QQuestionChemistry
QuestionChemistry
Draw the Lewis structure for the polyatomic phosphite $\left(\mathrm{PO}_{3}^{3 -}\right)$ anion. Be sure to include all resonance structures that satisfy the octet rule.
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Answer
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Step 1:: Determine the total number of valence electrons in the $\text{PO}_3^{3 -}$ anion.
Phosphorus (P) is in the 5th period and group 5A, so it has 5 valence electrons. Each oxygen (O) atom is in the 2nd period and group 6A, so each has 6 valence electrons. Since there are 3 oxygen atoms and the overall charge is - 3, we have: \text{Total valence electrons} = 5 (\text{P}) + 6 (3 \times \text{O}) + 3 (-) = 24
Step 2:: Place the atoms in the Lewis structure and distribute the valence electrons.
Place the phosphorus atom in the center with the three oxygen atoms surrounding it. Draw a single bond between the phosphorus and each oxygen atom, which uses up 6 electrons. \mathrm{P}-\mathrm{O}-\mathrm{O}-\mathrm{O}
Step 3:: Add remaining valence electrons as lone pairs on the oxygen atoms.
Distribute the remaining 18 electrons as lone pairs on the oxygen atoms. Each oxygen atom needs 2 more electrons to reach an octet, so we will add 6 electrons as lone pairs on the oxygens. \mathrm{P}-\overset{\cdot}{\mathrm{O}}: : \mathrm{O}-\mathrm{O}
Step 4:: Calculate formal charges for each atom.
Calculate the formal charge for each atom by subtracting the number of valence electrons each atom would have in its neutral state from the number of electrons assigned to that atom in the Lewis structure. \begin{aligned} &\text { Formal charge of P }= 5 -(5 + 0 - 2)= 0 \ &\text { Formal charge of O with a double bond }= 6 -(6 + 2 - 6)= 0 \ &\text { Formal charge of O with a single bond and a lone pair }= 6 -(6 + 1 - 2)=- 1 \end{aligned}
Step 5:: Draw resonance structures.
Since the formal charges are not evenly distributed, we need to draw resonance structures to satisfy the octet rule. Move the double bond to the other two oxygen atoms and redraw the Lewis structure. \mathrm{O}-\mathrm{P}=\overset{\cdot}{\mathrm{O}}: : \quad \mathrm{O}=\mathrm{P}-\overset{\cdot}{\mathrm{O}}:
Final Answer
The Lewis structure for the polyatomic phosphite $\left(\mathrm{PO}_{3}^{3 -}\right)$ anion is: \mathrm{O}-\mathrm{P}=\overset{\cdot}{\mathrm{O}}: : \quad \mathrm{O}=\mathrm{P}-\overset{\cdot}{\mathrm{O}}: \quad \mathrm{O}-\mathrm{P} \equiv \overset{\cdot}{\mathrm{O}} These three resonance structures represent the Lewis structure for the $\text{PO}_3^{3 -}$ anion, where all atoms satisfy the octet rule.
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