All the following compounds are characterized by ionic bonding between a group I metal cation and a tetrahedral anion. Write an appropriate Lewis structure for each anion, remembering to specify formal charges where they exist. (a) NaBF4 (b) LiAIH4 (c) K2SO4 (d) Na3PO4
The Correct Answer and Explanation is:
Let’s break down each of these compounds and determine the appropriate Lewis structures for their anions:
(a) NaBF4 (Sodium tetrafluoroborate)
The anion in this compound is BF4^-.
- Boron (B) is in group 13 and can form three bonds, so it will form three bonds with the four fluorine atoms, leaving one additional electron.
- Fluorine (F) is highly electronegative, so it will have a full octet with a negative charge.
- We place four fluorine atoms around the boron atom, each forming a single bond. Since there are four fluorine atoms, and one of them carries a negative charge (the extra electron), this will give the overall anion a charge of -1.
Lewis structure:
- Boron is in the center, surrounded by four fluorine atoms, each having three lone pairs and one bond to the boron atom.
- The negative charge is placed on one of the fluorine atoms to indicate the extra electron.
(b) LiAIH4 (Lithium aluminum hydride)
The anion in this compound is AlH4^-.
- Aluminum (Al) is in group 13 and typically forms three bonds. In this case, it bonds to four hydrogen atoms (each forming a single bond).
- Since aluminum has a +3 charge from the cation (Li+), the overall anion must have a negative charge of -1, which is balanced by the hydride ions (H^-).
Lewis structure:
- Aluminum is the central atom, with four hydrogen atoms bonded to it. Each hydrogen has two electrons, and the aluminum carries a negative charge to balance the +3 charge from the metal cation.
(c) K2SO4 (Potassium sulfate)
The anion in this compound is SO4^2-.
- Sulfur (S) is in group 16, so it typically forms six bonds (but in this case, it forms four bonds in the sulfate ion).
- The oxygen atoms are arranged around the sulfur atom, each forming a single bond with sulfur. However, each oxygen atom has two lone pairs.
- The sulfur is double-bonded to two oxygens, and the other two oxygens are single-bonded. The overall anion has a charge of -2 due to the presence of the two single-bonded oxygen atoms with their lone pairs.
Lewis structure:
- Sulfur is the central atom, bonded to four oxygen atoms. Two oxygens have double bonds to sulfur, and two have single bonds. The oxygens with single bonds each carry a -1 formal charge.
(d) Na3PO4 (Sodium phosphate)
The anion in this compound is PO4^3-.
- Phosphorus (P) is in group 15 and can form five bonds, but in phosphate, it forms four bonds with oxygen atoms.
- The oxygen atoms are arranged around the phosphorus atom. Three of the oxygen atoms form single bonds with phosphorus, while one oxygen atom forms a double bond with phosphorus.
- Each oxygen atom with a single bond has a negative charge, contributing a -1 formal charge to the overall anion.
Lewis structure:
- Phosphorus is the central atom, bonded to four oxygen atoms. Three of the oxygen atoms are single-bonded, each with a formal negative charge. One oxygen is double-bonded to phosphorus and does not carry a charge.
Conclusion:
- NaBF4: The anion BF4^- has four fluorines, with a negative charge on one of them.
- LiAIH4: The anion AlH4^- has four hydrogens bonded to aluminum, with a negative charge.
- K2SO4: The anion SO4^2- has sulfur bonded to four oxygens, with two negative charges on two oxygens.
- Na3PO4: The anion PO4^3- has phosphorus bonded to four oxygens, with three negative charges on three oxygens.
These anions exhibit various types of bonding with their respective metal cations, and their Lewis structures show formal charges to reflect the electron distribution in each anion.
