Question

How many nonbonding electron pairs are there in each of the following molecules: \((\mathrm{a})\left(\mathrm{CH}_{3}\right)_{2} \mathrm{S},(\mathbf{b}) \mathrm{HCN},(\mathbf{c}) \mathrm{C}_{2} \mathrm{H}_{2}\) \((\mathbf{d}) \mathrm{CH}_{3} \mathrm{F} ?\)

Short answer

In summary, the number of nonbonding electron pairs in each molecule are as follows: (a) (CH3)2S: 3 nonbonding electron pairs (1 on each C atom and 1 on the S atom), (b) HCN: 1 nonbonding electron pair (on the N atom), (c) C2H2: 0 nonbonding electron pairs, and (d) CH3F: 3 nonbonding electron pairs (on the F atom).

Step-by-step solution

(a) (CH3)2S

The molecule (CH3)2S consists of two CH3 groups bonded to a sulfur (S) atom. Each carbon (C) atom has 4 valence electrons, each hydrogen (H) atom has 1 valence electron, and the sulfur (S) atom has 6 valence electrons. In each CH3 group, the carbon atom makes 3 single bonds with 3 hydrogen atoms (total of 6 bonding electrons) leaving 2 electrons as a nonbonding pair on carbon. The sulfur atom in (CH3)2S forms bonds with two carbon atoms from each CH3 group, and shares 2 electrons per bond (total of 4 bonding electrons). The sulfur atom has 2 remaining electrons, which form a nonbonding electron pair. In total, there are 3 non-bonding electron pairs in the molecule: 1 for each carbon atom and 1 for the sulfur atom.

(b) HCN

The molecule HCN consists of a hydrogen (H) atom, a carbon (C) atom, and a nitrogen (N) atom. Hydrogen has 1 valence electron, carbon has 4 valence electrons, and nitrogen has 5 valence electrons. The hydrogen atom forms a single bond with the carbon atom, with a total of 2 bonding electrons. The carbon atom forms a triple bond with the nitrogen atom, with a total of 6 bonding electrons. There are no nonbonding electron pairs on the hydrogen or carbon atoms, as all their valence electrons are involved in bonding. Nitrogen has one lone pair remaining after the triple bond with carbon, so there is 1 nonbonding electron pair in HCN.

(c) C2H2

The molecule C2H2 consists of two carbon (C) atoms and two hydrogen (H) atoms. Carbon has 4 valence electrons and hydrogen has 1 valence electron. Each hydrogen atom forms a single bond with a carbon atom, with a total of 2 bonding electrons for the hydrogen atoms. The two carbon atoms are joined by a triple bond, with a total of 6 bonding electrons. The two carbon atoms and the two hydrogen atoms have no remaining valence electrons, so there are no nonbonding electron pairs in the C2H2 molecule.

(d) CH3F

The molecule CH3F consists of a carbon (C) atom, three hydrogen (H) atoms, and a fluorine (F) atom. Carbon has 4 valence electrons, hydrogen has 1 valence electron, and fluorine has 7 valence electrons. The carbon atom forms 3 single bonds with hydrogen atoms, with a total of 6 bonding electrons. The carbon atom also forms a single bond with the fluorine atom, sharing 2 bonding electrons. There are no nonbonding electron pairs on the carbon or hydrogen atoms, as all their valence electrons are involved in bonding. Fluorine has 3 lone pairs remaining after the single bond with carbon, so there are 3 nonbonding electron pairs in the CH3F molecule.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom. They play a crucial role in chemical bonding and reactions. Understanding them is essential for determining how atoms connect to form molecules. These electrons are found in the highest energy level of an atom and are the most loosely held. This makes them available for forming chemical bonds with other atoms.

• Each element has a different number of valence electrons, which can usually be determined based on its position in the periodic table.
• For example, carbon has four valence electrons, and hydrogen has one.
• Sulfur, found in group 16, has six valence electrons, while fluorine has seven.

Knowing the number of valence electrons helps predict the types of bonds an atom can form. For instance, in CH3F, carbon uses its four valence electrons to create one bond with fluorine and three bonds with hydrogen atoms. The remaining electrons on fluorine form nonbonding electron pairs (or lone pairs). Understanding valence electrons is key in inferring stability and reactivity of molecules.

Molecular Structure

The molecular structure reveals how atoms are organized in a molecule. Each molecule has a distinct shape that directly influences its physical and chemical properties. Molecular structures are often represented in two dimensions via Lewis structures, which help visualize bonding patterns and nonbonding electrons.

• For example, in the molecule \((\text{CH}_3)_2\text{S}\), the structure consists of a central sulfur atom bonded to two CH3 groups. The sulfur atom retains a nonbonding pair of electrons.
• In contrast, the molecule \(\text{HCN}\) has a linear structure, where carbon is in the middle, bonded to hydrogen at one end and triple bonded to nitrogen at the other end. This linear alignment is due to the triple bond stabilizing the molecule.
• The molecule \(\text{C}_2\text{H}_2\) consists of a triple bond between two carbon atoms, with single bonds to hydrogen atoms, forming a symmetrical linear shape.
• Meanwhile, \(\text{CH}_3\text{F}\) has a tetrahedral structure due to the single bonds between each hydrogen and carbon, and between fluorine and carbon.

Understanding the molecular structure is crucial to infer how a molecule will interact with others, predicting behavior and the role of nonbonding electron pairs.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom, typically expressed in a sequence of numbers and letters. This configuration provides insights into the chemical behavior of an atom, dictating how it bonds and interacts with other atoms.

• This configuration is pivotal in comprehending the distribution of valence electrons.
• A sulfur atom’s electron configuration is \([Ne] 3s^2 3p^4\), indicating six electrons in the 3s and 3p orbitals, which classify as its valence electrons in molecules like \((\text{CH}_3)_2\text{S}\).
• Fluorine’s electron configuration is \([He] 2s^2 2p^5\), showing that after forming a single bond, it maintains three nonbonding pairs of electrons in \(\text{CH}_3\text{F}\).
• Nitrogen, in \(\text{HCN}\), uses its electron configuration \([He] 2s^2 2p^3\) to form a triple bond and still has one lone pair of electrons.

These configurations highlight the tendency of atoms to complete orbitals for stability, often following the Octet Rule, which states that atoms tend to have eight electrons in their valence shell. This principle influences the arrangement and number of nonbonding electron pairs in a molecule.