Explain the following:
(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

(a)
Lithium, Sodium and Potassium have following similarities:

(i) They all have valence electrons equal to 1 so they all belong to same group, i.e. Group 1st in Periodic Table.

(ii) They all are highly reactive and form ionic bonds with non-metals.

(iii) They all are highly electropositive..

(iv)All of these react with water to form alkalis.

(b)

Following properties are common in the atoms of Helium and Neon:

(i) Both have completely filled outermost orbit and have stable electronic configuration.

(ii) Both belong to the same group, i.e. zero group in Modern Periodic Table.