Draw the Lewis structure for the selenium dioxide (
) molecule. Be sure to include all resonance structures that satisfy the octet rule.
The Correct Answer and Explanation is:
Lewis Structure of Selenium Dioxide (SeO₂)
Correct Lewis Structures (Resonance Forms):
Selenium dioxide (SeO₂) is a bent molecule with resonance. Below are the two main resonance structures that satisfy the octet rule:
Resonance Structure 1:
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O = Se - O⁻
..
(with a negative formal charge on the single-bonded oxygen and a positive charge on Se)
Resonance Structure 2:
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⁻O - Se = O
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(with the negative formal charge on the other single-bonded oxygen and a positive charge on Se)
These structures represent delocalized bonding where the double bond can exist with either oxygen atom.
Explanation
Selenium dioxide (SeO₂) is a linear molecule in the gas phase but often represented with a bent structure in its Lewis form to emphasize electron geometry and resonance. Selenium belongs to Group 16, like oxygen, and has 6 valence electrons, while each oxygen atom also contributes 6 electrons. The total number of valence electrons in SeO₂ is:
- Se = 6 electrons
- O = 6 electrons × 2 = 12 electrons
- Total = 18 valence electrons
In constructing the Lewis structure, we start by placing selenium (the central atom) bonded to two oxygen atoms. Each bond requires 2 electrons, using 4 of the 18 available. The remaining 14 electrons are distributed to satisfy the octets, usually by placing lone pairs on the oxygen atoms first.
If both oxygen atoms are single-bonded to selenium, selenium would need more than 8 electrons to hold all lone pairs and bonds — violating the octet rule. However, selenium is a third-period element and can expand its octet. Still, we aim to draw resonance structures that satisfy the octet rule where possible.
To satisfy the octet rule for each atom, one oxygen is double-bonded to selenium, and the other is single-bonded with a full negative charge. The single-bonded oxygen has three lone pairs (6 electrons), the double-bonded oxygen has two lone pairs (4 electrons), and selenium has one lone pair in addition to its bonding electrons.
Because either oxygen can form the double bond, the structure resonates between two forms. These resonance structures stabilize the molecule by delocalizing the electrons over both oxygen atoms.
