What is the significance of atomic number in the modern periodic table ?

Arrange the following in order of :

(i) increasing radii.

(a) Cl^-, Cl
(b) Mg²⁺, Mg, Mg⁺
(c) N, O, P

(ii) increasing ionisation energy.

(a) P, Na, Cl
(b) F, O, Ne
(c) Ne, He, Ar

Explain your choice.

Give reasons for the following assertions :

(a) The oxidising power of elements increases from left to right along a period.

(b) Ionisation potential of elements increases across a period from left to right.

(c) Alkali metals are good reducing agents.

Chlorine in the Periodic Table is surrounded by the elements with atomic number 9, 16, 18, and 35.

(a) Which of these have Physical and Chemical properties resembling chlorine ?

(b) Which is more electronegative than chlorine ?