1.The resonance structures of thiocyanate ion, SCN− depicted in Figure are equivalent.
a. True
b. False
2.The best Lewis structure, based on formal charge is:
a. the one in which atoms bear the smallest possible formal charges
b. the one in which the negative formal charges are on the most electronegative atoms
c. the one in which elements have the most possible covalent bonds
d. the one in which elements have the most lone pairs
e. the one in which none of the elements satisfy the rules of valence
The Correct Answer and Explanation is :
- The resonance structures of thiocyanate ion, SCN−, depicted in the figure are equivalent: Answer: b. False Explanation: The resonance structures of the thiocyanate ion (SCN−) are not equivalent, even though they both represent the same molecule. This is because, in different resonance structures, the distribution of electrons and the formal charges on the atoms may vary. In the case of thiocyanate, one resonance structure places a negative charge on the sulfur atom (S), while the other places it on the nitrogen (N) atom. The structures are not equivalent because the sulfur atom and the nitrogen atom have different electronegativities, which affects their ability to accommodate the negative charge. The sulfur atom is less electronegative than nitrogen, so it is less favorable for it to hold the negative charge compared to nitrogen. Therefore, these resonance structures are not equivalent because the placement of formal charges affects the stability of the molecule and does not follow the same distribution in both structures.
- The best Lewis structure, based on formal charge, is: Answer: a. the one in which atoms bear the smallest possible formal charges, and b. the one in which the negative formal charges are on the most electronegative atoms. Explanation: A good Lewis structure should minimize formal charges as much as possible because formal charges represent the deviation from the ideal electron distribution and indicate instability. The formal charge formula is: [
\text{Formal charge} = \text{Valence electrons} – (\text{Lone electrons} + \frac{1}{2} \text{Bonding electrons})
] To achieve the best Lewis structure:
- (a) The Lewis structure should have atoms bearing the smallest possible formal charges. This minimizes the instability that would arise from atoms holding large charges. A structure with formal charges close to zero is more stable.
- (b) Negative formal charges should be placed on the most electronegative atoms because electronegative atoms (such as oxygen, nitrogen, or fluorine) are more likely to accept and hold onto extra electrons. This leads to a more stable structure. The other options:
- (c) Having the most possible covalent bonds isn’t necessarily ideal because too many bonds can lead to overly positive formal charges, destabilizing the structure.
- (d) The number of lone pairs does not directly correlate to the stability of the structure; more lone pairs can contribute to less bonding and potentially higher formal charges.
- (e) None of the elements satisfying the rules of valence would lead to a less stable, incorrect structure. In summary, the best Lewis structure minimizes formal charges, places negative charges on electronegative atoms, and ensures the structure follows the valence rules.