Problem 18 - Chemistry The Central Science In Si Units

Question

Would you expect the nonbonding electron-pair domain in \(\mathrm{NCl}_{3}\) to be greater or smaller in size than the corresponding one in \(\mathrm{PCl}_{3}\)?

Step-by-Step Solution

Answer

The nonbonding electron-pair domain in \(\mathrm{NCl}_3\) is smaller than in \(\mathrm{PCl}_3\).

Step 1: Understanding the Problem

We need to compare the size of nonbonding electron-pair domains (also known as lone pairs) for nitrogen in \(\mathrm{NCl}_3\) and phosphorus in \(\mathrm{PCl}_3\). Since both are in Group 15, they can form three bonds with chlorine atoms and have one lone pair of electrons.

Step 2: Compare Atomic Sizes

Consider the atomic sizes of nitrogen and phosphorus. Nitrogen is smaller than phosphorus because it is higher up in Group 15 of the periodic table. Smaller atoms tend to hold their electron pairs more tightly due to the increased effective nuclear charge.

Step 3: Analyze Nonbonding Electron-Pair Domain Sizes

The lone pair in nitrogen is held more tightly and is closer to the nucleus due to the smaller atomic size and effective nuclear charge compared to phosphorus. Therefore, the nonbonding electron-pair domain in \(\mathrm{NCl}_3\) is expected to be smaller than in \(\mathrm{PCl}_3\).

Key Concepts

Atomic Size

In the periodic table, atomic size refers to the distance between the nucleus of an atom and its outermost electron shell. As you move down a group, the atomic size increases because additional electron shells are added. However, when you move across a period from left to right, the atomic size generally decreases. This is because more protons are added to the nucleus, which increases the positive charge and pulls the electron cloud closer.
In the context of \(\mathrm{NCl}_3\) and \(\mathrm{PCl}_3\), nitrogen and phosphorus are both found in Group 15. Although they are in the same group, nitrogen sits above phosphorus on the periodic table, making it smaller in size. This means that nitrogen's outer electrons, including nonbonding electron pairs, are held more closely to its nucleus due to the smaller atomic radius. Contrast this with phosphorus, whose larger size allows its electrons to sit farther from the nucleus, resulting in a larger atomic radius.

Effective Nuclear Charge

The effective nuclear charge is the net positive charge experienced by electrons in an atom. It is influenced by the number of protons in the nucleus and the extent to which electron-electron repulsions shield outer electrons from this nuclear charge. As you go across a period in the periodic table, the effective nuclear charge tends to increase because of the addition of more protons.
For nitrogen and phosphorus, nitrogen has a smaller atomic size, meaning its outer electrons, including the nonbonding electron pair, feel a stronger pull from the nucleus. This happens because the effective nuclear charge is higher in smaller atoms where outer electrons are less shielded by inner electron layers. In larger atoms, like phosphorus, the additional inner electron shells provide more shielding. Consequently, its outer electrons are less tightly held, resulting in a larger nonbonding electron-pair domain in compounds like \(\mathrm{PCl}_3\).

Group 15 Elements

Group 15 elements, also known as the pnictogens, include nitrogen, phosphorus, arsenic, antimony, and bismuth. These elements all have five valence electrons, which commonly leads them to form three covalent bonds like in \(\mathrm{NCl}_3\) and \(\mathrm{PCl}_3\), and an additional lone pair.
Each of these elements has distinct properties due to their position in the periodic table. Generally, as you descend the group, the atomic size increases and the electronegativity decreases. This group trend affects their chemical properties and their electron pairs' behavior. For example, the nonbonding electron pairs in \(\mathrm{NCl}_3\) are held more closely due to nitrogen's smaller size and higher effective nuclear charge compared to phosphorus in \(\mathrm{PCl}_3\). Understanding these properties helps predict reactivity and the geometry of the molecules formed by these elements.