Question

Write a Lewis structure for each of the following polyatomic ions. Show all bonding valence electron pairs as lines and all nonbonding valence electron pairs as dots. For those ions that exhibit resonance, draw the various possible resonance forms. a. sulfate ion, \(\mathrm{SO}_{4}^{2-}\) b. phosphate ion, \(\mathrm{PO}_{4}^{3-}\) c. sulfite ion, \(\mathrm{SO}_{3}^{2-}\)

Short answer

The Lewis structures for the given polyatomic ions are as follows: a. Sulfate ion, \(\mathrm{SO}_{4}^{2-}\): The sulfate ion consists of a central sulfur atom single-bonded to four oxygen atoms and has four possible resonance structures. b. Phosphate ion, \(\mathrm{PO}_{4}^{3-}\): The phosphate ion consists of a central phosphorus atom single-bonded to three oxygen atoms and double-bonded to one oxygen atom. It has four possible resonance structures. c. Sulfite ion, \(\mathrm{SO}_{3}^{2-}\): The sulfite ion consists of a central sulfur atom single-bonded to two oxygen atoms and double-bonded to one oxygen atom. It has three possible resonance structures.

Step-by-step solution

1. Determine the Total Number of Valence Electrons for Each Ion

- Sulfate ion, \(\mathrm{SO}_{4}^{2-}\): Sulfur has 6 valence electrons, each Oxygen atom has 6, and there are two additional electrons due to the 2- charge. Total valence electrons = 1(6) + 4(6) + 2 = 32 - Phosphate ion, \(\mathrm{PO}_{4}^{3-}\): Phosphorus has 5 valence electrons, each Oxygen atom has 6, and there are three additional electrons due to the 3- charge. Total valence electrons = 1(5) + 4(6) + 3 = 32 - Sulfite ion, \(\mathrm{SO}_{3}^{2-}\): Sulfur has 6 valence electrons, each Oxygen atom has 6, and there are two additional electrons due to the 2- charge. Total valence electrons = 1(6) + 3(6) + 2 = 26

2. Distribute Electrons and Check for Octets

a. Sulfate Ion: 1. Place a single bond between the central atom (S) and each oxygen atom. This uses 8 electrons. 2. Fill the octets of each oxygen atom using the remaining electrons, leaving 6 electrons for the central atom, completing its octet. b. Phosphate Ion: 1. Place a single bond between the central atom (P) and each oxygen atom. This uses 8 electrons. 2. Fill the octets of each oxygen atom using the remaining electrons, leaving 5 electrons for the central atom. 3. Add a double bond to one oxygen atom to complete the central atom's octet. c. Sulfite Ion: 1. Place a single bond between the central atom (S) and each oxygen atom. This uses 6 electrons. 2. Fill the octets of each oxygen atom using the remaining electrons, leaving 8 electrons for the central atom. 3. Add a double bond to one oxygen atom to complete the central atom's octet.

3. Check for Resonance Structures

a. Sulfate Ion: The double bond in the sulfate ion can be placed between any of the oxygen atoms, resulting in 4 possible resonance structures. b. Phosphate Ion: The double bond in the phosphate ion can be placed between any of the oxygen atoms, resulting in 4 possible resonance structures. c. Sulfite Ion: The double bond in the sulfite ion can be placed between any of the oxygen atoms, resulting in 3 possible resonance structures.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how an atom can bond with others. In chemical bonding, these electrons participate in forming bonds because they're the most accessible to other atoms. Let's take sulfur as an example. It belongs to the sixth group of the periodic table, hence it has six valence electrons. Oxygen, found in group 16, also has six valence electrons. For polyatomic ions like \(\mathrm{SO}_{4}^{2-}\), the charge of the ion suggests additional electrons that must be taken into account.

During the formation of bonds, an atom can either share, gain or lose valence electrons to achieve a full outer shell, similar to that of the noble gases. This concept is key to understanding Lewis structures, which are diagrams showing potential bonds and the arrangement of valence electrons among atoms within a molecule or ion.

Resonance Structures

Resonance structures come into play when a single Lewis structure does not accurately represent the distribution of electrons in a molecule or ion. The true form of the molecule is a hybrid of all possible resonance structures. For example, with the sulfate ion \(\mathrm{SO}_{4}^{2-}\), we can draw multiple valid Lewis structures that vary only in the placement of the double bonds. Each structure satisfies the octet rule, yet none of them alone represents the actual distribution of electrons. In truth, the electrons are said to be delocalized, shared by more than two atoms. Resonance is significant because it helps explain the stability and behavior of molecules, while also showing that the real structure is an average of the possible forms.

Octet Rule

The octet rule is a guideline in chemistry suggesting that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The term 'octet' refers to this group of eight electrons. Atoms will share, lose, or gain electrons to fulfill this rule. In our exercise, after placing the bonds in the sulfate \(\mathrm{SO}_{4}^{2-}\) and sulfite \(\mathrm{SO}_{3}^{2-}\) ions, we had to ensure that every oxygen atom and the central sulfur atom obeyed the octet rule. In instances where the central atom could not fulfill its octet through single bonds, as with the phosphate ion \(\mathrm{PO}_{4}^{3-}\), a double bond was introduced. It is important to note that there are exceptions to the octet rule, particularly with elements that can have more than eight electrons in their valence shell.

Chemical Bonding

Chemical bonding is the process by which atoms combine to form compounds. This occurs through the interactions between valence electrons. There are several types of chemical bonds, including ionic, covalent, and metallic bonds. Ionic bonds result from the electrostatic attraction between ions of opposite charges. Covalent bonds are formed by the sharing of electron pairs between atoms. In the Lewis structures of polyatomic ions, the covalent bonds are represented by lines indicating shared pairs of electrons.

The strength and type of the bonds determine the physical and chemical properties of the substance formed. In our exercises involving sulfate, phosphate, and sulfite ions, we observed how electrons were shared to fulfill the octet rule, emphasizing the concept of stability in chemical bonding. By completing Lewis structures and checking for resonance, we gain a deeper insight into the nature of chemical bonding within these ions.