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. nitrite ion b. hydrogen carbonate ion c. hydroxide ion

Short answer

The Lewis structures for the given polyatomic ions are as follows: a. Nitrite ion (NO2-): There are two resonance structures for the nitrite ion. 1. O\(= \)N - O\(^-\) 2. O - N\(= \)O\(^-\) b. Hydrogen carbonate ion (HCO3-): There are two resonance structures for the hydrogen carbonate ion. 1. O\(= \)C - O - H | O\(^-\) 2. O - C\(= \)O - H | O\(^-\) c. Hydroxide ion (OH-): Lewis structure: O - H | There is only one Lewis structure for the hydroxide ion, as there are no resonance structures.

Step-by-step solution

Calculate the total number of valence electrons for each ion

First, we need to determine the total number of valence electrons in each ion. For this step, remember that each atom has a specific number of valence electrons: - Nitrogen (N) has 5 valence electrons - Oxygen (O) has 6 valence electrons - Carbon (C) has 4 valence electrons - Hydrogen (H) has 1 valence electron In addition, we need to account for the charges on the ions. a. Nitrite ion (NO2-): 1 Nitrogen atom, 2 Oxygen atoms, and 1 extra electron due to the negative charge Valence electrons: 5 (N) + 6 (O) * 2 + 1 = 18 b. Hydrogen carbonate ion (HCO3-): 1 Hydrogen atom, 1 Carbon atom, 3 Oxygen atoms, and 1 extra electron due to the negative charge Valence electrons: 1 (H) + 4 (C) + 6 (O) * 3 + 1 = 24 c. Hydroxide ion (OH-): 1 Oxygen atom, 1 Hydrogen atom, and 1 extra electron due to the negative charge Valence electrons: 6 (O) + 1 (H) + 1 = 8

Draw the Lewis structures for the ions

Next, we will draw the Lewis structures for each ion, arranging the valence electrons to complete the octets for all atoms. a. Nitrite ion (NO2-): Nitrogen is the central atom, connected to both Oxygen atoms. Place a single bond between N and each O atom. There are two resonance structures for the nitrite ion. Resonance structure 1: O\(= \)N - O\(^-\) Resonance structure 2: O - N\(= \)O\(^-\) b. Hydrogen carbonate ion (HCO3-): Carbon is the central atom, connected to all three Oxygen atoms and the Hydrogen atom. The hydrogen atom is connected to one of the oxygen atoms. There are two resonance structures for the hydrogen carbonate ion. Resonance structure 1: O\(= \)C - O - H | O\(^-\) Resonance structure 2: O - C\(= \)O - H | O\(^-\) c. Hydroxide ion (OH-): Oxygen is the central atom, connected to the Hydrogen atom. Place a single bond between O and H, and assign two lone pairs to oxygen. Lewis structure: O - H | (There is only one Lewis structure for the hydroxide ion, as there are no resonance structures.)

Key concepts

Valence Electrons

Valence electrons play a pivotal role in chemical bonding and the properties of atoms and molecules. They are the electrons in the outermost shell of an atom and are primarily responsible for the chemical reactions that occur. In the context of drawing Lewis structures for polyatomic ions, such as nitrite (NO₂^-), hydrogen carbonate (HCO₃^-), and hydroxide (OH^-), valence electrons are depicted as dots (for non-bonding pairs) and lines (for bonding pairs).

To determine the number of valence electrons in ions, we begin by adding up the valence electrons from each atom. For ions, we must also consider the charge; a negative charge means an additional electron should be added, while a positive charge means an electron must be subtracted. Once the total count of valence electrons is known, it can be used in the subsequent steps to construct the Lewis structure, ensuring all elements (except hydrogen) follow the octet rule whenever possible.

Resonance Structures

Some molecules and ions exhibit what is called resonance – they have two or more valid Lewis structures that effectively describe the same molecule. These resonance structures represent the delocalization of electrons within a molecule and help in understanding the distribution of electrons more accurately.

In our exercise for the nitrite and hydrogen carbonate ions, we observe that these ions can be represented by more than one valid Lewis structure. The different resonance forms, such as for the nitrite ion (NO₂^-), are not separate structures but are theoretical constructs that help to represent a real structure that is a hybrid of all the resonance forms. Resonance structures are typically shown by using a double-headed arrow between the different forms. It's essential to recognize that the actual structure has an electron density that is a blend of the resonance forms, and the more stable the individual resonance form, the more it contributes to the resonance hybrid.

Octet Rule

The octet rule is a chemical rule of thumb that reflects a pattern of bonding for atoms of the main group elements. According to this rule, atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electron configuration as a noble gas. The rule is applicable to the second period elements and beyond, with the exception of hydrogen, which seeks to fill its valence shell with only two electrons.

When we draw Lewis structures, such as those for the nitrite, hydrogen carbonate, and hydroxide ions, we strive to give most of the atoms in these structures an octet of electrons. However, there could be exceptions such as expanded octets for elements in and beyond the third period (phosphorous, sulfur, etc.), and incomplete octets for certain molecules. The exercise of drawing Lewis structures for polyatomic ions essentially involves distributing the total valence electrons in a way that fulfills the octet rule for each atom, considering any constraints given by the ion's charge or molecular structure.