Question

Write the Lewis structure for each molecule (octet rule not followed). \begin{equation}\mathrm { a. } {BBr}_{3} \quad \text { b. NO } \quad \text { c. } \mathrm{ClO}_{2}\end{equation}

Short answer

For BBr3, Boron is at the center with three single bonds to Bromine atoms. For NO, Nitrogen forms a double bond with Oxygen and has one unpaired electron. For ClO2, Chlorine is central, with single bonds to two Oxygen atoms and has a lone pair of electrons, exceeding the octet rule.

Step-by-step solution

Lewis Structure for BBr3

Bromine (Br) has 7 valence electrons and Boron (B) has 3. Since Boron is less electronegative, it will be the central atom. Place Boron in the center and distribute three Bromine atoms around it. Connect each Bromine atom to Boron with a single bond, using 6 of Boron's valence electrons. Boron does not follow the octet rule and is stable with 6 valence electrons while Bromine atoms have complete octets.

Lewis Structure for NO

Nitrogen (N) has 5 valence electrons and Oxygen (O) has 6. Place Nitrogen in the center and connect it to Oxygen with a double bond, using 4 of Nitrogen's and 4 of Oxygen's valence electrons. This gives Oxygen a full octet and leaves Nitrogen with one unpaired electron, which is shown as a single dot. NO does not follow the octet rule because Nitrogen has only 7 valence electrons.

Lewis Structure for ClO2

Chlorine (Cl) has 7 valence electrons and each Oxygen (O) has 6. Place Chlorine in the center and connect it to two Oxygen atoms with a single bond each, using 4 of Chlorine's valence electrons. Then complete the octet of both Oxygen atoms by adding six electrons around each. Since Chlorine is capable of having more than 8 electrons, place a lone pair on the Chlorine, ending up with 7 valence electrons for Chlorine. ClO2 does not follow the octet rule because Chlorine has an expanded valence shell.

Key concepts

Valence Electrons

Valence electrons are the electrons present in the outermost shell of an atom that can participate in the formation of chemical bonds. These electrons play a crucial role in determining how atoms interact with each other. In the Lewis structure, valence electrons are represented by dots surrounding the elements' symbols. Each dot signifies one valence electron available for bonding or as a lone pair.

Understanding the number of valence electrons an element has is critical for drawing its Lewis structure. For example, in the compound BBr3, Bromine (Br) has 7 valence electrons, while Boron (B) has only 3. The way these electrons are distributed among the atoms in a molecule determines the chemical bonds and the structure of the molecule. Therefore, getting familiar with the concept of valence electrons is the first step in mastering the ability to draw accurate Lewis structures.

Octet Rule

The octet rule is a fundamental concept in chemistry that refers to the stability achieved when an atom has 8 valence electrons in its outermost shell, mimicking the electron configuration of noble gases. This tends to make the atom more stable from a chemical standpoint.

The rule is often applied when drawing Lewis structures, as atoms typically bond in a manner that results in each atom (especially for the main group elements) having a complete valet of electrons. However, there are exceptions to the octet rule, which become evident when dealing with certain elements or molecules. For instance, the molecule NO does not satisfy the octet rule, as Nitrogen (N) ends up having only 7 electrons after forming a double bond with Oxygen (O). These exceptions are important to recognize because they highlight the limitations of the rule and help us to understand the actual behavior of atoms within different chemical environments.

Expanded Valence Shell

The expanded valence shell refers to the ability of certain elements to have more than 8 electrons in their outermost shell, which is a departure from the octet rule. This phenomenon occurs primarily with non-metal elements from the third period of the periodic table and beyond, as these have d-orbitals available for bonding that can accommodate extra electrons.

Chlorine (Cl) in ClO2 is a classic example of an expanded valence shell. It can form bonds with two Oxygen (O) atoms and still have room for a lone pair of electrons. This results in Chlorine having more than 8 valence electrons around it. The concept of an expanded valence shell is an extension of the octet rule and helps explain the structures of many compounds, especially those involving heavier elements that are able to go beyond the 'octet' due to the available d-orbitals in their valence shell.