CBSE - Chemistry - Structure of the Atom

9. The ion of an element has 3 positive charges. Mass number of the 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
Explanation: Mass number = 27 and number of neutrons = 14
Hence, number of electrons in atom = 27 � 14 = 13
Now, ion has 3 positively charges, so number of electrons in the ion = 13 � 3 = 10

10. Identify the Mg2+ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons respectively
(a) (b)(c)(d)

(d)
Explanation: Electronic configuration of Mg atom is: 2, 8, 2
So, electronic configuration of Mg2+ ion is; 2, 8.

11. In a sample of ethyl ethanoate (CH₃COOC₂H₅) the two oxygen atoms have the same number of electrons but 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

Explanation: Isotopes are elements with same atomic number but different mass numbers. The different in mass number is because of different number of neutrons in them.

12. Elements 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
Explanation: If an element show positive valency then it is a metal; otherwise it is a nonmetal.

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

14. An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4
Ans. (c) 1
Explanation: Electronic configuration of this element: 2, 1
Since number of electron in outermost shell is 1, hence valency is 1.

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. (a) 2, 8, 3
Explanation: Atomic number of aluminium is 13 and the first shell can have at the most two electrons in it only. Hence option (a) is correct.

16. Which of the following in Fig. 4.2 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)
Explanation: The second shell cannot have more than 88 electrons and first shell cannot have more than 2 electrons.

17. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Ans. (a) An atom has equal number of electrons and protons.

18. Atomic models have been improved over the years. Arrange the following atomic 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)