Question

Using the VSEPR theory, predict the molecular struc ture 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

The molecular structures of the given polyatomic ions are as follows: a. Sulfate ion, \(\mathrm{SO}_{4}^{2-}\): seesaw shape. b. Phosphate ion, \(\mathrm{PO}_{4}^{3-}\): tetrahedral. c. Ammonium ion, \(\mathrm{NH}_{4}^{+}\): tetrahedral.

Step-by-step solution

Determine electron pairs

Sulfur is the central atom in this ion. It has 6 valence electrons and forms double bonds with four oxygen atoms. Each double bond contributes 2 electrons. Since there are two additional electrons in the ion (indicated by the 2- charge), these make up one lone pair. Therefore, there are a total of 4 bonding pairs and 1 lone pair around the sulfur atom.

Assign geometry

According to VSEPR theory, with 5 electron pair domains (4 bonding pairs and 1 lone pair), the electron pair geometry around the sulfur atom will be trigonal bipyramidal.

Identify molecular structure

Since there is one lone pair in the trigonal bipyramidal geometry, it will occupy an equatorial position, and the molecular structure of the sulfate ion will be a seesaw shape. #b. phosphate ion, \(\mathrm{PO}_{4}^{3-}\)

Determine electron pairs

Phosphorus is the central atom in this ion. It has 5 valence electrons and forms single bonds with four oxygen atoms. Each of these bonds contributes one electron pair. Since there are three additional electrons in the ion (indicated by the 3- charge), these make up three lone pairs, one on each of the three oxygen atoms. There are a total of 4 bonding pairs and no lone pairs around the phosphorus atom.

Assign geometry

According to VSEPR theory, with 4 electron pair domains (4 bonding pairs and 0 lone pairs) around the phosphorus atom, the electron pair geometry will be tetrahedral.

Identify molecular structure

Since there are no lone pairs on the phosphorus atom in the tetrahedral geometry, the molecular structure of the phosphate ion will also be tetrahedral. #c. ammonium ion, \(\mathrm{NH}_{4}^{+}\)

Determine electron pairs

Nitrogen is the central atom in this ion. It has 5 valence electrons and forms single bonds with four hydrogen atoms. Each single bond contributes one electron pair. Since there is one less electron in the ion (indicated by the +1 charge), the nitrogen atom has no lone pairs. There are a total of 4 bonding pairs and no lone pairs around the nitrogen atom.

Assign geometry

According to VSEPR theory, with 4 electron pair domains (4 bonding pairs and 0 lone pairs) around the nitrogen atom, the electron pair geometry will be tetrahedral.

Identify molecular structure

Since there are no lone pairs on the nitrogen atom in the tetrahedral geometry, the molecular structure of the ammonium ion will also be tetrahedral.

Key concepts

Molecular Geometry

Molecular geometry is all about the 3D shape of a molecule. This shape is determined by the arrangement of atoms around a central atom. Using VSEPR theory can help predict this arrangement. It stands for Valence Shell Electron Pair Repulsion theory. This theory says that electron pairs, including those in bonds and lone pairs, want to be as far apart as possible.

For instance, in the sulfate ion \((\mathrm{SO}_{4}^{2-})\), the electron pairs push apart from each other, giving the molecule a seesaw shape. Similarly, the phosphate \((\mathrm{PO}_{4}^{3-})\) and ammonium \((\mathrm{NH}_{4}^{+})\) ions both end up with a tetrahedral shape. Understanding this helps us predict how molecules behave and interact.

Polyatomic Ions

Polyatomic ions are ions that contain more than one atom. Examples include the sulfate, phosphate, and ammonium ions. These ions carry a charge because of extra or missing electrons.

• Sulfate \((\mathrm{SO}_{4}^{2-})\): Contains sulfur and oxygen atoms.
• Phosphate \((\mathrm{PO}_{4}^{3-})\): Includes phosphorus and oxygen atoms.
• Ammonium \((\mathrm{NH}_{4}^{+})\): Made up of nitrogen and hydrogen atoms.

In these ions, the central atom bonds with surrounding atoms. These bonds and any extra electrons due to the charge define their shape and properties. Polyatomic ions are essential in various chemical reactions and industries.

Electron Pair Geometry

Electron pair geometry focuses on the spatial arrangement of all electron pairs around the central atom, including both bonding and lone pairs. According to VSEPR theory, these pairs arrange to minimize repulsion.

For example:

• In the sulfate ion \((\mathrm{SO}_{4}^{2-})\), with 5 electron pairs, the geometry starts as trigonal bipyramidal.
• For the phosphate ion \((\mathrm{PO}_{4}^{3-})\), the 4 bonding pairs create a tetrahedral geometry.
• The ammonium ion \((\mathrm{NH}_{4}^{+})\) also has 4 bonding pairs, leading to a tetrahedral shape as well.

These electron pair geometries help us understand potential interactions and stability of the molecules.

Bonding Pairs

Bonding pairs are pairs of electrons shared between two atoms, forming a bond. These pairs play a key role in determining the structure of a molecule. In VSEPR theory, they help decide the geometry based on their number and arrangement.

Consider the following cases:

• The sulfate ion \((\mathrm{SO}_{4}^{2-})\) has 4 bonding pairs between sulfur and oxygen.
• Phosphate ion \((\mathrm{PO}_{4}^{3-})\) also has 4 bonding pairs, connecting phosphorus with oxygen.
• Ammonium ion \((\mathrm{NH}_{4}^{+})\) consists of 4 bonding pairs, each between nitrogen and hydrogen.

These bonding pairs shape the molecule and stabilize the ions by allowing electron sharing.

Lone Pairs

Lone pairs are pairs of electrons that are not involved in bonding. They reside on an atom and can influence the shape of the molecule. These pairs are significant because they take up space and repel bonding pairs, thus affecting molecular geometry.

In our examples:

• Sulfate ion \((\mathrm{SO}_{4}^{2-})\): Contains 1 lone pair, altering the shape to a seesaw.
• Phosphate ion \((\mathrm{PO}_{4}^{3-})\): Has no lone pairs; maintains a regular tetrahedral shape.
• Ammonium ion \((\mathrm{NH}_{4}^{+})\): Also lacks lone pairs, resulting in a perfect tetrahedral structure.

Understanding lone pairs is crucial for predicting and explaining molecular shapes and behaviors.