Lewis structures for a polyatomic ion

Lewis Structure for a Polyatomic Ion Draw the correct Lewis structure for the polyatomic ion phosphate (P04 ). 

Most Lewis structures give bonding pictures that are consistent with experimental information on bond strength and length. There are some molecules and polyatomic ions for which no single Lewis structure consistent with all characteristics and bonding information can be written. For example, consider the nitrate ion, NO3. To write a Lewis structure for this polyatomic ion, we use the following steps. 

Sometimes when writing the Lewis structure of a species, we may draw more than one possible correct Lewis structure for a molecule. The nitrate ion, N03 , is a good example. The structures that we write for this polyatomic anion differ in which oxygen has a double bond to the nitrogen. None of these three truly represents the actual structure of the nitrate ion—it is an average of all three of these Lewis structures. We use resonance theory to describe this situation. Resonance occurs when more than one Lewis structure (without moving atoms) is possible for a molecule. The individual structures are called resonance structures (or forms) and are written with a two-headed arrow (<- ) between them. The three resonance forms of the nitrate ion are ... 

You can use the procedure outlined below to draw the Lewis structures for molecules and ions that have a central atom, with other atoms around it. The Sample Problems and additional text that follow show how to apply these steps for several molecules and polyatomic ions that obey the octet rule. Afterwards, use Practice Problems 9 to 13 to practice drawing Lewis structures. 

Formal charge can also help answer the question where is the charge located that is frequently asked about polyatomic ions. Thus by writing out the Lewis structure for the ammonium ion NH4+, you should be able to convince yourself that the nitrogen atom has a formal charge of +1 and each of the hydrogens has 0, so we can say that the positive charge is localized on the central atom. 

The cyanide polyatomic ion, CN is similar in structure to carbon monoxide, CO. Although they work by different mechanisms, they are both poisons that can disrupt the use of oxygen, O2, in organisms. Draw a reasonable Lewis structure for the cyanide ion. 

Tip-off In this chapter, you are given a Lewis structure for a molecule or polyatomic ion (or a formula from which you can draw a Lewis structure), and asked to (1) name the electron group geometry arormd one or more atoms in the structure, (2) draw a geometric sketch of the structrue, and/or (3) name the molecular geometry around one or more of the atoms in the structure. You will find in later chapters that there are other tip-offs for these tasks. 

Resonance structures Two or more Lewis structures for a single molecule or polyatomic ion that differ in the positions of lone pairs and multiple bonds but not in the positions of the atoms in the structure. It is as if the molecule or ion were able to shift from one of these structures to another by shifting pairs of electrons from one position to another. 

After the Lewis structure for a molecule or polyatomic ion is developed, the shape of the species can be determined using the information in this table as a guide. 

The Lewis structures for polyatomic ions are developed in the same way as in the illustrated problems. However, in the case of a positive ion like NH4, there is one less than the total number of valence electrons, and in the case of a negative ion like CO/, there are two more than the total number of valence electrons. Lewis structures of typical polyatomic ions are illustrated by the... 

Consider the Lewis structure for the polyatomic oxyanion shown here, where X is an element fiom the third period (Na—Ar). By changing the overall charge, 11, from 1— to 2— to 3— we get three different polyatomic ions. For each of these ions (a) identify the central atom, X (b) determine the formal charge of the central atom, X (c) draw a Lewis structure that makes the formal charge on the central atom equal to zero. [Sections 8.5,8.6, and 8.7 ]... 

The sum of the formal charges in a Lewis structure must equal zero for a neutral molecule and must equal the magnitude of the charge for a polyatomic ion. [Thus for structure (10.17), this sum is +1 + 1 - 1 = +1.]... 

One form of the polyatomic ion I, has an unusual V-shaped structure one I atom lies at the point of the V, with a linear chain of two I atoms extending on each side. The bond angles are 88° at the central atom and 180° at the two atoms in the side chains. Draw a Lewis structure for L that explains its shape and indicate the hybridization you would assign to each nonterminal atom. 

It is also possible to write a Lewis structure for polyatomic anions or cations. The N - A = S rule can be used, but for an anion extra electrons equal to the magnitude of the negative charge must be added to the electrons available. If the ion is a cation, you will need to subtract number of electrons equal to the charge. 

An advantage of VSEPR is its foundation upon Lewis electron-pair bond theory. No mention need be made of orbitals and overlap. If you can write down a Lewis structure for the molecule or polyatomic ion in question, with all valence electrons accounted for in bonding or nonbonding pairs, there should be no difficulty in arriving at the VSEPR prediction of its likely shape. Even when there may be some ambiguity as to the most appropriate Lewis structure, the VSEPR approach leads to the same result. For example, the molecule HIO, could be rendered, in terms of Lewis theory as ... 

Using the same sequence of atoms, it is possible to have more than one correct Lewis structure when a molecule or polyatomic ion has both a double bond and a single bond. Consider the polyatomic ion nitrate (N03 ) shown in Figure 9-11a. Three equivalent structures can be used to represent the nitrate ion. Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. The two or more correct Lewis structures that represent a single molecule or ion are often referred to as resonance structures. Resonance structures differ only in the position of the electron pairs, never the atom positions. The location of the lone pairs and bonding pairs differs in resonance structures. The molecule O3 and the polyatomic ions N03, N02, 803 and C03 commonly form resonance structures. 

You now know how to draw Lewis structures for molecules and polyatomic ions. You can use them to determine the number of bonding pairs between atoms and the number of lone pairs present. Next, you will learn to describe molecular structure and predict the angles in a molecule, both of which determine the three-dimensional molecular shape. 

Given a formula for a molecule or polyatomic ion, draw a reasonable Lewis structure for it. 

Lewis structures can tell a lot about the bonding in covalent species, and they can be derived for almost all small molecules and polyatomic ions by following a prescribed sequence of operations. Let s go through this sequence and derive the Lewis structures for both the nitrate ion, N03 , and the formaldehyde molecule, H2CO, at the same time. 

To write a Lewis structure, we decide on the relative placement of the atoms in the molecule (or polyatomic ion)—that is, which atoms are adjacent and become bonded to each other—and distribute the total number of valence electrons as bonding and lone pairs. Let s begin by examining Lewis structures for species that obey the octet rule—those in which each atom fills its outer level with eight electrons (or two for hydrogen). 

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. 

Molecules or polyatomic ions often have many nonequivalent Lewis structures, all which obey the rules for writing Lewis structures. For example, as we will see in detail, the sulfate ion has a Lewis structure with all single bonds and several Lewis structures that contain double bonds. How do we decide which of the many possible Lewis structures best describes the... 

In the vast majority of molecules and polyatomic ions, the total number of valence electrons is even, and complete pairing of electrons occurs. However, in a few molecules and polyatomic ions, such as CIO2, NO, NO2, and O2, the number of valence electrons is odd. Complete pairing of these electrons is impossible, and an octet around each atom cannot be achieved. For example, NO contains 5 -I- 6 = 11 valence electrons. The two most important Lewis structures for this molecule are... 

---