Q85 / 200 · NEET 2010 (Screening) · Chemistry
Which one of the following species does not exist under normal conditions ?
- A$\mathrm{Be}_2^{+}$
- Correct answerB$\mathrm{Be}_2$
- C$\mathrm{B}_2$
- D$\mathrm{Li}_2$
Solution
Key Idea Molecules with zero bond order, do not exist.
(a) $\mathrm{Be}_2^{+}(4+4-1=7)=$
$$
\begin{aligned}
& \sigma 1 \mathrm{~s}^2, \sigma 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^1 \\
& \mathrm{BO}=\frac{4-3}{2}=0.5
\end{aligned}
$$
(b) $\mathrm{Be}_2(4+4=8)=\sigma 1 \mathrm{~s}^2, \sigma 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2$
$$
\mathrm{BO}=\frac{4-4}{2}=0
$$
(c) $\mathrm{B}_2(5+5=10)$
$$
\begin{gathered}
=\sigma 1 \mathrm{~s}^2, \sigma 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2, \pi 2 p_{\mathrm{x}}^1 \approx \pi 2 \mathrm{p}_{\mathrm{y}}^1 \\
\mathrm{BO}=\frac{6-4}{2}=1
\end{gathered}
$$
(d)
$$
\begin{aligned}
& \operatorname{Li}_2(3+3=6) \\
& =\sigma 1 \mathrm{~s}^2, \sigma 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2 \\
& =\frac{4-2}{2}=1
\end{aligned}
$$
$\mathrm{BO}=\frac{4-2}{2}=1$
Thus, $\mathrm{Be}_2$ does not exist under normal conditions.
Hence, the correct answer is option: 'B'.