Question

Is phosphorus trichloride, \(P C l_{3}\), a polar molecule? If it is, draw the dipole moment vector for the entire molecule and show where the \(\delta+\) and \(\delta-\) regions of the molecule are.

Short answer

Phosphorus trichloride (PCl3) is a polar molecule due to its trigonal pyramidal structure and the electronegativity difference between Phosphorus (approx. \(2.19\)) and Chlorine (approx. \(3.16\)). There is a net dipole moment with the δ+ region located at the Phosphorus atom and δ- regions at the Chlorine atoms.

Step-by-step solution

Find the molecular structure of PCl3

To determine the polarity of PCl3, we first need to find its molecular structure. Phosphorus has 5 valence electrons, and each Chlorine atom has 7. When they bond, they form a trigonal pyramidal structure, with the Phosphorus atom in the center and the three Chlorine atoms surrounding it.

Determine Electronegativity Difference

Next, consider the electronegativity of Phosphorus and Chlorine atoms. Electronegativity is a measure of how strongly an atom attracts electrons in a bond. The greater the difference in electronegativity, the more polar the bond will be. The electronegativity value of Phosphorus is approximately \(2.19\) and that of Chlorine is approximately \(3.16\). There is an electronegativity difference of \(0.97\), which implies that the P-Cl bond is polar.

Identify the Direction of the Dipole Moment in P-Cl bonds

As Chlorine is more electronegative than Phosphorus, the electrons in the P-Cl bond are pulled closer to Chlorine, giving a partial negative charge (δ-) to the Chlorine atoms and a partial positive charge (δ+) to the Phosphorus atom. The P-Cl bond dipoles have their positive ends towards the Phosphorus atom and their negative ends towards the Chlorine atoms.

Determine the Overall Dipole Moment of PCl3

To find the overall dipole moment of PCl3, we need to consider the dipole moments of all three P-Cl bonds in the trigonal pyramidal structure. Since the P-Cl bond dipoles do not cancel each other out due to the molecular geometry, there is a net dipole moment present in PCl3. This means PCl3 is a polar molecule.

Draw the Dipole Moment Vector and Identify δ+ and δ- Regions

To represent the dipole moment in PCl3: 1. Draw the PCl3 molecule in its trigonal pyramidal structure. 2. Mark the Phosphorus atom with a δ+ symbol and the Chlorine atoms with δ- symbols. 3. Draw an arrow from the Phosphorus atom to each Chlorine atom, indicating the direction of the dipole moment in each P-Cl bond. 4. Draw a resultant dipole moment vector from the Phosphorus atom (δ+) towards the center of the triangle formed by the three Chlorine atoms (δ-). As a result, PCl3 is a polar molecule with the δ+ region at the Phosphorus atom and the δ- regions at the Chlorine atoms.

Key concepts

Molecular Structure

Understanding the molecular structure of a compound is crucial in determining its properties, including its polarity. For phosphorus trichloride ( PCl_3), it consists of a phosphorus atom at the center surrounded by three chlorine atoms, forming a trigonal pyramidal shape. This structure is similar to a pyramid with a triangular base when visualized in three dimensions.
This specific arrangement is due to the presence of lone pairs of electrons on the phosphorus atom, which push the chlorine atoms closer, forming bond angles less than 109.5°. The spatial arrangement provides insight into how the molecules interact and influence each other, especially in terms of polarity.
In summary, the molecular structure of PCl_3 is a fundamental factor in understanding whether or not the molecule is polar, as the shape contributes significantly to the distribution of charges in the molecule.

Electronegativity Difference

Electronegativity refers to an atom's ability to attract electrons towards itself within a chemical bond. When looking at phosphorus trichloride ( PCl_3), the crucial aspect is the difference in electronegativity between the phosphorus and chlorine atoms. Phosphorus has an electronegativity of approximately 2.19, while chlorine is about 3.16.
The electronegativity difference, calculated as approximately 0.97, indicates that the P-Cl bonds are indeed polar. A significant difference in electronegativity often results in one atom having a partial negative charge ( δ- ) and the other a partial positive charge ( δ+ ).
This disparity is key to determining the polarity of not just individual bonds but the overall molecule, as it affects how electrons are distributed and creates potential zones of positive and negative charge within the molecule.

Dipole Moment

A dipole moment arises in a molecule when there is an uneven distribution of charges, creating a positive end and a negative end. In the case of phosphorus trichloride ( PCl_3), the dipole moment results from its polar P-Cl bonds due to the electronegativity difference between phosphorus and chlorine.
Each P-Cl bond has its electronic charges oriented with the negative end towards the chlorine, resulting in polar bonds. When visualizing dipole moments, arrows are typically drawn from the positive charge ( δ+ ) towards the negative charge ( δ- ).
In PCl_3, the individual dipole moments of the three P-Cl bonds add up because of the trigonal pyramidal arrangement. This net dipole moment means that the molecule is polar as a whole. The direction and magnitude of this net dipole influence physical properties like solubility and boiling point.

Trigonal Pyramidal Structure

The trigonal pyramidal structure of phosphorus trichloride ( PCl_3) is crucial in understanding its polarity. Unlike a symmetrical arrangement that might result in dipole moments canceling out, the trigonal pyramidal shape creates an asymmetrical distribution of electron density.
In this structure, the phosphorus atom is located at the apex, while the three chlorine atoms form a base. The presence of a lone pair of electrons on phosphorus contributes to pushing the chlorine atoms downward, resulting in this shape.
This configuration allows the bond dipoles to add up rather than cancel out, ensuring that PCl_3 retains its net dipole moment and is polar. Understanding the trigonal pyramidal structure helps us see why some molecules behave the way they do under varying conditions.