Ncert Chapter 4 Structure of Atom Solution
INTEXT QUESTIONS (Page No. 47)
Q. 1. What are the canal rays?
Ans. The positively charged radiations produced in a discharge tube containing a gas at low pressure when a high potential difference is applied between the electrons are called canal rays.
Q. 2. If an atom contains one electron and one proton, will it carry any charge or not?
Ans. The atom will not carry any charge because electron carries one unit negative charge. The net charge on the atom will, therefore, be zero
INTEXT QUESTIONS (Page No. 49)
Q. 1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole. Ans. According to Thomson’s model, the atom is considered as a sphere of positive charge in which negatively charged electrons are embedded like seeds in a watermelon. The total negative charge on the electrons is equal to the total positive charge on the sphere. Therefore, the atom as a whole is electrically neutral.
Q. 2. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans. Proton is the subatomic particle present in the nucleus of an atom.
Q. 3. Draw a sketch of Bohr’s model of an atom with three shells.
Ans,
Q. 4. What do you think would be the observation if the alpha-particle scattering experiment is carried out using a foil of a metal other than gold.
Ans. With the foil of any heavy metal like gold, for example, platinum, silver etc., the observation will be same but with the foil of a light metal, for example, lithium, the massive alpha-particles may push the nucleus and may not be deflected back.
INTEXT QUESTIONS (Page No. 49)
Q. 1. Name the three subatomic particles of an atom.
Ans. Electron, proton and neutron.
Q. 2. A helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Ans. Number of neutrons = Mass number – Number of protons = (4 – 2) = 2
INTEXT QUESTIONS (Page No. 50)
Q. 1. Write the distribution of an electron in carbon and sodium atoms.
Ans. For Carbon atom : Number of electrons = Atomic number = 6
Hence, Electron distribution,K-shell = 2 electrons L-shell = (6 – 2) = 4 electrons For sodium atom : Number of electrons = Atomic number = 11 Electron distribution,K-shell = 2 electrons L-shell = 8 electrons M-shell = 1 electrons .
Q. 2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Ans. Number of electrons in full K-shell = 2 Number of electrons in full
L-shell = 8 So, total number of electrons in an atom = (2 + 8) = 10 electrons
INTEXT QUESTIONS (Page No. 52 A)
1. How will you find the valency of chlorine, sulphur and magnesium?
Ans. Valency of chlorine :
Number of electrons in chlorine atom = 17
Electronic configuration = K L M
2 8 7
Valency of chlorine = 8 – 7 = 1
Valency of sulphur
Number of electrons in sulphur atom = 16
Electronic configuration = K L M
2 8 6
Valency of sulphur = 8 – 6 = 2
Valency of magnesium
Number of electrons in magnesium atom = 12
Electronic configuration = K L M
2 8 2
Valency of magnesium = number of electrons in the valence shell = 2
Intext Questions (Page No. 52B)
Q. 1. If a number of electrons in an atom is 8 and the 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?
Ans. (i) Atomic number = Number of protons Then atom has 8 protons Hence, the atomic number of the atom is 8.
(ii) There are 8 electrons and 8 protons in the atom. An electron has a unit
–ve charge while the proton has unit +ve charge. Thus Total negative charge = 8 (–1) = – 8 Total positive charge = 8 × 1 = 8 Net charge = – 8 + 8 = 0 Thus, the atom has no net charge.
INTEXT QUESTIONS (Page No. 52)
Q. 1. How will you find the valency of chlorine, sulphur and magnesium?
Ans. Valency of chlorine: Atomic number of chlorine = 17
Valency of chlorine = 8 – 7 = 1 Valency of sulphur Atomic number of sulphur = 16
Electronic configuration = 2 8 6 (K L M) Valency of sulphur = 8 – 6 = 2 Valency of magnesium Atomic number of magnesium = 12
Valency of magnesium = number of electrons in the valence shell = 2
Q. 2. If a number of electrons in an atom is 8 and the 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?
Ans. (i) An atomic number of the atom = number of protons in its nucleus (ii) The charge on the atom is zero because a total number of positive charges due to the protons is equal to a total number of negative charges due to electrons.
Q. 3. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Ans. For oxygen atom : Number of protons = 8 Number of neutrons = 8
The mass number of oxygen = Number of proton + Number of neutrons
= 8 + 8 = 16 u For sulphur atom : Number of protons = 16 Number of neutrons = 16 Mass number of sulphur = Number of protons + Number of neutrons = 16 + 16 = 32 u
Intext Questions (Page No. 52B)
2. If a number of electrons in an atom is 8 and the 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?
Ans. (i) The atomic number of the atom = number of protons in its nucleus = 8
(ii) The charge on the atom is zero because a total number of positive charges due to the protons is equal to the total number of negative charges due to electrons.
3. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Oxygen O 8 8 8
Sulphur S 16 16 16
| Element | Symbol | No. of Protons | No. of Neutrons | No. of Electrons |
| Oxygen | 0 | 8 | 8 | 8 |
| Sulphur | S | 16 | 16 | 16 |
INTEXT QUESTIONS (Page No. 53) Q. 1. For the symbol H, D and T tabulate three subatomic particles found in each of them :
Ans:
Q. 2. Write the electronic configuration of any one pair of isotopes and isobars.
Ans. Electronic configuration of a pair of isotopes :
