Define an 'isotope'. Give reasons why isotopes have same chemical but different physical properties.

An element 'A' has mass number 23 and atomic number 11. State the —

(i) no. of neutrons in it's shell,
(ii) electronic configuration of the element 'A'.

What are 'energy levels'? Explain the arrangement and distribution of electrons in the various shells with reference to an atom in general and to an atom of potassium $^{39}_{19}\text{K}$ with special reference to the 2n² rule.

Draw the geometric atomic structure of each of the following atoms showing the number of electrons, protons and neutrons in each of them :

(i) $^{12}_6 \text{C}$

(ii) $^{23}_{11} \text{Na}$

(iii) $^{31}_{15}\text{P}$

(iv) $^{39}_{19} \text{K}$

(v) $^{40}_{20} \text{Ca}$