The atomic number of an element X is 8 and that of element Y is 4. Both these elements can exhibit a valency of:

The four atomic species can be represented as follows. Out of these, the two species which can be termed isobars are:

A.

B.

C.

D.

There are four elements P, Q, R and S having atomic numbers of 4, 18, 10 and 16 respectively. The element which can exhibit covalency as well as electrovalency will be:

The isotopes of an element contain:

What is the number of valence electrons in:

(a) sodium ion, Na⁺

(b) oxide ion, O^2-