Question

Draw Lewis structures for each of the following compounds: (a) ethane \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)\) (b) ethylene \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)\) (c) acetylene \(\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)\)

Short answer

Ethane (C2H6) has a single bond between the carbon atoms and three single bonds to hydrogen atoms for each carbon. Ethylene (C2H4) has a double bond between carbon atoms and two single bonds to hydrogen atoms for each carbon. Acetylene (C2H2) has a triple bond between carbon atoms and one single bond to a hydrogen atom for each carbon.

Step-by-step solution

Understanding the basics

Lewis structures are diagrams that represent the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. Each atom in a molecule shares a certain number of electrons to achieve the noble gas electron configuration. Hydrogen wants to have 2 electrons to be like helium, and carbon wants to have 8 electrons to be like neon.

Drawing Lewis Structure for Ethane (C2H6)

Ethane is comprised of two carbon atoms and six hydrogen atoms. The carbon atoms are bonded to each other by a single bond, and each carbon atom has three hydrogen atoms bonded to it, also by single bonds. Each bond represents a pair of shared electrons. The carbon atoms share a pair of electrons between them, and each carbon-hydrogen bond represents a carbon sharing a pair of electrons with a hydrogen.

Drawing Lewis Structure for Ethylene (C2H4)

Ethylene has two carbon atoms double bonded to each other. Each carbon atom also has two hydrogen atoms single bonded to it. The double bond between the carbon atoms accounts for four shared electrons (two pairs), and each carbon-hydrogen bond consists of a pair of shared electrons.

Drawing Lewis Structure for Acetylene (C2H2)

Acetylene consists of two carbon atoms triple bonded to each other, with each carbon atom also single bonded to one hydrogen atom. The triple bond between the carbon atoms consists of six shared electrons (three pairs), and each carbon-hydrogen bond consists of a pair of shared electrons.

Key concepts

Chemical Bonding

Chemical bonding is a fundamental concept in chemistry that explains how atoms join together to form molecules. Bonds are formed due to the interactions between the valence electrons of different atoms. These interactions result in molecules taking form in three main types of chemical bonds: ionic, covalent, and metallic bonds.

In the case of organic compounds like ethane, ethylene, and acetylene, covalent bonding is key. Covalent bonds involve the sharing of electron pairs between atoms, allowing each atom to achieve a stable electron configuration similar to that of noble gases. The type of bond—single, double, or triple—refers to the number of shared electron pairs. Ethane illustrates single bonds, ethylene has double bonds, and acetylene features a triple bond between carbon atoms. Understanding the differences in bonding helps explain the properties and reactivity of different molecules.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom that are involved in forming bonds. The number of valence electrons an atom has determines its bonding capacity and thus its role in molecules. For understanding Lewis structures, recognizing the count of these electrons is crucial.

Hydrogen has one valence electron and seeks to form a bond to resemble the helium configuration. Carbon, with four valence electrons, forms enough bonds to achieve eight valence electrons, mimicking the electron configuration of neon. In Lewis structures, valence electrons are represented as dots around the elements' symbols, with lines or pairs of dots signifying shared pairs in bonds. For instance, in ethane \(\mathrm{C}_{2}\mathrm{H}_{6}\), each carbon shares three pairs with hydrogen and one pair with another carbon.

Molecular Geometry

Molecular geometry, or the 3-dimensional shape of molecules, arises from the spatial arrangements of atoms and the chemical bonds that hold them together. The geometry is determined by the number of bond pairs and lone pairs of electrons around a central atom.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the geometry of molecules. It posits that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. For example, ethane has a tetrahedral geometry around each carbon atom, due to the four regions of electron density (three hydrogen atoms and one carbon). Ethylene has a planar geometry, as the double bond consists of bonded electron pairs spread out over a larger area, and acetylene has a linear geometry since the carbon atoms are connected by a triple bond with electron pairs directly opposite each other.