Question

Write the Lewis structure for phosphorus trichoride \((\mathrm{PC} / 3) .\) Is \(\mathrm{PCl}_{3}\) a lewis acid, a Lewis base, or neither?

Short answer

The Lewis structure for phosphorus trichloride (\(\mathrm{PCl}_3\)) consists of a central phosphorus atom surrounded by three chlorine atoms, each connected to the phosphorus by a single bond. Since the phosphorus atom can still accept an electron pair and has less than 8 electrons in its outer shell, \(\mathrm{PCl}_3\) can act as a Lewis acid.

Step-by-step solution

Count the total number of valence electrons

Phosphorus (P) belongs to group 15 and thus has 5 valence electrons. Chlorine (Cl) belongs to group 17 with 7 valence electrons each. In \(\mathrm{PCl}_3\), there are 3 Chlorine atoms. Total valence electrons = 5 (from P) + 3 * 7 (from three Cl atoms) = 5 + 21 = 26 valence electrons.

Distribute the electrons and create bonds

Place the central atom (P) in the middle and surround it with the three Cl atoms. Then form single bonds between the central atom (P) and each of the surrounding atoms (Cl) by placing 2 electrons between them. Now, 6 valence electrons have been used to form the three single P-Cl bonds. Remaining valence electrons = 26 - 6 = 20 electrons.

Complete the octets of the surrounding atoms

Distribute the remaining 20 valence electrons among the three Cl atoms to complete their octets. Each Cl atom needs 6 more electrons since the single bond is providing them with 2 electrons each. After filling the octets of the Cl atoms, we have utilized all 26 valence electrons.

Verify the central atom's octet

Now, we need to check if the central atom (P) has an octet. Each single bond around P contributes 2 electrons. Thus, the central atom has 3 * 2 = 6 electrons in its outer shell. However, phosphorus can expand its octet beyond 8 electrons, so this configuration is an acceptable Lewis structure for \(\mathrm{PCl}_3\).

Identify if \(\mathrm{PCl}_3\) is a Lewis acid, base, or neither

A Lewis acid is a molecule that can accept an electron pair, while a Lewis base is a molecule that can donate an electron pair. The central atom (P) has a total of 5 valence electrons, and there are 6 electrons around it from the single bonds. As it can have up to 8 electrons in the outer shell (since it can expand its octet), \(\mathrm{PCl}_3\) can still accept an electron pair. Therefore, \(\mathrm{PCl}_3\) can act as a Lewis acid.

Key concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom that are involved in chemical bonding. They are significant because they determine how atoms will interact with each other. In a Lewis structure, valence electrons are depicted as dots around the chemical symbols of the elements. For example, when writing the Lewis structure for phosphorus trichloride ((PCl_3)), we count the valence electrons to assess how the atoms are bonded.

Phosphorus, being in group 15 of the periodic table, has 5 valence electrons, while each chlorine atom, found in group 17, comes with 7 valence electrons. By summing the valence electrons from one phosphorus atom and three chlorine atoms, we determine that (PCl_3) has a total of 26 valence electrons. This count dictates the number of total electrons available for bonding and is essential in constructing the molecule's Lewis structure accurately. Understanding this concept is crucial for students as it lays the foundation for predicting molecular shapes, bond formation, and chemical reactivity.

Lewis Acid and Base

The concepts of Lewis acid and base are rooted in the ability of compounds to donate or accept electron pairs. A Lewis acid is a compound that can accept an electron pair, whereas a Lewis base can donate an electron pair. These definitions go beyond the traditional Arrhenius definitions, which require the generation of hydrogen ions (H^+) in water for acids and hydroxide ions (OH^-) for bases.

In the case of phosphorus trichloride ((PCl_3)), it's important to analyze the structure to determine if it can accept or donate electrons. Phosphorus in (PCl_3) can expand its octet, meaning it can have more than eight electrons in its outer electron shell. This expansion possibility allows (PCl_3) to act as a Lewis acid since it can accept additional electrons into its expanded valence shell. Students must grasp this classification system, as it directly impacts their understanding of chemical interactions, such as catalysts in reactions and active sites in enzymes.

The Octet Rule

The octet rule is a chemical rule of thumb that states atoms tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. Atoms will share, gain, or lose electrons to fulfill this rule, which often explains the stability of compounds.

When building the Lewis structure of a molecule like phosphorus trichloride ((PCl_3)), it is crucial to ensure that the chlorine atoms fulfill the octet rule, thereby obtaining full valence shells with eight electrons. However, as seen with phosphorus in (PCl_3), there are exceptions to this rule. Certain elements can have fewer (as in the case of hydrogen) or more (as in the case of elements in the third period and beyond) than eight electrons in their valence shell. The ability to expand the valence shell is due to the availability of the d orbitals. These subtleties are paramount when discussing the reactivity and stability of compounds, and they underline the importance for students to recognize both the rule and its exceptions.