Problem 68 - Chemistry The Central Science In Si Units

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, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-},\) in which the \(\mathrm{H}\) atoms are bonded to \(\mathrm{O}\) atoms. \((\mathbf{a})\) What is the predicted dominant Lewis structure if satisfying the octet rule is the top criterion? (b) What is the predicted dominant Lewis structure if achieving the best formal charges is the top criterion?

Step-by-Step Solution

Answer

(a) Prioritizing the octet rule results in a structure with phosphorus central and each oxygen obeying the octet rule. (b) Prioritizing best formal charges often results in one double bond, balancing charges more effectively than satisfying the octet.

Step 1: Determine the Total Number of Valence Electrons

Dihydrogen phosphate ion, \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\), consists of 2 hydrogen atoms, 1 phosphorus atom, and 4 oxygen atoms, and it carries a -1 charge. Hydrogen contributes 1 valence electron each, phosphorus contributes 5, and oxygen contributes 6 each. The additional -1 charge adds 1 more electron. Thus, the total number of valence electrons is:\[2\times1 + 5 + 4\times6 + 1 = 32.\]

Step 2: Construct a Lewis Structure Prioritizing the Octet Rule

To satisfy the octet rule, every atom (except hydrogen) should be surrounded by 8 electrons. Start by placing phosphorus in the center and connect it to four oxygen atoms. Assign hydrogen to two of the oxygen atoms. Distribute the remaining electrons around the oxygen atoms so that each oxygen, except those bonded with hydrogen, satisfies the octet rule, but with the least negative formal charges possible. The structure should have phosphorus with no lone pairs and all atoms (except hydrogen) should aim to satisfy the octet.

Step 3: Calculate Formal Charges for Octet Rule Structure

The formal charge of an atom is calculated using the formula:\[\text{Formal charge} = \text{Valence electrons} - (\text{Lone pair electrons} + 0.5\times\text{Bonding electrons}).\] Apply this to each atom in the octet-satisfying structure to ensure the distribution. This typically results in the central phosphorus having a formal charge different from zero, while the outer oxygen atoms strive to balance the charges.

Step 4: Construct a Lewis Structure Prioritizing Best Formal Charges

To achieve the best formal charges, start again with phosphorus in the center bonded to four oxygen atoms and place two hydrogen atoms on two oxygen atoms. Adjust the arrangement such that the formal charges are minimized, often resulting in some atoms not perfectly following the octet rule if it results in better overall formal charges (usually a double bond forms between phosphorus and one oxygen to help balance formal charges).

Step 5: Calculate Formal Charges for Best Formal Charges Structure

Re-calculate the formal charges using the same formula from the previous step to ensure that this structure has the charges closest to zero for each atom. Typically, this will involve one or two atoms having a formal charge of 0, departing only slightly from the octet requirement for maximizing overall stability through charge distribution.

Key Concepts

Octet Rule

The octet rule is a fundamental guideline in chemistry that helps predict the bonding behavior of atoms. It states that atoms tend to prefer having eight electrons in their valence shell, mimicking the electron configuration of a noble gas, which is stable.
Atoms achieve this stable configuration through various types of bonding:

• Covalent bonds: Sharing electrons with other atoms.
• Ionic bonds: Donating or accepting electrons to/from other atoms.
• Coordinate covalent bonds: One atom donates both electrons in a shared pair.

When creating a Lewis structure, fulfilling the octet rule for each atom (except hydrogen, which only needs two electrons) is often prioritized. This rule helps predict the locations of bonds and lone pairs, leading to a stable molecular structure.

Formal Charge

The concept of formal charge aids in determining the most probable structure of a molecule or ion by minimizing charge separation. The formal charge of an atom is calculated using the formula:\[\text{Formal charge} = \text{Valence electrons} - (\text{Lone pair electrons} + 0.5\times\text{Bonding electrons}).\]When assessing Lewis structures, achieving minimal formal charges across atoms can indicate the most stable arrangement.
It can sometimes mean allowing exceptions to the octet rule if it results in a favorable charge distribution:

• Minimizing formal charges: A structure is favored if formal charges are closest to zero.
• Consistency of total charge: Total charge on the molecule or ion should match the given charge, such as -1 for \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\).

Optimizing formal charges often requires rearranging electrons, such as forming double bonds, to distribute charge more evenly.

Dihydrogen Phosphate Ion

The dihydrogen phosphate ion, \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\), is a polyatomic ion that plays a crucial role in biochemical processes and industrial applications. It consists of:

• Two hydrogen atoms, which bond with two of the oxygen atoms.
• One phosphorus atom at the center, bonded to four oxygen atoms.
• Four oxygen atoms, helping form the backbone of the structure.
• An overall -1 charge, critical for its chemical behavior.

Understanding the structure of this ion is important for predicting its reactivity and behavior in different chemical environments. When forming its Lewis structure, focusing on both the octet rule and formal charges helps highlight its potential as a mediator in acid-base reactions and as a ligand in metal complexes.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how that atom bonds with others. These electrons play key roles in defining the chemical properties of an element.
During the formation of molecules or ions:

• Electron sharing or transfer: Valence electrons are either shared or transferred to other atoms to fulfill the octet rule.
• Counting total valence electrons: For the \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) ion, the total number of valence electrons is calculated by adding contributions from all constituent atoms and accounting for the ion's charge.

The precise arrangement and number of valence electrons allow chemists to construct Lewis structures that explain molecular geometry and reactivity patterns, providing a window into understanding an ion’s role in chemical systems.