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Ch.8 - Basic Concepts of Chemical Bonding
All textbooksBrown 14th EditionCh.8 - Basic Concepts of Chemical BondingProblem 68a
Chapter 8, Problem 68a

Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, H2PO4-, in which the H atoms are bonded to O atoms. (a) What is the predicted dominant Lewis structure if satisfying the octet rule is the top criterion?

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Step 1: Begin by determining the total number of valence electrons available for the dihydrogen phosphate ion, H₂PO₄⁻. Phosphorus (P) has 5 valence electrons, each oxygen (O) has 6 valence electrons, and each hydrogen (H) has 1 valence electron. Additionally, the negative charge on the ion contributes 1 extra electron.
Step 2: Calculate the total number of valence electrons: 5 (from P) + 4 × 6 (from O) + 2 × 1 (from H) + 1 (from the negative charge) = 32 valence electrons.
Step 3: Arrange the atoms with phosphorus as the central atom, since it is less electronegative than oxygen. Connect each oxygen atom to the phosphorus atom with a single bond. Attach each hydrogen atom to one of the oxygen atoms.
Step 4: Distribute the remaining electrons to satisfy the octet rule for each atom. Start by completing the octets for the oxygen atoms, then ensure the phosphorus atom also achieves an octet. Remember that hydrogen only needs 2 electrons to be satisfied.
Step 5: Verify that all atoms satisfy the octet rule. Adjust the placement of electrons if necessary, ensuring that the total number of electrons used equals the number calculated in Step 2. The dominant Lewis structure will have all atoms satisfying the octet rule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Octet Rule

The octet rule is a chemical guideline that states atoms tend to bond in such a way that they each have eight electrons in their valence shell, achieving a stable electron configuration similar to that of noble gases. This rule is particularly relevant for main group elements and helps predict the bonding behavior and structure of molecules, including the arrangement of electrons in Lewis structures.
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Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They are used to visualize the arrangement of electrons and to predict the geometry and reactivity of molecules. In constructing Lewis structures, one must consider the octet rule, formal charges, and the overall charge of the molecule or ion.
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Formal Charge

Formal charge is a theoretical charge assigned to an atom in a molecule, calculated based on the number of valence electrons, the number of non-bonding electrons, and the number of bonds. It helps in determining the most stable Lewis structure by minimizing the formal charges across the molecule. A structure with the lowest formal charges is often preferred, but it must also satisfy the octet rule for the atoms involved.
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Related Practice
Textbook Question

(a) Describe the molecule xenon trioxide, XeO3, using four possible Lewis structures, one each with zero, one, two, or three Xe¬O double bonds. (b) Do any of these resonance structures satisfy the octet rule for every atom in the molecule? (c) Do any of the four Lewis structures have multiple resonance structures? If so, how many resonance structures do you find? (d) Which of the Lewis structures in part (a) yields the most favorable formal charges for the molecule?

Textbook Question

There are many Lewis structures you could draw for sulfuric acid, H2SO4 (each H is bonded to an O). (a) What Lewis structure(s) would you draw to satisfy the octet rule?

Textbook Question

There are many Lewis structures you could draw for sulfuric acid, H2SO4 (each H is bonded to an O). (b) What Lewis structure(s) would you draw to minimize formal charge?

Textbook Question

Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, H2PO4-, in which the H atoms are bonded to O atoms. (b) What is the predicted dominant Lewis structure if achieving the best formal charges is the top criterion?

Textbook Question

State whether each of these statements is true or false. (a) The longer the bond, the stronger the bond. (c) A typical double bond length is in the 500–1000 pm range.

Textbook Question

State whether each of these statements is true or false. (e) The longer the bond, the more energy is stored chemical bonds.