Four elements A, B, C, D are given :

A shows the presence of 20 neutrons, 17 protons and 17 electrons.

B shows the presence of 18 neutrons, 17 protons and 17 electrons.

C shows the presence of 10 neutrons, 9 protons and 10 electrons.

D shows the presence of 4 neutrons, 3 protons and 2 electrons.

State which of the above is —

(a) an anion

(b) a cation

(c) a pair of isotopes.

Write the formula of the compound formed between D and C.

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

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

What are noble gases. Give a reason why noble gases have stable electronic configuration.