Problem 124 - Chemistry: An Atoms-Focused Approach

Question

How many electrons are there in the covalent bonds surrounding the phosphorus atom in the following species? (a) \(\mathrm{POCl}_{3} ;\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4} ;\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{3} ;\) (d) \(\mathrm{PF}_{6}-.\)

Step-by-Step Solution

Answer

Answer: In POCl3, H3PO4, and H3PO3, there are 10 electrons in the covalent bonds surrounding the phosphorus atom. In PF6-, there are 12 electrons surrounding the phosphorus atom.

Step 1: Identify surrounding atoms

In this species, the phosphorus atom (P) is directly bonded to one oxygen atom (O) and three chlorine atoms (Cl).

Step 2: Determine bonding pairs

Phosphorus forms one double bond with the oxygen atom and three single bonds with the chlorine atoms.

Step 3: Calculate total electrons

With one double bond (2 pairs) and three single bonds (1 pair each), there are a total of 5 bonding pairs. So, there are 10 electrons in covalent bonds surrounding the phosphorus atom. (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\)

Step 1: Identify surrounding atoms

Phosphorus is directly bonded to one oxygen atom (O) and three hydrogen atoms (H).

Step 2: Determine bonding pairs

Phosphorus forms three single bonds with the hydrogen atoms and one double bond with the oxygen atom.

Step 3: Calculate total electrons

With one double bond (2 pairs) and three single bonds (1 pair each), there are a total of 5 bonding pairs. So, there are 10 electrons in the covalent bonds surrounding the phosphorus atom. (c) \(\mathrm{H}_{3} \mathrm{PO}_{3}\)

Step 1: Identify surrounding atoms

Phosphorus is directly bonded to one oxygen atom (O) and three hydrogen atoms (H).

Step 2: Determine bonding pairs

Phosphorus forms three single bonds with the hydrogen atoms and one double bond with the oxygen atom.

Step 3: Calculate total electrons

With one double bond (2 pairs) and three single bonds (1 pair each), there are a total of 5 bonding pairs. So, there are 10 electrons in the covalent bonds surrounding the phosphorus atom. (d) \(\mathrm{PF}_{6}-\)

Step 1: Identify surrounding atoms

Phosphorus is directly bonded to six fluorine atoms (F).

Step 2: Determine bonding pairs

Phosphorus forms six single bonds with the fluorine atoms.

Step 3: Calculate total electrons

With six single bonds (1 pair each), there are a total of 6 bonding pairs. So, there are 12 electrons in the covalent bonds surrounding the phosphorus atom.

Key Concepts

Phosphorus Compounds

Phosphorus compounds are a fascinating and diverse group of chemicals where phosphorus typically forms covalent bonds with other atoms. These compounds include substances like phosphates and halides. Central to the chemistry of phosphorus compounds is their ability to create bonds with elements such as oxygen and halogens like chlorine and fluorine. For instance, in phosphorus oxychloride (\(\mathrm{POCl}_3\)), phosphorus is bonded to oxygen and chlorine atoms, while in phosphorus pentafluoride anion (\(\mathrm{PF}_6^-\)), phosphorus is bonded to fluorine atoms.

These compounds often serve critical roles in biological and industrial processes. For example, phosphates are essential in biological systems for energy transfer and storage, being components of ATP. Inorganic phosphorus compounds are also used in fertilizers, detergents, and as flame retardants. Understanding the way phosphorus bonds in its compounds is key to grasping their reactivity and functionality.

Electron Pairing

Electron pairing is the foundational concept in covalent bonding, where atoms share pairs of electrons to achieve a stable electron configuration. In the formation of phosphorus compounds, phosphorus typically shares its electrons to form single or double bonds.

• Single bonds involve the sharing of one pair of electrons, as seen in \(\mathrm{H}_3\mathrm{PO}_4\) where phosphorus forms single bonds with hydrogen.
• Double bonds involve sharing two pairs of electrons, important when phosphorus is bonded with oxygen, like in \(\mathrm{POCl}_3\).

These bond arrangements ensure that every atom involved achieves a full outer electron shell, which usually consists of eight electrons for most elements, known as the octet rule. However, phosphorus can sometimes expand its octet, as seen in \(\mathrm{PF}_6^-\), allowing more than eight electrons in its valence shell due to the availability of d-orbitals.

This flexibility in electron pairing contributes to the diversity in the structures and reactivities of phosphorus compounds.

Molecular Structure

The molecular structure of phosphorus compounds determines their chemical and physical properties. The arrangement of atoms and the type of covalent bonds present define the shape and reactivity of these molecules. For example, \(\mathrm{POCl}_3\) has a tetrahedral shape, with the phosphorus center bonded to one oxygen and three chlorine atoms.

This tetrahedral arrangement is a common theme in phosphorus chemistry, allowing for a diverse array of complex molecular formations. In \(\mathrm{H}_3\mathrm{PO}_4\), phosphorus is bonded to oxygen in a tetrahedral structure, forming a phosphoric acid molecule that is critical in biological energetics and industrial applications.

• The shape of \(\mathrm{PF}_6^-\) can be described as octahedral because phosphorus is centrally located, surrounded by six equivalent fluorine atoms.

The molecular geometry influences properties such as polarity, boiling and melting points, and solubility, which dictate how these molecules interact with other substances. Understanding these structures allows chemists to predict the behavior of phosphorus compounds in various chemical environments.