Question

Using the VSEPR theory, predict the molecular structure of each of the following polyatomic ions. a. sulfate ion, \(\mathrm{SO}_{4}{ }^{2-}\) b. phosphate ion, \(\mathrm{PO}_{4}^{3-}\) c. ammonium ion, \(\mathrm{NH}_{4}^{+}\)

Short answer

Using VSEPR theory, the molecular structures of the polyatomic ions are as follows: a. Sulfate ion (SO₄²⁻): Tetrahedral with bond angles of approximately 109.5°. b. Phosphate ion (PO₄³⁻): Tetrahedral with bond angles of approximately 109.5°. c. Ammonium ion (NH₄⁺): Tetrahedral with bond angles of approximately 109.5°.

Step-by-step solution

Identify the central atom and electron domains for each ion

For each ion, we need to identify the central atom and the number of electron domains surrounding it. The central atom is usually the least electronegative one. a. For the sulfate ion, the central atom is S (sulfur), surrounded by four O (oxygen) atoms. b. For the phosphate ion, the central atom is P (phosphorus), surrounded by four O (oxygen) atoms. c. For the ammonium ion, the central atom is N (nitrogen), surrounded by four H (hydrogen) atoms.

Calculate the electron domains

We will count the total number of electron domains (lone pairs and bond pairs) around the central atom in each ion. a. In the sulfate ion, the central atom S has four single bonds to four O atoms with a formal charge of -2. Thus, the central atom has 4 electron domains (4 bond pairs). b. In the phosphate ion, the central atom P has four single bonds to four O atoms with a formal charge of -3. Thus, the central atom has 4 electron domains (4 bond pairs). c. In the ammonium ion, the central atom N has four single bonds to four H atoms with a formal charge of +1. Thus, the central atom has 4 electron domains (4 bond pairs).

Apply VSEPR theory to predict molecular structures

Now we will use VSEPR theory to predict the molecular structure of each ion based on the number of electron domains around the central atom. a. In the sulfate ion, the central atom has four electron domains (4 bond pairs). According to the VSEPR theory, this leads to a tetrahedral molecular structure with bond angles of approximately 109.5°. b. In the phosphate ion, the central atom has four electron domains (4 bond pairs). According to the VSEPR theory, this leads to a tetrahedral molecular structure with bond angles of approximately 109.5°. c. In the ammonium ion, the central atom has four electron domains (4 bond pairs). According to the VSEPR theory, this leads to a tetrahedral molecular structure with bond angles of approximately 109.5°.

Conclusion:

Using VSEPR theory, we predicted the molecular structures of the ions as follows: a. Sulfate ion (SO₄²⁻): Tetrahedral with bond angles of approximately 109.5°. b. Phosphate ion (PO₄³⁻): Tetrahedral with bond angles of approximately 109.5°. c. Ammonium ion (NH₄⁺): Tetrahedral with bond angles of approximately 109.5°.

Key concepts

Molecular Structure Prediction

Predicting the molecular structure of compounds is a fundamental aspect of chemistry that helps us understand how molecules interact with each other and their environment. Valence Shell Electron Pair Repulsion (VSEPR) theory offers a practical method for such predictions, focusing on the idea that electron domains around a central atom repel each other and thus want to be as far apart as possible.

This leads to specific molecular geometries based on the number of electron domains, which is a combination of lone pairs and bonding pairs of electrons. For instance, four electron domains lead to a tetrahedral structure due to the equal repulsion experienced by the electron domains, resulting in bond angles of about 109.5°. This pattern remains consistent in predicting the structure of polyatomic ions, each with distinctive electron configurations and formal charges.

Electron Domains

Electron domains are regions around a central atom where electrons are likely to be found. They include bond pairs, which are shared by atoms in a chemical bond, and lone pairs, which are electrons that are not shared with another atom. The geometry that minimizes the repulsion among these domains dictates the overall shape of the molecule or ion.

In the exercise examples, the sulfate (SO₄²⁻), phosphate (PO₄³⁻), and ammonium (NH₄⁺) ions all exhibit four bonding pairs and no lone pairs, leading to a tetrahedral geometry since the repulsions between the bond pairs are equal. Understanding electron domains is crucial as it also helps predict molecular polarity and the resulting intermolecular forces which define the physical properties of substances.

Polyatomic Ions

Polyatomic ions are charged species composed of two or more atoms covalently bonded together, and they carry an overall charge. The charge results from an excess or shortage of electrons when compared to the number of protons in the nuclei of the atoms making up the ion. The sulfate (SO₄²⁻), phosphate (PO₄³⁻), and ammonium (NH₄⁺) ions are common examples mentioned in the exercise.

The structure of polyatomic ions can be predicted using VSEPR theory, which helps us understand their chemical reactivity and solubility. For instance, the sulfate ion has a tetrahedral geometry which allows it to form strong electrostatic interactions with cations in solution, leading to the formation of compounds like sodium sulfate (Na₂SO₄). Such ions are prevalent in various fields, from biological molecules to materials science.