Question

A common form of elemental phosphorus is the white phosphorus, where four \(\mathrm{P}\) atoms are arranged in a tetrahedron. All four phosphorus atoms are equivalent. White phosphorus reacts spontaneously with the oxygen in air to form \(\mathrm{P}_{4} \mathrm{O}_{6} .\) (a) How many valance electron pairs are in the \(\mathrm{P}_{4} \mathrm{O}_{6}\) molecule? (b) When \(\mathrm{P}_{4} \mathrm{O}_{6}\) is dissolved in water, it produces a \(\mathrm{H}_{3} \mathrm{PO}_{3}\), molecule. \(\mathrm{H}_{3} \mathrm{PO}_{3}\) has two forms, \(\mathrm{P}\) forms 3 covalent bonds in the first form and \(\mathrm{P}\) forms 5 covalent bonds in the second form. Draw two possible Lewis structures of \(\mathrm{H}_{3} \mathrm{PO}_{3}\). (c) Which structure obeys the octet rule?

Short answer

There are 28 valence electron pairs in the P4O6 molecule. Two possible Lewis structures of H3PO3 are: (i) First form: P forming 3 covalent bonds O || H - O - P - O - H | H (ii) Second form: P forming 5 covalent bonds O || H - O - P - O - H || H The first form (P forming 3 covalent bonds) of H3PO3 obeys the octet rule.

Step-by-step solution

(a) Counting valence electron pairs in P4O6

To determine the number of valence electron pairs in P4O6, we first need to determine the number of valence electrons. There are 4 phosphorus (P) atoms and 6 oxygen (O) atoms in the molecule. Phosphorus has 5 valence electrons, while oxygen has 6 valence electrons. Total valence electrons in P4O6 = (4 * 5) + (6 * 6) = 20 + 36 = 56 Now we will divide the total valence electrons by 2 to find the number of valence electron pairs: Number of valence electron pairs = 56 / 2 = 28 There are 28 valence electron pairs in the P4O6 molecule.

(b) Drawing two possible Lewis structures of H3PO3

(i) First form: P forming 3 covalent bonds In this form, the phosphorus atom is surrounded by three oxygen atoms. Two oxygen atoms are bonded to hydrogen atoms, and the third oxygen has a double bond with phosphorus and two lone pairs. The Lewis structure is: O || H - O - P - O - H | H (ii) Second form: P forming 5 covalent bonds In this form, the phosphorus atom is surrounded by three oxygen atoms, two of these oxygen atoms are bonded to hydrogen atoms, and one hydrogen atom is bonded directly to the phosphorus atom. There are two double bonds between phosphorus and two of the oxygen atoms. The Lewis structure is: O || H - O - P - O - H || H

(c) Determining which structure obeys the octet rule

Octet rule states that atoms tend to achieve eight electrons in their outer shell, similar to noble gases' electron configuration. In the first form, the central phosphorus atom has 5 electrons in its outer shell (3 bonds and 1 lone pair), making a total of 8 electrons, which obeys the octet rule. In second form, the central phosphorus atom has 10 electrons in its outer shell (5 bonds and no lone pairs), making a total of 10 electrons, which does not obeys the octet rule. Therefore, the first form (P forming 3 covalent bonds) of H3PO3 obeys the octet rule.

Key concepts

Lewis Structures

Lewis structures are a simple yet powerful way to visualize molecules and their bonding. They show how atoms in a molecule are arranged and how valence electrons are distributed. The valence electrons are those found in an atom's outermost shell, and they are pivotal in forming chemical bonds.
When drawing Lewis structures, each symbol represents an atom, and lines or pairs of dots represent the bonds or lone pairs of electrons. For example, in the molecule \( ext{H}_3 ext{PO}_3 \), we draw phosphorus surrounded by oxygen and hydrogen atoms.
In the first form of \( ext{H}_3 ext{PO}_3 \), phosphorus forms 3 covalent bonds, making it central in the Lewis structure. This form is represented with one double bond between phosphorus and oxygen, and single bonds with other oxygen atoms connected to hydrogens. In the second form, some atoms can form more bonds, showing double bonds between phosphorus and two oxygen atoms. This gives insight into the potential flexibility and complexity in molecular structure.

Octet Rule

The octet rule is a guiding principle in chemistry that explains how atoms strive to have eight electrons in their valence shell. This rule is a reflection of the stability found in noble gases, which naturally have full valence shells.
In \( ext{H}_3 ext{PO}_3 \), according to the octet rule, the first form where phosphorus forms 3 covalent bonds fulfills the rule. By achieving a stable configuration similar to that of noble gases, the phosphorus atom in this arrangement reaches 8 electrons in its valence shell by sharing electrons with other atoms.
However, this rule isn't absolute. There are exceptions, particularly with elements like phosphorus, which can exceed the octet in some compounds. The second form of \( ext{H}_3 ext{PO}_3 \) is such an example, where phosphorus ends up with 10 electrons, reflecting its ability to expand its valence shell.

Covalent Bonds

Covalent bonds are chemical bonds where atoms share pairs of electrons. These bonds enable atoms to achieve full valence shells, adhering to rules like the octet rule.
In the \( ext{H}_3 ext{PO}_3 \) molecule, phosphorus and oxygen form covalent bonds, fulfilling their needs for stability. Phosphorus often forms covalent bonds with non-metals like oxygen to create stable compounds such as phosphites.
Covalent bonds can be single, double, or triple, depending on the number of pairs of electrons shared. In \( ext{H}_3 ext{PO}_3 \), the first form includes both single and a double bond. Double bonds, in particular, involve two shared pairs of electrons, adding robustness and rigidity between atoms. Recognizing how these bonds influence the structure and stability of a molecule helps understand molecular properties and reactions.