Question

Which element was not known when Mendeleev proposed his classification?

(a) Germanium

(b) Sodium

(c) Hydrogen

(d) Oxygen​

Answer 1

Answer:

your ans is (a)

Explanation:

he elements which were unknown when Mendeleev gave his classification were geranium, gallium and scandium. He left gaps for the unknown elements and predicted correctly the properties of some of the unknown elements.