(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?

Explanation

(a) Yes, there is a similarity in the atoms of lithium, sodium and potassium elements. All these elements have similar electronic configurations having 1 electron each in their valence shells. The electronic configurations of lithium, sodium and potassium are given below:
Lithium : 2,1
Sodium : 2,8,1
Potassium : 2,8,8,1
(b) Helium and neon are noble gases. The common thing betwen and neon is that both of them have the valence shells (outermost shells) completely filled with electrons. Helium has 2 electrons in its `K` valence shell (`K` shell can accommodated only a maximum of 2 electrons), whereas neon has 8 electrons in its valence shell.