SEBA Class 9 Science: Structure of the Atom
INTEXT QUESTION
1. What are canal rays?
Answer: Canal rays are positively charged radiations seen in a gas discharge. They were discovered by E. Goldstein and helped in identifying the proton.
2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: If an atom has one electron and one proton, it will not carry any charge because the positive and negative charges cancel each other.
INTEXT QUESTION
1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: According to Thomson’s model, an atom is neutral because the positive and negative charges are equal in magnitude. The atom has a positively charged sphere, and the electrons are embedded in it. Since both charges balance each other, the atom as a whole becomes electrically neutral.
2. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: According to Rutherford’s model of an atom nucleus is positively charged; therefore, protons are present inside the nucleus.
3. Draw a sketch of Bohr’s model of an atom with three shells.
Answer: Three shells or orbits are presented by the letters K, L, M, N (or the numbers n=1,2,3,4
4. 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: If α-particle scattering experiment is carried out using a foil of a metal other than gold, there would be no change in the observation. Because the basic structure of an atom remains the same, irrespective of the material.
In the original experiment by Rutherford, a gold foil was taken because gold is malleable and can be easily made as thin as possible when compared to other metals.
INTEXT QUESTION
1. Write the distribution of electrons in carbon and sodium atoms.
Answer: The distribution of electrons in the given atoms is as follows:
Carbon has 6 electrons, so its distribution is 2, 4.
Sodium has 11 electrons, so its distribution is 2, 8, 1.
2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: If the K and L shells of an atom are full, then:
• K shell holds 2 electrons
• L shell holds 8 electrons
So, the total number of electrons in the atom = 2 + 8 = 10 electrons.
INTEXT QUESTION
1. How will you find the valency of chlorine, sulphur, and magnesium?
Answer: To find the valency of chlorine, sulphur, and magnesium, we use the number of electrons in their outermost shell.
Chlorine has 7 electrons in its outermost shell, so it needs 1 more electron to complete 8. Its valency is 1.
Sulphur has 6 electrons in its outermost shell, so it needs 2 more electrons to complete 8. Its valency is 2.
Magnesium has 2 electrons in its outermost shell, so it loses these 2 electrons. Its valency is 2.
INTEXT QUESTION
1. If the 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?
Answer: (i) The atomic number of the atom is 8, because atomic number is equal to the number of protons.
(ii) The charge on the atom is zero, because the number of electrons and protons are equal, so their charges cancel each other.
2. With the help of Table 4.1, find out the mass number of oxygen and sulphur atoms.
Answer: Mass number = Number of protons + Number of neutrons.
Oxygen has 8 protons and 8 neutrons.
Mass number = 8 + 8 = 16
Sulphur has 16 protons and 16 neutrons.
Mass number = 16 + 16 = 32
INTEXT QUESTION
EXERCISES QUESTION
2. What are the limitations of J.J. Thomson’s model of the atom?
Answer: The limitations of J.J. Thomson’s model of the atom are:-
(i) It could not explain the experimental results of the other scientists, such as Rutherford, as there is no nucleus in the atomic model proposed by Thomson.
(ii) It could not explain the stability of an atom, i.e., how positive and negative charges could remain so close together.
3. What are the limitations of Rutherford’s model of the atom?
Answer: Limitations of Rutherford’s model of the atom are as follows:-
(i) It could not explain why a moving charge (electrons) does not lose energy and fall into the nucleus.
(ii) It could not explain the stability of an atom when charged electrons are moving under the attractive force of a positively charged nucleus.
(iii) It could not explain how electrons are arranged in the space outside the nucleus.
4. Describe Bohr’s model of the atom.
Answer: Bohr’s model of the atom can be described as follows:
(i) Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
(iii) These orbits are represented by the letters, K,L,M,N or the numbers n=1,2,3,4
5. Compare all the proposed models of an atom given in this chapter.
6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Answer: The rules for writing the electron distribution are:
(i) The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number.
For the first three shells:
• K–shell (n = 1): 2 × 1² = 2 electrons
• L–shell (n = 2): 2 × 2² = 8 electrons
• M–shell (n = 3): 2 × 3² = 18 electrons
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell unless the inner shells are filled.
7. Define valency by taking examples of silicon and oxygen.
Answer: Valency is the combining capacity of an atom. It depends on how many electrons an atom gains, loses, or shares to complete its outermost shell.
• Silicon has 4 electrons in its outermost shell, so its valency is 4.
• Oxygen has 6 electrons in its outermost shell, so it needs 2 more electrons to complete 8. Therefore, its valency is 2.
8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Answer: (i) Atomic Number:
The atomic number is the total number of protons present in the nucleus of an atom.
Example: Hydrogen has atomic number 1 because it has 1 proton.
(ii) Mass Number:
The mass number is the sum of the total number of protons and neutrons in the nucleus.
Example: Carbon has 6 protons and 6 neutrons, so its mass number is 12.
(iii) Isotopes:
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.
9. Na⁺ has completely filled K and L shells. Explain.
Answer: A sodium atom has 11 electrons with a distribution of 2, 8, 1.
When it becomes Na⁺, it loses one electron from the outermost shell.
So the new electron distribution becomes 2, 8.
This means the K shell has 2 electrons and the L shell has 8 electrons, and both shells are completely filled.
