are the cathode rays?

Ans. A beam of negatively charged particles is called cathode rays.

Q. 6. At what pressure of a gas in the discharge tube are the cathode rays produced?

Ans. At about 10–5 atm. pressure.

Q. 7. What is the most important discovery made through the alpha-particle scattering experiment?

Ans. The alpha-particle scattering experiment proved the presence of a high, dense, positively charged core (called nucleus) at the centre of an atom.

Q. 8. What determines the number of positive charges on the nucleus?

Ans. A number of protons an atom contains.

Q. 9. Which subatomic particle is not present in a hydrogen atom?

Ans. Neutron.

Q. 10. Electrons present around the nucleus carry negative charge whereas the nucleus carries a positive charge, then why the electrons do not fall into the nucleus?

Ans. Electrons keep on revolving around the nucleus. The centrifugal force acting outwards balances the force of attraction and therefore, electrons do not fall into the nucleus.

Q. 11. Write down the electrons distribution of chlorine atom. How many electrons are there in the L-shell? (Atomic number of chlorine is 17).

Q. 12. What is meant by nuclear composition in an atom?

Ans. The number of neutrons and protons present in the nucleus of an atom is called nuclear composition in an atom.

Q. 13. The atomic number of neon is 10. Write its electronic configuration.

Q. 14. If both K and L-shells of an atom are full, what is the total number of electrons contained in them?

Ans. No. of electrons = 10 (K = 2, L= 8).

Q. 15. Write the distribution of electrons in carbon and sodium atoms.


Q. 16. Define valency.

Ans. It is the combining capacity of the element. The number of electrons gained, lost or shared by an atom to complete the octet in the outermost shell is the valency of the element.

Q. 17. If the molecule has 5 electrons in the outermost M-shell, what valency or valencies can it have?

Ans. If the outermost shell is M containing 5 electrons, its electronic configuration is 2, 8, 5. It can have valency 5 or (8 – 5) = 3.

Q. 18. How do we calculate the valencies of cations or anions?

Ans. Valency of a cation or an anion is equal to the number of electrons lost or gained by the atom complete to its octet, i.e., it is equal to the units of charge present on the ion.

Q. 19. What is the atomic number of a species which contains 20 protons, 20 neutrons and 18 electrons?

Ans. Atomic number = number of protons = 20.

Q. 20. There are 15 protons and 16 neutrons in the nucleus of an element. Calculate the atomic number and mass number.

Ans. Atomic number = number of protons = 15

Mass number = no. of protons + no. of neutrons

= 15 + 16 = 31

Q. 21. The number of electrons in the outermost shell of chlorine is 7. What is its valency and why?

Ans. It valency is 1 because it can gain 1 electron to complete its octet

Q. 22. Which subatomic particles of an atom are responsible for atomic mass?

Ans. Protons and neutrons.

Q. 23. Write the mass number of the isotope of the element AXZ

X which has two excess neutrons.

Ans. A + Z

Ans. The species A and B are isobars since they have the same atomic mass but different atomic numbers.

Q. 25. Give any two uses of isotopes.

Ans. (i) For estimating the age of archaeological samples.

(ii) For estimating the age of glaciers.

Q. 26. Write the symbol of the element having atomic number 16 and mass number 32.


Q. 27. Which isotope of hydrogen contains one neutron?

Ans. Deuterium contains one proton and one neutron in its nucleus.

(i) What does the numeral 23 indicate?

(ii) What does the numeral 11 indicate?

Ans. (i) 23 indicates the mass number of X.

(ii) 11 indicates the atomic number of X.

(i) What does the numeral 23 indicate?

(ii) What does the numeral 11 indicate?

Ans. (i) 23 indicates the mass number of X.

(ii) 11 indicates the atomic number of X.