Problem 164 - Introductory Chemistry Atoms First

Question

The phosphorus atom in \(\mathrm{PCl}_{3}\) should have a (a) \(\delta^{+}\) charge (b) \(\delta^{-}\) charge (c) \(3+\) charge (d) 3 - charge Explain.

Step-by-Step Solution

Answer

The phosphorus atom in PCl3 should have a \( \delta^{+} \) charge because its electronegativity value (2.19) is lower than that of chlorine (3.16), meaning that chlorine attracts the electrons within the P-Cl covalent bond more than phosphorus does, leading to a partial positive charge on the phosphorus atom.

Step 1: Understand electronegativity and charge distribution

Electronegativity is the ability of an atom to attract shared electrons to itself in a covalent bond. The charge on an atom in a molecule is determined by the difference in electronegativity between the atoms in each bond. In general, when an atom attracts bonding electrons more than another atom in a bond, it gains a partial negative charge (δ⁻), and the other atom gains a partial positive charge (δ⁺).

Step 2: Determine the electronegativity values of phosphorus and chlorine

To find the charge on a phosphorus atom in PCl3, we need to consider electronegativity values of phosphorus (P) and chlorine (Cl). We can consult an electronegativity chart or periodic table to find the values: Electronegativity of phosphorus (P): 2.19 Electronegativity of chlorine (Cl): 3.16

Step 3: Compare the electronegativity values

Since the electronegativity of chlorine (3.16) is higher than that of phosphorus (2.19), chlorine is more electronegative than phosphorus. This means that chlorine attracts the electrons within the P-Cl covalent bond more than phosphorus does, leading to a partial negative charge (δ⁻) on chlorine and a partial positive charge (δ⁺) on phosphorus.

Step 4: Choose the correct answer

Based on the above analysis, we can conclude that the phosphorus atom in PCl3 should have a partial positive charge (δ⁺). Thus, the correct answer is: (a) δ⁺ charge.

Key Concepts

Partial Charges in Molecules

When atoms with different electronegativity values come together to form a covalent bond, the electrons are not shared equally. The more electronegative atom pulls the shared electrons towards itself more strongly, causing the electron density around it to increase. This leads to the formation of a partial negative charge (denoted as \( \delta^{-} \) ) on that atom while the less electronegative atom, now having fewer electrons near it, ends up with a partial positive charge (denoted as \( \delta^{+} \) ).

The concept of partial charges is crucial in understanding the molecular structure, reactivity, and properties such as solubility and boiling points. For students aiming to visualize this, picture two ice skaters holding hands: if one is stronger and pulls harder, they'll draw the other closer – similar to how the more electronegative atom pulls the shared electrons.

Comparing Electronegativity Values

Electronegativity is a quantitative measure of an atom's ability to attract and hold onto shared electrons in a bond. The Pauling scale, the most widely used, assigns numerical values to the electronegativity of elements. To compare electronegativity values, one can look up these numbers on a periodic table or a dedicated chart.

Elements with higher electronegativity values attract shared electrons more strongly, helping us predict bond polarity and chemical reactivity. For example, in a molecule of water (H2O), oxygen has a higher electronegativity than hydrogen, indicating that the shared electrons are closer to oxygen, resulting in a polar molecule. Students can think of electronegativity values as the ‘pulling power’ rankings among atoms in a tug-of-war contest over electrons.

Covalent Bond Polarity

A covalent bond is formed when two atoms share a pair of electrons. If the atoms involved have identical or very close electronegativity values, they share the electrons equally, and the bond is classified as nonpolar covalent. However, if there's a significant difference in electronegativity, the electron sharing is unequal, and the bond becomes polar covalent.

The degree of polarity can influence the molecule's physical properties and the way it interacts with other molecules. In a classroom setting, you might think of each bond as a shared class project where the outcome depends on the input of each participant (atom). If one dominates (is more electronegative), the end result (electron distribution) reflects their greater contribution. Covalent bond polarity is a central concept in chemical bonding and molecular geometry and is fundamental in understanding organic mechanisms and biochemistry.