General Chemistry Exam Study Guide: Bonding, Lewis Structures, and Molecular Geometry

Q8. Choose the best Lewis structure for PO43−.

Background

Topic: Lewis Structures and Resonance

This question tests your ability to identify the most accurate Lewis structure for the phosphate ion (PO43−), considering formal charges and resonance.

Key Terms and Formulas:

• Lewis Structure: A diagram showing the arrangement of atoms and electrons in a molecule or ion.

• Formal Charge: The charge assigned to an atom in a molecule, calculated as:

$\text{Formal charge} = \text{Valence electrons} - \text{Nonbonding electrons} - \frac{1}{2} \times \text{Bonding electrons}$

• Resonance: When more than one valid Lewis structure can be drawn for a molecule or ion.

Step-by-Step Guidance

1. Count the total number of valence electrons for PO43−. Phosphorus (P) has 5, each oxygen (O) has 6, and the ion has a -3 charge (add 3 electrons).

2. Draw the skeleton structure with phosphorus in the center and four oxygens around it.

3. Distribute the electrons to satisfy the octet rule for each atom, starting with the outer atoms (oxygen).

4. Assign formal charges to each atom in the different possible Lewis structures. Compare which structure minimizes formal charges and places negative charges on the most electronegative atoms (oxygen).

5. Consider resonance: If multiple structures have similar formal charges, the actual structure is a hybrid of these resonance forms.

Try solving on your own before revealing the answer!

Final Answer: The best Lewis structure for PO43− is the one with three single bonds and one double bond to oxygen, with formal charges minimized and resonance among the oxygens.

In this structure, phosphorus forms one double bond with an oxygen and single bonds with the other three oxygens. The negative charges are placed on the single-bonded oxygens, which are more electronegative.