Class 9 Science

Structure of Atom

In Text Question Answer

Question 1: What are canal rays?

Answer: In 1886; E. Goldstein discovered the presence of new radiations in a gas discharge. He called them canal rays. Canal rays were positively charged radiations.

Question 2: If an atom contains one electron and one proton, will it carry any charge or not?

Answer: The negative charge on electron will balance the positive charge on proton. Hence, this atom will carry no charge.

Question 3: On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Answer: According to Thomson; the negative and positive charges in an atom are equal in magnitude. So, the atom is electrically neutral.

Question 4: On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

Answer: Proton

Question 5: Draw a sketch of Bohr’s model of an atom with three shells.


Neils Bohr Model of Atom

Question 6: What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?

Answer: Most of the other metals are not as malleable as gold. The results of this experiment depend on how thin the foil can be. If foil of any other metal can be almost as thin as that of gold; there would be no change in observations. In other cases, observations would be erratic.

Question 7: Name the three sub-atomic particles of an atom.

Answer: Electron, Proton and Neutron

Question 8: Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Answer: We know; Atomic Mass = Number of Protons + Number of neutrons
Or, 4 u = 2 + Number of neutrons
Or, Number of neutrons = 4 – 2 = 2

Question 9: Write the distribution of electrons in carbon and sodium atoms.

Answer: Carbon (6): 2, 4
Sodium (11): 2, 8, 1

Question 10: If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Answer: Maximum number of electrons in K = 2n2 = 2 x 12 = 2
Maximum number of electrons in L = 2 x 22 = 8
Hence, total number of electrons = 2 + 8 = 10

Question 11: How will you find the valency of chlorine, sulphur and magnesium?

Answer: Electronic configuration of chlorine (17) = 2, 8, 7
Hence, valency of chlorine = 1

Electronic configuration of sulphur (16) = 2, 8, 6
Hence, valency of sulphur = 2

Electronic configuration of magnesium (12) = 2, 8, 2
Hence, valency of magnesium = 2

Question 12: If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? and (ii) what is the charge on the atom?

Answer: Atomic number = number of protons = 8
Charge on atom = 0 because no. of electrons and protons is equal.

Question 13: With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.

Answer: Number of protons in oxygen atom = 8
Number of neutrons in oxygen atom = 8
Hence, mass number of oxygen = 8 + 8 = 16

Number of protons in sulphur = 16
Number of neutrons in sulphur = 16
Hence, mass number of sulphur = 16 + 16 = 32

Question 14: For the symbol H,D and T tabulate three sub-atomic particles found in each of them.

Answer: Hydrogen: proton (1), neutron (0)
Deuterium: propton (1), neutron (1)
Tritium: proton (1), neutron (2)

Question 15: Write the electronic configuration of any one pair of isotopes and isobars.

Answer: Electronic configuration of isotopes of an element shall be the same because of same number of electrons in their atoms. However, electronic configuration of isobars can be different as illustrated in this example;
4018Ar: 2, 8, 8
4020Ca: 2, 8, 8, 2