Question

Phosphorous acid is listed in Appendix D as a diprotic acid. Propose a Lewis structure for phosphorous acid that is consistent with this fact.

Short answer

Phosphorus is the central atom bonded with three oxygen atoms. Two of these oxygen atoms are further bonded with one hydrogen atom each. The two protons donated originate from these hydrogen atoms, indicating that phosphorus acid is a diprotic acid.

Step-by-step solution

Identify the central atom.

Phosphorus is the central atom because it is the least electronegative element in phosphorus acid.

Determine the total number of valence electrons

Phosphorous has 5 valence electrons, each oxygen has 6 and each hydrogen has 1. In total there are: \(5+(3 \times 6)+(3 \times 1)= 23\) valence electrons.

Draw the structure

Place Phosphorus in the center and arrange oxygen and hydrogen atoms around it. For a start, connect them with single bonds. Then fill the octets of other atoms starting with the most electronegative atom (Oxygen) before returning to the central atom.

Check the formal charges

Considering Formal charges: Phosphorus has +1, each oxygen bonded with hydrogen has 0, and the oxygen bonded only with phosphorus has -1. This implies that the structure satisfies the Formal Charge criterion, having charges as close as possible to zero.

Key concepts

Diprotic Acid

A diprotic acid is a type of acid that can donate two hydrogen ions (protons) per molecule when dissolved in water. Phosphorous acid, the subject of the original exercise, is an example of a diprotic acid. This means it can undergo two ionization stages, losing one proton at a time.

In the Lewis structure of phosphorous acid, you'll see two hydrogen atoms bonded to oxygen atoms. Each of these is capable of being independently released as a proton. Understanding the concept of diprotic acids is crucial because it affects the acid's behavior in solution and its acid dissociation constants. The first proton is usually more readily dissociable compared to the second one due to molecular stability.

Learning that phosphorous acid falls into this category helps in predicting chemical reactions and solutions' behavior, which is significant in fields like biology and chemistry.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are important in determining how the atom interacts with others. In the case of phosphorous acid, we need to calculate the total number of valence electrons in the molecule to propose a valid Lewis structure.

Phosphorus, being the central atom here, has five valence electrons. Oxygen, known for its high electronegativity, has six valence electrons each. The hydrogen atoms add one more valence electron each. Altogether, the molecule has 23 valence electrons, calculated as:

• Phosphorus (P): 5 electrons
• Oxygen (O): 3 × 6 = 18 electrons
• Hydrogen (H): 3 × 1 = 3 electrons

Adding these, we get 23 valence electrons. Understanding the distribution of valence electrons helps in predicting how the molecule can form bonds and ensures that the octet rule is satisfied for the constituent atoms.

Formal Charge

Formal charge is a concept used to determine the most stable Lewis structure for a molecule. It helps us reflect the distribution of electrons in molecules, especially when several possible structures can be drawn. The goal is to have a formal charge as close to zero as possible for all atoms in the molecule.

In the phosphorous acid molecule, we need to calculate the formal charge to verify the found Lewis structure. Here’s how it works for each atom type:

• Phosphorus (P) ends up with a formal charge of +1.
• Each hydrogen-bonded oxygen (which participates in hydrogen donation) has a formal charge of 0.
• The oxygen bonded solely to phosphorus has a formal charge of -1.

This calculation indicates that the overall structure maintains equilibrium and assigns charges that minimize the potential energy, usually making the structure stable. If discrepancies exist, adjusting bonds or electron pairs can bring formal charges closer to zero.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. It is a crucial factor in determining how bonds form between atoms type and influences their chemical and physical properties significantly.

In phosphorous acid, phosphorus is the least electronegative element among phosphorus, oxygen, and hydrogen. Therefore, it acts as the central atom in the Lewis structure. Oxygen, being highly electronegative, will pull shared electrons towards itself. This results in polar covalent bonds, which is a characteristic feature of this molecule.

The difference in electronegativity between bonded atoms can also help predict the molecule's polarity. Generally, a significant difference results in polar bonds, affecting how the molecule interacts with others, including how it dissolves in water. Understanding electronegativity also helps in predicting the molecule's reactivity and the strength of its bonds, making it a fundamental concept in chemistry.