Topic outline

    • Structure Of The Atom   

      Q27. Number of valence electrons in Cl ion are:

      (a) 16 (b) 8 (c) 17 (d) 18

      Ans. (b) 8

      Explanation: Electronic configuration of Cl (17) = K-2, L-8, M-7. Thus, it has 7 valence electrons. Cl gains 1 electron to form Cl ion. So, number of valence electrons = 7 + 1 = 8.


      Q28. Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17)

      Ans. Atomic number of chlorine atom = 17

      So, its electronic configuration is

      K L M

      2 8 7

      L shell of chlorine will have 8 electrons.


      Q29. Which of the following statements about Rutherford’s model of atom are correct?

      (i)   Considered the nucleus as positively charged.

      (ii)  Established that the a-particles are four times as heavy as a hydrogen atom.

      (iii) Can be compared to 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)     

      Rutherford’s model of the atom proposed that, there is a positively charged centre in an atom called the nucleus and the electrons revolve around the nucleus in well-defined orbits.


      Q30. In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

      Ans. The given statement is not correct. Number of neutrons can be equal to or greater than number of protons but number of protons is equal to number of electrons for an atom as it is neutral.


      Q31. Will 35Cl and 37Cl have different valencies? Justify your answer.

      Ans. No, both 35Cl and 37Cl will have same valencies, as they are isotopes. The isotopes have same number of electrons and protons. They differ only in the number of neutrons.


      Q32. 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. 

      Ans. (a) False

      (b) False

      (c) True

      (d) False


      Q33. Fill in the blanks in the following statements.

      (a) Rutherford’s a-particle scattering experiment led to the discovery of the_____.

      (b) Isotopes have same ______ but different ______.

      (c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be______ and _______ respectively.

      (d) The electronic configuration of silicon is _____ and that of sulphur is______.

      Ans. (a) nucleus

      (b) atomic number, mass number

      (c) 0 and 1

      (d) silicon – 2, 8, 4

      sulphur – 2, 8, 6


      Q34. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.




      Mass No.

      Atomic No.

      No. of


      No. of protons

      No. of neutrons























      Q35. Draw a sketch of Bohr’s model of an atom with three shells.



      Q36. What are canal rays?

      Ans. E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle called proton.


      Q37. Na+ has completely filled K and L shells. Explain.

      Ans. Sodium atom (Na), has atomic number =11

      Electronic configuration of Na = K-2, L-8, M-1 

      Sodium atom (Na) looses 1 electron to become stable and form Na+ ion. Hence it has completely filled K and L shells.


      Q38. Helium atom has 2 electrons in its valence shell but its valency is not 2. Explain.

      Ans. Helium atom has only one shell (K shell) which can have maximum 2 electrons. The atoms of elements, having a completely filled outermost shell show little chemical activity. Thus, their combining capacity or valency is zero.


      Q39. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

      Ans. Thomson proposed that:

      (i) An atom consists of a positively charged sphere and the electrons are embedded in it.

      (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.


      Q40. Find out the valency of the atoms represented by the Fig. (a) and (b).

      Ans. (a) It has electronic configuration = 2, 8, 8. Its outermost shell has complete octet. Hence, its valency = 0.

      (b) It has electronic configuration = 2, 7. It can easily gain one electron to complete its outermost octet. Hence, its valency = 1.


      Q41. 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.

      Isotopes are atoms of the same element having the same atomic number but different mass numbers. The isotopes of an element differ in the number of neutrons in their nuclei.


      Q42. Why do Helium, Neon and Argon have a zero valency?

      Ans. Helium has only K shell and it is completely filled with 2 electrons. Argon and neon have 8 electrons in their outermost shell which is the maximum number of electrons that can be accommodated in the outermost shell. Thus, they have stable electronic configuration. They neither lose electrons nor gain electrons. Hence, their valency is zero.


      Q43. Composition of the nuclei of two atomic species X and Y are given as under














      Give the mass numbers of X and Y. What is the relation between the two species?

      Ans. Mass number of X = Protons + Neutrons

                                        = 6 + 6 = 12

      Mass number of Y = Protons + Neutrons 

                                 = 6 + 8 = 14

      As the atomic number is same i.e., = 6.

      Both X and Y are isotopes of same element.


      Q44. An element X has a mass number 4 and atomic number 2. Write the valency of this element?

      Ans. Atomic number = 2

      Hence, number of electrons = 2

      Element has only one shell (K shell) which can have maximum 2 electrons. The atoms of elements, having a completely filled outermost shell show little chemical activity. Thus, their combining capacity or valency is zero.


      Q45. The average atomic mass of a sample of an element X is 16.2 u.

      What are the percentages of isotopes in the sample?