NCERT Solution Class X Science Periodic Classification of Elements Question 4 (Ex 5.3)

Question 4:

(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?

Answer:

(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have

one electron in their outermost shells.

(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet

in its K shell, while neon has an octet in its L shell.

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