IX Atomic Structure CBSE Board Solved Questions

Q. Write two differences between isobars and isotopes.

Ans:  Isobars          (i) Isobars have the same mass number but different atomic numbers (ii) Isobars have different chemical properties Isotopes Isotopes have the same atomic number but different mass numbers. Isotopes have similar chemical properties.

Q. Why do isotopes shows similar chemical properties.

Ans: Due to same number of electron in valance shell

Q. How Rutherford concluded that the center part of any atom not empty?

Ans: When Rutherford experimented with alpha particles, then he realized that few particles bounced back directly. Rutherford’s result lead him to believe that most of the foil was made of empty space, but had extremely small, dense lumps of matter inside, which is present only at the center because from center, few particles bounced back. All other particles deflected at different angles. So Rutherford concluded that the center part of any atom is nucleus not empty.

Q. Why an atom of an element is very stable even electrons are in continuous motion?

Ans: The electron could revolve around the nucleus in certain orbits called energy shells and each orbit have different radius. The electrons which are close to nucleus have low energy and the electrons which are farther from the nucleus have high energy .Due to this the electrons do not loose energy and remain stable.

Q. What was the limitation of J .J. Thomson’s model of an atomic?

Ans: When an electron moves around a nucleus, it must continuously radiate out energy and hence, gradually move towards the nucleus in a spiral path, till it collides with the nucleus. Hence atoms are unstable.

Q. When a gold sheet is bombarded by a beam of Alpha particles, only a few of them get deflected whereas most go straight undeflected. Why?

Ans: This is because the nucleus has much smaller volume than that of an atom. The size of atom is 10-10m and size of nucleus is 10-15 m.

Q On the basis of Thomson’s model explains how the atom is neutral.

Ans: J.J. Thomson believed that an atom is made up of positively charged substance in the form of a sphere. Electrons are embedded same way as the apples are embedded in an apple pie. Furthermore, the total positive charge of the sphere is equal to total negative charge of the electrons and hence the atom remained electrically neutral.

Q.Why did Goldstein give the name canal rays to rays that travel from anode to that cathode

Ans: This is because these rays cross the canals of the cathode and reach the other side

Q.What is the limitations of Thomas atomic model?

Ans: Thomson’s model could successfully explain the electrical neutrality of atom. However, it failed to explain how the positively charged particles are shielded from the negatively charged electrons without getting neutralized.

MCQ Class IX – X Science (Chemistry)

1. The charge/ mass ratio of electron:

a. Depends on the nature of the electrodes

b. Depends upon nature of the gas c. Remains constant

d. Depends on both nature of the gas and nature of the electrode

2. A student weighs 30kg. Suppose his body is entirely made of electrons. How any electrons are there in his body? Mass of an electron= 9.1X10-31kg

a. 3.29 X1031

b. 3.29 X1030

c. 3.29 X1023

d. 3.29 X1032

3. Which of the following is correct?

Column 1

Column2

A. Electrons

i. Positive charge

B. Protons

ii. No charge

C. Neutrons

iii. Negative charge

a. A-iii, B-ii, C-i

c. A-ii, B-iii, C-i

b. A-iii, B-i, C-ii

d. A-ii, B-i, C-iii

4. If K, L, M, N shells of an atom are full, the total number of electrons in the atom are:

a. 60

b. 26

c. 42

d. 36

5. Which of the following are positively charged ions:

Atoms

Protons

Electrons

Neutrons

A

17

17

18

B

12

10

12

C

16

17

20

D

1

0

0

E

18

18

22

F

10

10

10

a. A and B

b. C and D

c. B and D

d. D and F

6. The electronic configuration of Cl(17) is:

a. 2,8,7

b. 2,2, 8,5

c. 2,8,2,5

d. 2,2,5,8

7. Composition of the nuclei of two atomic species are given:

X

Y

P

7

8

N

9

8

The mass number of x and Y and their relation is

a. 16,16; isotopes

b. 17,15; isotopes

c. 17,15; isotopes

d. 16,16; 1sobars

8. Na+ has 12 neutrons and 10 electrons. Which of the following statements is correct?

a. Na+ has atomic number 10 and mass number 22.

b. Na+  has atomic number 11 and mass number 23.

c. Na+  has atomic number 10 and mass number 23.

d. Na+  has atomic number 11 and mass number 22.

9. Which of the following statement is correct about proton?

a. It is the nucleus of deuterium                        b. It is an ionized hydrogen molecule

c. It is an ionized Hydrogen atom              d. It is an α particle with unit positive charge

10. The highest value of e/m ratio for anode rays is observed when the discharge tube is filled with:

a. N2

b. H2

c. He

d. Ar

11. When a gold foil is bombarded by a beam of α particle, only a few of them get deflected whereas most go straight undeflected. This is because

a. The force of attraction exerted on α particle by the electrons is insufficient

b. The volume of nucleus is much smaller than that of the atom

c. The force of repulsion acting on the fast moving α particles is very small

d. The neutrons have no effect on α particles

12. Which of the following statements does not belong to Bohr’s model?

a. Energy of the electrons in the orbit is quantized

b. The electrons in the orbit nearest to the nucleus is the lowest energy

c. Electrons revolve around the nucleus in different orbits having fixed energies

d. The electrons radiate energy during revolution due to force of attraction between nucleus and electrons

13. How many electrons, protons and neutrons are present in X+, if atomic number of X is 19 and its mass number is 39

a. E=19, P=19, N= 20

b. E=18, P=19, N= 20

c. E=18, P=19, N= 19

d. E=19, P=20, N= 20

14. Which of the following does not have 8 valence electrons:

a. He

b. Ne

c. Ar

d. Cl-

15. Which of the following does not have one electron in its valance shell

a. Na                                                  b. Li

c. H                                                    d. Ca

16. The electronic configuration of Cl- ion is:

a. 2,8,7

b. 2,8,8

c. 2,8,6

d. 2,8,8,1

17. Which of the following are isotopes:11H, 12D,13T,11 H+

a. 11 H, 12D,13 T

b. 11 H, 12D, 11 H+

c. 11H, 11H+

d. 11 H, 13 T, 11 H+

18. Which of the following are isobars:18Ar40,19 K39 ,20 Ca40, 19[K+]39

a. 19 K39 , 19+[K+]39

b. 18 Ar40,19 K39

c. 18 Ar40,20 Ca40,

d. 18 Ar40,19 K39 ,20 Ca40

19. Cathode rays have :

a. Charge only

b. Mass only

c. Charge as well as mass

d. Neither charge nor mass

20. The number of valence electrons determines:

a. Physical properties of elements

b. Chemical properties of elements

c. Both physical and chemical properties of elements

d. Neither physical nor chemical properties of elements

Answer:

1. c 2. a 3. b 4. a 5. c 6. a 7. d 8. b 9. c 10. b 11. b 12. d 13. a 14. a 15. d 16. b 17. a 18. c 19. a 20. b

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