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The Toxicity and Versatility of PCl3: Exploring Valence Electrons and Molecular Geometry

Valence Electrons and Hybridization of PCl3

Phosphorus trichloride, also known as PCl3, is a colorless and volatile liquid that is highly reactive with water. It has the chemical formula PCl3 and, as the name suggests, is composed of three atoms of chlorine and one of phosphorus.

What distinguishes PCl3 from other phosphorus compounds is its toxicity. The substance can cause severe chemical burns, and it can also become hazardous after exposure to moisture or oxygen.

But despite its dangerous quality, PCl3 finds usefulness in a variety of industrial applications. This article will delve into its chemical composition, properties, and uses, as well as explore the valence electrons and hybridization of PCl3.

Chemical Composition and Properties

Phosphorus trichloride is a covalent molecule that contains two single and one double bond between the three atoms. The double bond exists between P and one of the chlorine atoms, significantly altering the molecule’s overall polarity.

It is also a peculiar molecule because it is classified as both a Lewis acid and a Lewis base. It is a Lewis acid because it can receive a pair of electrons from a Lewis base, and a Lewis base because it can donate a pair of electrons to a Lewis acid.

This property makes PCl3 a versatile chemical in many organic synthesis reactions. Due to its high reactivity and volatility, it is stored and transported in tightly sealed containers.

When exposed to moisture or water, PCl3 reacts vigorously, generating toxic and acidic hydrogen chloride (HCl) gas. Hence, it is crucial to handle the substance with the utmost care and under strict laboratory conditions.

Industrial Applications

Phosphorus trichloride plays a vital role in the production of organophosphorus compounds. It is used to create a wide range of products, including pesticides, flame retardants, chemical intermediates, and plasticizers.

One of the most critical applications of PCl3 is in the production of phosphites. Phosphites are organophosphorus compounds that serve as antioxidants.

They are added to lubrication oil, plastics, and rubber to prevent degradation. Another relevant application of PCl3 is in the production of trialkyl and triaryl phosphates.

Trialkyl and triaryl phosphates are used as plasticizers in different industries, such as textiles, leather, and flooring. They increase the plastics’ flexibility and durability, making them more malleable and resistant to fractures.

Valence Electrons

Phosphorus has five valence electrons, and each chlorine atom has seven valence electrons. To determine the total number of valence electrons in PCl3, you add the valence electrons of each atom (5+7+7+7=26).

The number 26 represents the total number of electrons that serve as building blocks for creating the bonding structures between the different atoms.


When we talk about hybridization, we refer to the process in which atomic orbitals combine to form hybrid orbitals. In the case of PCl3, the phosphorus atom hybridizes its three 3d orbitals and one 3p orbital to give four sp3 hybrid orbitals.

This hybridization results in four equivalent hybrid orbitals, forming the shape of a tetrahedron. The three chlorine atoms occupy three corners of the tetrahedron, with the fourth corner occupied by an unpaired electron.

Therefore, PCl3 has a trigonal pyramidal shape.


Phosphorus trichloride is a versatile and useful chemical compound despite its toxic and volatile properties. Its properties make it essential in the production of organophosphorus compounds through the use of trialkyl and triaryl phosphates and phosphites.

The chemical composition of PCl3 consists of three atoms of chlorine and one of phosphorus, and valence electrons play a crucial role in its hybridization. With the knowledge of its properties, PCl3 can be safely handled for laboratory and industrial purposes.

Bond Angles and Molecular Geometry of PCl3

Phosphorus trichloride (PCl3) is a molecule that has a trigonal pyramidal molecular geometry. Since the molecule has a tetrahedral electron pair geometry that is compatible with its sp3 hybridization, this results in a trigonal pyramidal molecular shape.

In addition, each of the three chlorine atoms present in the molecule are equally spaced around the phosphorus atom, forming a base of the pyramid. The fourth region of electron density, which includes the lone pair of electrons in the hybrid orbital, is situated above the plane of the triangle, leading to a characteristic bent or pyramidal shape.

Bond Angles

The molecular geometry of PCl3 provides clues about the bond angles that exist between the different atoms in the molecule. One of the fundamental principles of molecular geometry is that the bond angles are determined by the repulsion between electrons that are placed between two atoms.

In this case, the central phosphorus atom in PCl3 is surrounded by three separate chlorine atoms, each chlorine with one lone pair of electrons. The molecule also has an unshared pair of electrons on the phosphorus atom.

Due to electron repulsion, the bond angles present in PCl3 are less than the 109.5 degrees expected for the tetrahedral, sp3 hybrid orbitals. The bond angles between the phosphorus and chlorine atoms in PCl3 are approximately 107 degrees.

The bent shape of the molecule makes the bond angles slightly smaller than those observed in the tetrahedral, sp3 hybridization.

Molecular Geometry

The molecular geometry of PCl3 is determined by the electron geometry. The electron geometry is determined by the hybridization of the central atom and its surrounding atoms.

In this case, the electron geometry of PCl3 is tetrahedral, with phosphorus hybridizing its three 3d orbitals and one 3p orbital to give four sp3 hybrid orbitals. PCl3 has four electron pairs, three of which are bonding pairs, and the fourth is a lone pair.

The presence of the lone pair of electrons in the hybrid orbital above the trigonal plane, above the three chlorine atoms, changes the molecular shape to a trigonal pyramid. Therefore, the molecular geometry of PCl3 is a trigonal pyramidal one.

Reaction with water and release of HCl gas

PCl3 is a highly reactive substance with a vigorous reaction with water, which can release hydrochloric acid (HCl) gas. The reaction with water proceeds as follows:

PCl3 + 3H2O → 3HCl + H3PO3

The reaction with water is an exothermic one, meaning it releases energy when it occurs.

As soon as even small amounts of water are added to PCl3, a vapor state starts to form, which can be irritating to the eyes, skin, and respiratory tract due to the volatile nature of the compound. The hydrolysis reaction of PCl3, in addition to the heavy smoke of HCl released, can also be thermally dangerous.

Hence, the reaction with water should always be performed in a laboratory setting and under strict control, as these reactions can be highly exothermic and may be explosive. The reaction of PCl3 with water forms phosphorous acid (H3PO3) and hydrochloric acid (HCl).

Throughout the hydrolysis of PCl3, the electron pair from one of the phosphorus-chlorine bonds is utilized to form a new bond between the phosphorus atom and one oxygen atom. The new bond leads to the formation of phosphorous acid, which is a weak acid.

Simultaneously, it releases hydrogen chloride or HCl gas, which is highly toxic and corrosive.


Phosphorus trichloride or PCl3 is a highly reactive and toxic compound with industrial usefulness in the production of organophosphorus compounds. The molecule’s electronic and molecular structure contributes significantly to the bond angles and molecular geometry, with the geometry being a trigonal pyramid shape.

The hydrolysis of PCl3 with water generates hazardous hydrochloric acid gas, which presents safety challenges and hazards. Thus, it is necessary to handle PCl3 rightly in laboratory settings to avoid physical and health risks.

In conclusion, understanding the chemical composition, properties, molecular geometry, and reaction of PCl3 is crucial to handle the substance safely in laboratory and industrial environments. PCl3 has a trigonal pyramidal molecular geometry, with bond angles of approximately 107 degrees.

The compound is highly volatile and can release hydrochloric acid when exposed to water. Therefore, strict safety protocols need to be in place when handling and transporting PCl3.


  • Q: What is PCl3?
  • A: PCl3 is a colorless and volatile liquid composed of three atoms of chlorine and one of phosphorus.
  • Q: What are the industrial applications of PCl3?
  • A: PCl3 is used to create organophosphorus compounds, including phosphites and trialkyl and triaryl phosphates, which are used in pesticides, lubrication oils, plastics, and textiles.
  • Q: What is the molecular geometry of PCl3?
  • A: PCl3 has a trigonal pyramidal molecular geometry, with bond angles of approximately 107 degrees.
  • Q: Why is PCl3 hazardous?
  • A: PCl3 is hazardous because it is highly reactive with water and can produce toxic and acidic hydrogen chloride gas.
  • Q: What is the importance of understanding the properties of PCl3?
  • A: Understanding the properties of PCl3 is essential for safely handling the substance in a laboratory or industrial setting, thereby reducing any potential safety hazards.

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