Problem 18 - Chemistry: The Central Science in SI Units, Global Edition

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 (lone pair) in NCl₃ is smaller than the corresponding one in PCl₃. This is because Nitrogen has a smaller atomic size and a more covalent bond character with Chlorine (due to smaller electronegativity difference) than Phosphorus, resulting in a higher electron density around the central atom and a smaller electron-pair domain size.

Step 1: Identify the central atom and its electronegativity in each molecule

In NCl₃, Nitrogen is the central atom and has an electronegativity of 3.04. In PCl₃, Phosphorus is the central atom and has an electronegativity of 2.19.

Step 2: Examine the atomic size of Nitrogen and Phosphorus

Both Nitrogen(N) and Phosphorus(P) are in Group 15 of the periodic table, but Phosphorus is in the third period, while Nitrogen is in the second period. As we move down a group, atomic size increases due to the addition of energy levels (shells). Therefore, Phosphorus has a larger atomic size than Nitrogen.

Step 3: Compare the electronegativity difference between N-Cl and P-Cl bonds

To understand the size of the electron-pair domain, we need to examine the electronegativity difference of both molecules. The electronegativity of Chlorine is 3.16. So, the electronegativity difference for N-Cl bond is |3.04 - 3.16| = 0.12. And the electronegativity difference for P-Cl bond is |2.19 - 3.16| = 0.97. In NCl₃, the bond is more covalent due to the smaller electronegativity difference, which means electron-pair is more evenly shared between the Nitrogen and Chlorine atoms. In PCl₃, the bond is more polar as it has a larger electronegativity difference. Thus, the electron pair in P-Cl is more attracted to Chlorine atom.

Step 4: Analyze the impact of atomic size and electronegativity on electron-pair domain size

In NCl₃, the smaller atomic size of Nitrogen and higher covalent bond character result in higher electron density around the central atom, which shrinks the nonbonding electron-pair domain size. In PCl₃, the larger atomic size of Phosphorus and higher polarity of the bond result in a less electron-dense central atom, leading to an increase in the nonbonding electron-pair domain size.

Step 5: Conclusion

Based on the analysis of atomic size and electronegativity differences, we can conclude that the nonbonding electron-pair domain (lone pair) in NCl₃ is smaller than the corresponding one in PCl₃.

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that refers to the ability of an atom to attract shared electrons in a covalent bond. In essence, it describes how strongly an atom pulls on electrons when forming a chemical bond.
This property significantly influences molecular geometry and bond characteristics.

• Elements with high electronegativity, like Fluorine, tend to draw electrons closer, causing a greater electron density around them.
• Conversely, elements with lower electronegativity, such as metals, have a weaker pull on electrons.

Electronegativity helps predict the behavior of molecules during chemical reactions and the type of bonds two atoms might form, whether covalent or ionic. Understanding electronegativity allows us to grasp why some atoms in compounds have higher electron densities, influencing the size and shape of the formed molecules.

Atomic Size

Atomic size, often referred to as atomic radius, is the distance from the atom's nucleus to the outermost boundary of its electron cloud. It can vary depending on the atom's environment, but periodic trends give us a general understanding.

Trends in the Periodic Table

As you move from left to right across a period in the periodic table, atomic size decreases because the increasing nuclear charge pulls electrons closer.
Moving down a group, the atomic size increases as new electron shells are added, expanding the distance the outermost electrons are from the nucleus.

• Nitrogen, being in the second period, has relatively smaller atomic size compared to Phosphorus in the third period.
• This increased size for Phosphorus affects the spacing available for electron-pair domains around the atom.

Understanding atomic size helps significantly in predicting and explaining the spatial arrangement and resulting geometry of molecules.

Covalent Bond

A covalent bond forms when two atoms share electrons to achieve a stable electron configuration, commonly that of a noble gas. Covalent bonds are crucial in understanding molecular geometry since they maintain a shared electron pair between two atoms.

• When the electronegativity difference between two atoms is small, a covalent bond is more likely to form because electrons are shared equally.
• This can result in non-polar covalent bonds if atoms have identical or very similar electronegativities.

Covalent bonds are often present in organic molecules and are responsible for the creation of complex structures like proteins and DNA. These bonds hold atoms together but also influence the molecule's reactivity, boiling and melting points, as well as its solubility in different substances.

Electronegativity Difference

The electronegativity difference between two bonded atoms determines the bond's nature—whether it's covalent, polar covalent, or ionic. This difference plays a critical role in the bond's strength and polarity.

Bond Characterization

• If the difference is < 0.5, the bond is usually considered nonpolar covalent, with electrons shared equally.
• Differences between 0.5 and 1.7 typically indicate a polar covalent bond, where electrons are shared unequally, leading to partial charges.
• A difference > 1.7 suggests an ionic bond with electrons transferring from one atom to another.

Understanding electronegativity difference is key to determining molecule properties, like solubility, boiling and melting points. For example, the N-Cl bond in NCl₃ has a smaller electronegativity difference than the P-Cl bond in PCl₃, rendering it less polar, which influences the electron-pair domain within the molecule.