Question

Would you expect the nonbonding electron-pair domain in \(\mathrm{NH}_{3}\) to be greater or less in size than for the corresponding one in \(\mathrm{PH}_{3}\) ? Explain.

Short answer

The nonbonding electron-pair domain in PH3 is greater in size than the corresponding one in NH3. This is due to the larger size of the phosphorus atom compared to nitrogen and the lower electron repulsion between the lone pair and bonding pairs in PH3.

Step-by-step solution

Understanding the nonbonding electron-pair domain concept

Nonbonding electron-pair domain is the region in a molecule where an electron pair is not involved in bonding between atoms. It is also called the lone pair of electrons. The size of the nonbonding electron-pair domain is influenced by the atom's size and the electron repulsion among the electron pairs. In our case, we need to compare the size of the nonbonding electron-pair domain in NH3 and PH3.

Electron configuration of nitrogen and phosphorus

To make a comparison between NH3 and PH3, we first need to know the electron configuration of the central atoms, nitrogen (N) and phosphorus (P). Nitrogen has an electron configuration of \(1s^{2}2s^{2}2p^{3}\), with 5 valence electrons. Phosphorus has an electron configuration of \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{3}\), with 5 valence electrons as well.

Molecular structure of NH3 and PH3

Both molecules, NH3 and PH3, have a central atom (N and P) surrounded by three hydrogen atoms and one nonbonding electron-pair domain (lone pair). Due to this lone pair, the molecular geometry of both molecules is trigonal pyramidal.

Comparing the size of nonbonding electron-pair domain in NH3 and PH3

Now, we need to compare the size of the nonbonding electron-pair domain in NH3 and PH3. The size of the nonbonding electron-pair domain depends on the size of the central atom and electron repulsion among electron pairs. Phosphorus is larger in size than nitrogen because it is located in the third period of the periodic table, while nitrogen is in the second period. As a result, the electron cloud in PH3 is more spread out, and the nonbonding electron-pair domain in PH3 will be larger than in NH3. The electron repulsion among electron pairs is another factor that affects the size of the nonbonding electron-pair domain. In NH3, the nitrogen atom has a high electronegativity, which means that it attracts the electrons in the bond more towards itself, thus causing more repulsion between the lone pair and the bonding pairs. In PH3, the electronegativity of phosphorus is lower, and consequently, the repulsion between the lone pair and bonding pairs is also lower. Considering both factors, we can conclude that:

Conclusion

The nonbonding electron-pair domain in PH3 is greater in size than the corresponding one in NH3. This is due to the larger size of the phosphorus atom compared to nitrogen and the lower electron repulsion between the lone pair and bonding pairs in PH3.

Key concepts

Electron Configuration

The electron configuration refers to the distribution of electrons in an atom's orbitals. Understanding the electron configuration of elements such as nitrogen and phosphorus helps us to comprehend the behavior and characteristics of molecules like ammonia (NH₃) and phosphine (PH₃).

- **Nitrogen (N):** Its electron configuration is \(1s^2 2s^2 2p^3\). This indicates that nitrogen has five valence electrons available for bonding with hydrogen atoms.- **Phosphorus (P):** Its electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^3\), also possessing five valence electrons suitable for forming chemical bonds.

Despite this similarity in valence electrons, the position of these elements in the periodic table influences the size and shape of their electron clouds when forming compounds.

Molecular Geometry

The molecular geometry describes the arrangement of atoms around a central atom in a molecule, which influences its shape and angles.

In both ammonia (NH₃) and phosphine (PH₃), the central atom (nitrogen or phosphorus) is bonded to three hydrogen atoms. This setup, along with a lone electron pair on the central atom, leads to a molecular geometry known as "trigonal pyramidal."
- **Trigonal Pyramidal Geometry:** This shape arises when there are three bonds and one lone pair of electrons around the central atom. The lone pair exerts a repulsive force, pushing the bonding pairs closer together and slightly altering the expected tetrahedral shape to a pyramid with a triangular base.
Understanding the molecular geometry is essential because it affects physical and chemical properties such as polarity and reactivity.

Electron Repulsion

Electron repulsion plays a significant role in determining the molecular shape and size of a molecule. The concept is based on the idea that electron pairs will orient themselves as far apart as possible due to their like charges repelling each other.

- **NH₃:** In ammonia, the lone pair of electrons is quite electronegative, meaning it draws bonding electrons towards the nitrogen atom. This results in larger repulsion forces, which crowd the hydrogen atoms and compress the bond angles. - **PH₃:** In phosphine, the phosphorus atom has a larger atomic radius, and its lone pair is less electronegative. This results in less repulsion compared to NH₃, allowing bond angles to remain wider and the electron cloud to spread out more.
These differences in electron repulsion and atomic radius lead to varying molecular dimensions and properties for NH₃ and PH₃.

Trigonal Pyramidal

Trigonal pyramidal is a term used to describe a specific geometric arrangement of atoms within a molecule. It essentially looks like a pyramid with a triangular base.

In this geometry:

• The central atom forms three bonds.
• It holds one nonbonding pair (lone pair) of electrons.

This results in a skewed shape because the lone pair is not visible in the molecular structure but influences the molecular geometry significantly.

For instance, both NH₃ and PH₃ have a trigonal pyramidal shape due to their lone pair, which affects the bond angles and overall spatial arrangement. However, the difference in the atom sizes (N vs. P) and their electronegativities adds further distinction between these molecules. Trigonal pyramidal geometry highlights how electron pair arrangements can dramatically influence molecular shape and behavior.