Which of the Lewis electron dot representations is correct for the cyanide ion, CN⁻¹?
[C=N]⁻¹ [C≡N]⁻¹
Answer (1)
Calculate the total number of valence electrons: 4 + 5 + 1 = 10 .
Determine the Lewis structure with a triple bond: [ : C ≡ N : ] − .
Calculate formal charges: Carbon (-1), Nitrogen (0).
The correct Lewis structure is [ : C ≡ N : ] − .
Explanation
Understanding the Problem The problem asks us to identify the correct Lewis structure for the cyanide ion, CN-. To do this, we need to consider the number of valence electrons, the octet rule, and formal charges.
Calculating Valence Electrons First, we need to calculate the total number of valence electrons in the cyanide ion (CN-). Carbon (C) has 4 valence electrons, and nitrogen (N) has 5 valence electrons. The negative charge indicates an additional electron. Therefore, the total number of valence electrons is: 4 + 5 + 1 = 10
Determining the Lewis Structure Now, let's consider possible Lewis structures. We want to satisfy the octet rule for both carbon and nitrogen. This means each atom should have 8 electrons around it. We can achieve this with a triple bond between carbon and nitrogen. The Lewis structure would be: [ : C ≡ N : ] − In this structure, carbon has 3 bonding electrons + 1 lone pair electron, totaling 4 + 4 = 8 electrons. Nitrogen has 3 bonding electrons + 1 lone pair electron, totaling 5 + 3 = 8 electrons.
Calculating Formal Charges Let's calculate the formal charges for this structure. Formal charge = (Valence electrons) - (Non-bonding electrons) - (1/2 * Bonding electrons) For carbon: 4 − 2 − ( 1/2 ∗ 6 ) = 4 − 2 − 3 = − 1 For nitrogen: 5 − 2 − ( 1/2 ∗ 6 ) = 5 − 2 − 3 = 0 The formal charges are -1 on carbon and 0 on nitrogen. This is a reasonable distribution of formal charges.
Final Answer The correct Lewis structure for the cyanide ion (CN-) is [ : C ≡ N : ] − . This structure satisfies the octet rule and has a reasonable distribution of formal charges.
Examples
Lewis structures are used in chemistry to visualize the bonding between atoms in a molecule or ion. For example, understanding the Lewis structure of carbon dioxide (CO2) helps explain why it is a nonpolar molecule, even though the C=O bonds are polar. Similarly, understanding the Lewis structure of ozone (O3) helps explain its reactivity and role in absorbing UV radiation in the stratosphere. Lewis structures are fundamental tools for predicting molecular properties and reactivity.
