## Class 9 Science Exemplar Problems

Multiple Choice Questions

Q. 1. Which of the following correctly represent the electronic distribution in the atom?

(a) 3, 8, 1

(b) 2, 8, 2

(c) 1, 8, 3

(d) 8, 2, 2

Ans. (b) 2, 8, 2

Q. 2. Rutherford’s alpha-particles scattering experiment resulted in the discovery of (a) Electron (b) Proton (c) Nucleus in the atom (d) Atomic mass

Ans. (c) Nucleus in the atom

Q. 3. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

Q. 4. Dalton’s atomic theory successfully explained :

(i) Law of conservation of mass

(ii) Law of constant composition

(iv) Law of multiple proportions

(a) (i), (ii) and (iii)

(b) (i), (iii) and (iv)

(c) (ii), (iii) and (iv)

(d) (i), (ii) and (iv)

Ans. (d) (i), (ii) and (iv)

Q. 5. Which of the following statements about Rutherford’s model of an atom are correct?

(i) considered the nucleus as positively charged

(ii) established that the –particles are four times as heavy as a hydrogen atom

(iii) can be compared by the solar system

(iv) was in agreement with Thomson’s model

(a) (i) and (iii) (b) (ii) and (iii) (c) (i) and (iv) (d) only (i)

Ans. (a) (i) and (iii)

Q. 6. Which of the following are true for an element?

(i) Atomic number = number of protons + number of electrons

(ii) Mass number = number of protons + number of neutrons

(iii) Atomic mass = number of protons = number of neutrons

(iv) Atomic number = number of protons = number of electrons

(a) (i) and (ii)

(b) (i) and (iii)

(c) (ii) and (iii)

(d) (ii) and (iv)

Ans. (d) (ii) and (iv)

Q. 7. In Thomson’s model of the atom, which of the following statements are correct?

(i) the mass of the atom is assumed to be uniformly distributed over the atom.

(ii) the positive charge is assumed to be uniformly distributed over the atom

(iii) the electrons are uniformly distributed in the positively charged sphere (iv) the electrons attract each other to stabilise the atom

(a) (i), (ii) and (iii)

(b) (i) and (iii)

(c) (i) and (iv) (c) (i), (iii) and (iv)

Ans. (a) (i), (ii) and (iii)

Q. 8. Rutherford’s alpha–particle scattering experiment showed that :

(i) electrons have a negative charge

(ii) the mass and positive charge of the atom is concentrated in the nucleus (iii) neutron exists in the nucleus

(iv) most of the space in an atom is empty. Which of the above statements are correct : (a) (i) and (iii) (b) (ii) and (iv) (c) (i) and (iv) (d) (iii) and (iv)

Ans. (b) (ii) and (iv)

Q. 9. The ion of an element has 3 positive charges. A mass number of an atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?

(a) 13 (b) 10 (c) 14 (d) 16

Ans. (b) 10

Q. 10. Identify the Mg2+ ion from Fig. where n and p represent the number of neutrons and protons respectively.

Q. 11. In a sample of ethyl ethanoate (CH3 COO C2H5 ) the two oxygen atoms have the same number of electrons but a different number of neutrons. Which of the following is the correct reason for it?

(a) One of the oxygen atoms has gained electrons (b) one of the oxygen atoms has gained two neutrons (c) the two oxygen atoms are isotopes (d) the two oxygen atoms are isobars.

Ans. (c) the two oxygen atoms are isotopes.

Q. 12. Element with valency 1 are :

(a) always metals

(b) always metalloids

(c) either metals or non-metals

(d) always non-metals

Ans. (c) either metals or non-metals

Q. 13. The first model of an atom was given by (a) N. Bohr (b) E. Gold steam (c) Rutherford (d) J.J. Thomson

Ans. (d) J.J. Thomson

Q. 14. An atom has 3 protons and 4 neutrons will have a valency of (a) 3 (b) 7 (c) 1 (d) 4

Ans. (c) 1

Q. 15. The electron distribution in an aluminium atom is : (a) 2, 8, 3

(b) 2, 8, 2

(c) 8, 2, 3

(d) 2, 3, 8

Ans. (d) 2, 3, 8.

Q. 16. Which of the following in figure do not represent Bohr’s model of an atom correctly?

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (ii) and (iv)

(d) (i) and (iv)

Ans. (c) (ii) and (iv)

Q. 17. Which of the following statement is always correct?

(a) An atom has an equal number of electrons and protons

(b) An atom has an equal number of electrons and neutrons

(c) An atom has an equal number of protons and neutrons

(d) An atom has an equal number of electrons, protons and neutrons.

Ans. (a) An atom has an equal number of electrons and protons

Q. 18. Atomic models have been improved over the years. Arrange the following models in the order of their chronological order.

(i) Rutherford’s atomic model

(ii) Thomson’s atomic model

(iii) Bohr’s atomic model

(a) (i), (ii) and (iii)

(b) (ii), (iii) and (i)

(c) (ii), (i) and (iii)

(d) (iii), (ii) and (i)

Ans. (c) (ii), (i) and (iii)