Question

Molecular formula of the compound formed by the combination of atoms A and B with 2 and 7 valence electrons in third orbit is ____.

• A. \(AB\)
• B. \(B_2A\)
• C. \(AB_2\)
• D. \(A_2B_7\)

Hint:

Atoms with 7 valence electrons generally have valency 1, while atoms with 2 valence electrons usually have valency 2.

Answer 1

The Correct Option is A

Concept:
Valence electrons determine the combining capacity or valency of atoms.
• An atom with 2 valence electrons tends to lose 2 electrons.
• An atom with 7 valence electrons tends to gain 1 electron. The formula of a compound depends on balancing the valencies of the combining atoms.

Step 1: Finding the valencies of A and B.
Atom \(A\) has 2 valence electrons. Therefore, its valency is: \[ 2 \] Atom \(B\) has 7 valence electrons. To complete octet, it needs 1 electron. Hence, its valency is: \[ 1 \] Thus, \[ A = 2, \qquad B = 1 \]

Step 2: Writing the molecular formula using valencies.
Using criss-cross method: \[ A^{2} \quad B^{1} \] After criss-crossing valencies: \[ AB_2 \] Therefore, the molecular formula formed is: \[ \boxed{AB_2} \] Hence, the correct option is: \[ \boxed{(3)\ AB_2} \]