Structure of Atom NCERT Questions

Structure of Atom
Class: 9 | Chapter: 4

NCERT Questions & Answer

1. What are canal rays?
Answer: Canal rays are positively charged particles emitted from anode of a discharge tube provided low pressure of a gas and high voltage current.

2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: No, it will not carry any charge.

3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: Atom is nuetral as a whole becouse it has equal number of protons ( positive charge) and electrons ( negative charge).

4. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: Protons are present in the nucleus of an atom.

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


6. What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer: The use of gold metal was due to its malleable narure, so we could get a very thin foil to obtain good observations but if we take foil of a metal other than gold then that can give other unexpected results.

7. Name the three sub-atomic particles of an atom.
Answer: Protons, electrons and nuetrons.

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, Mass No. = No. of protons + No. of nuetrons 4 = 2 + No. of nuetrons So, No. Of nuetrons = 2

9. Write the distribution of electrons in carbon and sodium atoms.
Answer: Carbon has atomic no. 6 . It's electronic configurarion is 2,4 .
Sodium has atomic no. 11 , It's electronic configurarion is 2,8,1.

10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: K shell can have maximum of 2 electrons and L shell can have 8 electrons. So, if these both shells are full then in total 2 + 8 = 10 electrons would be there in the atom.

11. How will you find the valency of chlorine, sulphur and magnesium?
Answer: Trick for valency - 1. If last shell containing 1/2/3/4 electrons, then No. Of electrons = Valency
2. If last shell containing more than 4 electrons,then 8 - no. of electrons = Valency
a. For chlorine atomic no. Is 17 . So, it's electronic configurarion is 2,8,7 . Therefore valency will be 8 - 7 = 1.
b. For Sulphur atomic no. Is 16. So, it's electronic configurarion is 2,8,6. Therefore valency will be 8 - 6 = 2.
c. For Magnesium atomic no. is 12, So, it's electronic configurarion is 2,8,2.Therefire valency will be 2.

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: (i) Atomic no. Will be equal to no. of protons i.e. 8 .
(ii) Charge will be zero becouse protons and electrons are equal.

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

Answer: Mass no. of oxygen = p + n => 8 + 8 = 16.
Mass no. of sulphur = p + n => 16 + 16 = 32.

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

	Electrons	Protons	Nutrons
Protium	1	1	0
Deuterium	1	1	1
Tritium	1	1	2

15. Write the electronic configuration of any one pair of isotopes and isobar.
Answer: Isoropes : Carbon -12 , Carbon - 13 , Carbon - 14 .All have same atomic no. 12. So, electronic configurarion will be 2,4 for all.
Isobars : Nitrogen (¹⁴₇N) and Carbon (¹⁴₆C)
¹⁴₇ Nitrogen ( electronic configurarion) 2,5 and ¹⁴₆Carbon ( electronic configurarion) 2,4.

Exercise Questions with Answer

1. Compare the properties of electrons, protons and neutrons.
Answer:

Electrons	Protons	Nutrons
negatively Charged	Positively Charged	Neutral
Present outside the nuclus	Present in the nuclus	Present in the nuclus

2. What are the limitations of J.J. Thomson’s model of Atom ?
Answer: The results of other scientists experiments were not in accordance with J. J. Thomson's model like scattering experiment of Rutherford.
It could not explain about the stability of the atom and presense of nucleus in the atom.

3. What are the limitations of Rutherford’s model of the atom?
Answer: 1. It could not explain law of electrodynamics. Acc. to this, the accelerated charged particles must emit radiation or energy and by
reducing radius it should fall into the nucleus but atom never collapses in this way.
2. It could not explain about line spectra of atoms.

4. Describe Bohr’s model of the atom.
Answer: 1. Only special orbits of fixed energy known as discrete orbits of electrons are allowed inside the atom.
2. These fixed energy orbits are called energy levels and are represented by K, L, M, N... etc.
3. The electrons while revolving in these discrete orbits don't radiate energy.

5. Compare all the proposed Bohr’s models of an atom given in this chapter.
Answer:

6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Answer: 1. The maximum no. of electrons present in a shell is 2n², where 'n' is the energy level no.
2. The maximum no. of electrons that can be accommodated in the outermost orbit is 8.
3. 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 : number of electrons lost or gained by an electron to achieve stable configurarion.
Silicon has atomic no. 14 , it's configurarion is 2,8,4. So, valency is 4 to become stable.
Oxygen has atomic no. 8 , it's configurarion is 2,6. So, valency is 2 to become stable.

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 : It is equal to no. of protons or no. of electrons in nuetral atom. It has symbol Z. Example : Atomic no. of Hydrogen is 1.
(ii) Mass number : It is equal to the sum of no. of protons and nuetrons. It has symbol A. Example : Mass no. of carbon is 12 ( 6 p + 6 n).
(iii) Isotopes : These are the atoms of an element having same atomic no. but different mass numbers. Example : Carbon - 12 , Carbon - 13 and Carbon - 14 have same atomic no. 6 but different mass numbers 12,13 and 14.
(iv) Isobars : These are the atoms of different elements having same mass numbers but different atomic numbers. Example : ⁴⁰₁₈Ar and ⁴⁰₁₉K.
Uses of isotopes : 1. Isotopes of uranium is used as a fuel in nuclear reactors.
2. Isotope of Cobalt is used in treatment of cancer.

9. Na⁺ has completely filled K and L shells. Explain
Answer: Na has electronic configurarion 2,8,1 . By losing last electron it has electronic configurarion 2 electrons in K shell and 8 electrons in L shell. Hence, Na+ has completely filled K and L shells.

10. If bromine atom is available in the form of say, two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom.
Answer: The average atomic mass of bromine atomic = 79 × 49.7 /100 + 81 × 50.3 / 100 => 80.006 = 80 amu.

11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample?
Answer: Let the percentage of ¹⁶₈ X be a and the percentage of ¹⁸₈ X be 100 - a.
Now, the average mass = ( 16 × a / 100 ) + [ 18 × 9 (100 - a) / 100 ] = 16.2
=> 16 a + 1800 - 18 a = 1620
=> a = 90
=> percentage of 188 X = 100 - 90 = 10
So, the isotope 168 X has 90 % and 188 X has 10 % .

12. If Z = 3, what would be the valency of the element? Also, name the element.
Answer: Z = 3 , it's electronic configurarion is 2,1 . So , valency is 1. The elements is Lithium.

13. Composition of the nuclei of two atomic species X and Y are given as under

	X	Y
Protons	6	6
Neutrons	6	8

Give the mass number of X and Y. What is the relation between the two species?
Answer: Mass no. of X = p + n => 6 + 6 = 12
Mass no. of Y = p + n = 6 + 8 = 14 .
X and Y are isotopes becouse these have same no. of protons.

14. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore,it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer: (a) F
(b) F
(c) T
(d) T

15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of (a) Atomic nucleus (b) Electron (b)Proton (d)neutron
Answer: (a) Atomic nucleus

16. Isotopes of an element have (a) the same physical properties (b)different chemical properties(c) different number of neutrons (d) different atomic numbers
Answer: (c) different number of neutrons

17. Number of valence electrons in Cl^-ion are : (a) 16 (b) 8 (c) 17 (d) 18
Answer: (b) 8

18. Which one of the following is a correct electronic configuration of sodium ?
(a) 2, 8 (b) 8, 2, 1 (c) 2, 1, 8 (d) 2, 8, 1
Answer: d) 2, 8, 1

19. Complete the following table :

Answer: