Consider isotopes of oxygen 16O and 18O. Would you able to place them in Mendeleev’s periodic table ?

Ans. No, we cannot place 16O and 18O in Mendeleev’s periodic table.

Q. Find resemblance between hydrogen and alkali metals by writing

compounds of both with chlorine, sulphur and oxygen. Ans. Alkali metals are : Li, Na, K, Rb, Cs, Fr.

H2O Na2O
2S Na2S
HCl NaCl
Hydrogen and alkali metals form similar formulae with, oxygen, sulphur and chlorine.

Q. Find out a pair of elements from the periodic table where higher mass element is placed before lower mass element.

Ans. Co Ni ; Te I 

(58.9) (58.7) (12.8) (12.7)