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Identify a molecule which does not exist.
easy
Chemical bonding and Molecular structure
2020
chemistry
Explanation To determine which molecule does not exist, we need to consider the molecular orbital theory. \\
Molecular orbital theory helps us understand the stability of diatomic molecules by calculating the bond order. \\
The bond order is given by the formula: \\
Bond order = N b − N a 2 = \frac{N_b - N_a}{2} \\
= 2 N b − N a
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where
is the number of electrons in bonding orbitals and
is the number of electrons in antibonding orbitals.
Let's analyze each option:
Option 1:
Li
• Li has 3 electrons. In
Li
there are 6 electrons in total.
• Electron configuration:
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 \sigma_{1s}^2 \sigma_{1s}^*{}^2 \sigma_{2s}^2 \\
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 • Bond order
= 4 − 2 2 = 1 = \frac{4 - 2}{2} = 1 \\
= 2 4 − 2 = 1 • Since the bond order is positive,
Li
exists.
Option 2:
C
• C has 6 electrons. In
C
there are 12 electrons in total.
• Electron configuration:
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 σ 2 s ∗ 2 π 2 p x 2 π 2 p y 2 \sigma_{1s}^2 \sigma_{1s}^*{}^2 \sigma_{2s}^2 \sigma_{2s}^*{}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \\
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 σ 2 s ∗ 2 π 2 p x 2 π 2 p y 2 • Bond order
= 8 − 4 2 = 2 = \frac{8 - 4}{2} = 2 \\
= 2 8 − 4 = 2 • Since the bond order is positive,
C
exists.
Option 3:
O
• O has 8 electrons. In
O
there are 16 electrons in total.
• Electron configuration:
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 σ 2 s ∗ 2 σ 2 p z 2 π 2 p x 2 π 2 p y 2 π 2 p x ∗ 1 π 2 p y ∗ 1 \sigma_{1s}^2 \sigma_{1s}^*{}^2 \sigma_{2s}^2 \sigma_{2s}^*{}^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^*{}^1 \pi_{2p_y}^*{}^1 \\
σ 1 s 2 σ 1 s ∗ 2 σ 2 s 2 σ 2 s ∗ 2 σ 2 p z 2 π 2 p x 2 π 2 p y 2 π 2 p x ∗ 1 π 2 p y ∗ 1 • Bond order
= 10 − 6 2 = 2 = \frac{10 - 6}{2} = 2 \\
= 2 10 − 6 = 2 • Since the bond order is positive,
O
exists.
Option 4:
He
• He has 2 electrons. In
He
there are 4 electrons in total.
• Electron configuration:
σ 1 s 2 σ 1 s ∗ 2 \sigma_{1s}^2 \sigma_{1s}^*{}^2 \\
σ 1 s 2 σ 1 s ∗ 2 • Bond order
= 2 − 2 2 = 0 = \frac{2 - 2}{2} = 0 \\
= 2 2 − 2 = 0 • Since the bond order is zero,
He
does not exist.
Therefore, the molecule that does not exist is
He
, which corresponds to Option 4.