Structure of the atom Worksheet-7

Structure of the atom Worksheet-7


  1. Tabulate the three sub atomic particles for Hydrogen, Deuterium, and Tritium.


  1. What are the limitations of J.J. Thomson's model of an atom?


  1. What are the limitations of Rutherford's model of an atom?


  1. Describe valency by taking the examples of silicon and oxygen.


  1. Na+ Ion has completely filled K and L shells. Explain.


  1. If the element bromine is in the form two isotopes which are 35Br79 (49.7%) and 35Br81 (50.3%), then calculate the average atomic mass of bromine.


  1. The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes 8X16 and 8X18 in the sample?


  1. If Z = 3, what would be the valency of the element?


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


  1. How many electrons are present in He2+ ion?





  1. Limitations of Thomson's model of an atom

(i) It could not explain the results of the Rutherford’s scattering experiment.

(ii) It did not have any experimental evidence in its support.


  1. On the basis of Maxwell radiation theory charged particles moving in circular orbits with fast speed emit energy in the form of radiations. With the loss of energy they must come closer and closer to the nucleus of atom. Ultimately they must fall into nucleus or become its part. But an atom is stable. Rutherford could not give suitable explanation for this.


  1. Valency of an element may be defined as the number of electrons its atom loses, gains or shares in order to complete its octet (or duet in case of Hydrogen).


It is the combining capacity of an element.

Valency of silicon (Si) : Atomic number = 14

Its electronic distribution is K (2), L (8), M (4).

It can share 4 electrons to acquire 8 electrons in its outermost orbit. Hence its valency is 4.

Valency of oxygen (O) : Atomic number = 8

Its electronic distribution is; K(2), L(6)

It can gain two electrons to complete its octet. Hence its valency is 2.


  1. Na+ ion is formed by the loss of one electron from its valence shell i.e. M shell. The Na+ ion formed has only two shells which are complete Na : K(2) L(8) M(1)  ;   Na+: K(2) L(8)


  1. Percentage of Br-79 isotope  = 49.7 %

Percentage of Br-81 isotope = 50.3 %

∴ Average atomic mass of bromine




  1. Let the percentage of X-16 isotope = x

∴ The percentage of X-18 isotope =(100 -x)

From the information, the average atomic mass or the element X is : =

But the given atomic mass of the element X = 16.2 u

16x + 1800 – 18x = 16.2 × 100 = 1620

2x = 1620 - 1800 = -180

x = 180/2 = 90

Percentage of X-16 isotope = 90

Percentage of X-18 isotope =10


  1. The configuration of the element with Z = 3 is K (2) L (1). It has one electron in the valence shell i.e. L-shell. Its valency is one. The element is lithium (Li).


  1. Mass number of X = protons + neutrons

          = 6 + 6 = 12

Mass number of Y = 6 + 8 = 14

The species X and Y are the isotopes because their atomic numbers are same and their mass numbers are different.


  1. He2+ Ion has no electrons. It is also called alpha α particle.