An element has an atomic number of 19 and atomic mass of 39.

(i) How many electrons are there in each atom of the element?

(ii) How are these electrons arranged in the atom?

                         Or

Write electronic distribution in an atom of potassium.

Compare the properties of electron, proton and neutron.

What are the limitations of J.J. Thomson’s model of atom?

What are the limitations of Rutherford model of the atom?

Describe Bohr's model of the atom ?

Compare all the proposed models of an atom given in this chapter.

Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Define valency by taking examples of silicon and oxygen.

Explain with examples, (i) Atomic number, (ii) Mass number, (iii) Isotopes, and (iv) Isobars. Give any two uses of isotopes also.

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