Question

Which of the following orders are correct against the property given? I) dipole moment : $NF_3 > NH_3 >BF_3$ II) Covalent bond length : $C - O > N - O > O - H$ III) Bond order : $C_2 > B_2 > He_2$

• A. I, II only
• B. II, III only
• C. I, III only
• D. I, II , III

Answer 1

Answer: (B)

II, III only

Answer 2

(I) In $BF _{3}$, electronegativity of $F$ -atom is greater than $B$.

In $NH _{3}$, electronegativity of $N$ -atom is greater than $H$. Therefore, direction of all dipole moment with lone pairs lies in same direction, hence net dipole moment increases.

In $NF _{3}$, electronegativity of $F$ is greater than $N$. Therefore, direction of dipole moment are opposite from the lone pair, hence, net dipole decreases.

Correct order of dipole moment will be

$NH _{3}>\, NF _{3}>\, BF _{3}$

(II) Covalent bond length

$C - O > \,N - O >\, O - H$

as electronegativity difference increases then bond length decreases but lone pair-lone pair repulsion increases the bond length.

(III ) Bond order = Number of bonding electron $\left(N_{b}\right)$
$\frac{\text { - Number of antibonding electron }}{2}$

$C _{2}=\sigma l s^{2}