Problem 21 - Chemistry in Focus

Question

Draw a Lewis structure for each of the following covalent compounds: a. \(\mathrm{I}_{2}\) b. \(\mathrm{NF}_{3}\) c. \(\mathrm{PCl}_{3}\) d. \(\mathrm{SCl}_{2}\)

Step-by-Step Solution

Answer

\(\mathrm{I}_{2}\): I-I with three lone pairs on each I atom. \(\mathrm{NF}_{3}\): N with a single bond to each of three F atoms and one lone pair, each F with three lone pairs. \(\mathrm{PCl}_{3}\): P with a single bond to each of three Cl atoms and one lone pair, each Cl with three lone pairs. \(\mathrm{SCl}_{2}\): S with a single bond to each of two Cl atoms and two lone pairs, each Cl with three lone pairs.

Step 1: Understanding the Lewis Structure

Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist. The Lewis structure is used to represent covalent bonding, where electrons are shared between atoms to achieve a full outer electron shell, following the octet rule (except for hydrogen, which follows the duplet rule).

Step 2: Drawing Lewis Structure for \(\mathrm{I}_{2}\)

Step 1: Count the valence electrons. Iodine (I) has 7 valence electrons and there are two iodines, so there are a total of 14 valence electrons. Step 2: Arrange the atoms. Two iodine atoms are bonded together. Step 3: Distribute the electrons. Each iodine shares one pair of electrons to form a single bond, and each will have three pairs of nonbonding electrons to complete the octet.

Step 3: Drawing Lewis Structure for \(\mathrm{NF}_{3}\)

Step 1: Count the valence electrons. Nitrogen (N) has 5 valence electrons and fluorine (F) has 7. Since there are 3 F atoms, the total is 5 + 3*7 = 26 valence electrons. Step 2: Arrange the atoms. N is the central atom with three F atoms around it. Step 3: Distribute the electrons. Form a single bond between N and each F, using 6 electrons. Distribute the remaining 20 electrons as lone pairs to fulfill the octet rule, giving each F 6 additional electrons, and the N one lone pair.

Step 4: Drawing Lewis Structure for \(\mathrm{PCl}_{3}\)

Step 1: Count the valence electrons. Phosphorus (P) has 5 valence electrons and chlorine (Cl) has 7. With three Cl atoms, the total is 5 + 3*7 = 26 valence electrons. Step 2: Arrange the atoms. P is the central atom with three Cl atoms around it. Step 3: Distribute the electrons. Form a single bond between P and each Cl. Use the remaining 20 electrons to give each Cl three pairs of nonbonding electrons, and P has one pair of nonbonding electrons.

Step 5: Drawing Lewis Structure for \(\mathrm{SCl}_{2}\)

Step 1: Count the valence electrons. Sulfur (S) has 6 valence electrons and each Cl has 7. With two Cl atoms, the total is 6 + 2*7 = 20 valence electrons. Step 2: Arrange the atoms. S is the central atom with two Cl atoms on either side. Step 3: Distribute the electrons. Form a single bond between S and each Cl. Use the remaining 16 electrons to provide three pairs of nonbonding electrons to each Cl and two pairs to S.

Key Concepts

Covalent Compounds

Covalent compounds are substances where atoms share electrons to achieve chemical stability. This sharing allows each atom to reach a state that resembles that of a noble gas, with a filled outer electron shell.

In covalent bonding, two atoms contribute one or more valence electrons to form a shared pair. These electrons constitute what we refer to as a molecule's bond. As these shared electrons count for both atoms' outer shells, covalent bonds enable atoms to adhere to the octet rule and result in the formation of a covalent compound.

For example, a molecule of water, H₂O, consists of two hydrogen atoms each sharing its one valence electron with an oxygen atom, which in turn shares two of its six valence electrons. Thus, the oxygen ends up with a full shell of eight electrons, aligning with the octet rule, and the compound itself is held together by these shared electrons.

Valence Electrons

Valence electrons are the electrons found in an atom's outer shell, and they play a crucial role in chemical reactivity and bonding. The number of valence electrons determines how an atom will interact with others; atoms bond in an effort to either fill or empty their valence shell to become stable, like noble gases.

The first step in drawing Lewis structures is to count these valence electrons since this count will guide the arrangement of electrons around atoms. Elements within the same group on the periodic table have the same number of valence electrons, which can be helpful to remember — for instance, all halogens have seven valence electrons, while all alkali metals have one. Understanding the arrangement of valence electrons is essential in predicting how atoms will bond and what shape the resulting molecule will take.

The Octet Rule

The octet rule is a principle that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, forming an electron configuration that is stable and similar to that of noble gases. With a full or satisfied shell, atoms reach a lower-energy, more stable state.

There are exceptions to the octet rule; for example, hydrogen and helium are stable with two valence electrons (the duet rule), and there are molecules with odd numbers of electrons that can't form an octet. However, in the majority of cases, especially when dealing with organic and simple inorganic compounds, the octet rule applies.

When we draw Lewis structures as shown in the textbook example, the objective is to pair up valence electrons between atoms to give each atom (except hydrogen) eight electrons through shared and unshared pairs. The rule serves as a guideline to predict the connectivity and distribution of electrons in a molecule which ultimately determines its shape and reactivity.