Question

How many electron pairs are shared when two carbon atoms form a double bond? What must be the geometric arrangement around the carbon atoms involved in a double bond? Give an example of a simple molecule in which there is a carbon- carbon double bond, and draw its Lewis structure.

Short answer

When two carbon atoms form a double bond, they share two electron pairs. The geometric arrangement around the carbon atoms involved in a double bond is linear, with a bond angle of 180 degrees. An example of a simple molecule with a carbon-carbon double bond is ethene (C2H4), which has the following Lewis structure: H H \ / C=C / \ H H The carbon atoms have a trigonal planar arrangement with a bond angle of approximately 120 degrees.

Step-by-step solution

Determine the number of electron pairs shared in a carbon-carbon double bond

In a covalent bond between two atoms, each atom contributes one electron to be shared. A single bond consists of one electron pair shared between two atoms. A double bond consists of two electron pairs shared between two atoms. Thus, when two carbon atoms form a carbon-carbon double bond, they share a total of two electron pairs.

Determine the geometric arrangement around the carbon atoms involved in a double bond

In a double bond, the two electron pairs are located in two different regions of electron density. The area around the carbon atom can be described using VSEPR (valence shell electron pair repulsion) theory. According to VSEPR, electron pairs arrange themselves to minimize repulsion. When two electron pairs are present around a central atom, they adopt a linear arrangement, with a bond angle of 180 degrees.

Provide an example of a simple molecule with a carbon-carbon double bond and draw its Lewis structure

An example of a simple molecule with a carbon-carbon double bond is ethene (C2H4). The Lewis structure is drawn as follows: 1. Determine the total number of valence electrons in the molecule. Ethene has two carbon atoms (each with 4 valence electrons) and four hydrogen atoms (each with 1 valence electron), for a total of 12 valence electrons. 2. Place the two carbon atoms in the center and draw a double bond (two pairs of electrons shared) between them. 3. Place the four hydrogen atoms around the carbon atoms, forming single bonds with the available unpaired electrons on each carbon atom. The Lewis structure for ethene (C2H4) is shown below: H H \ / C=C / \ H H In this structure, the carbon atoms are sp2-hybridized, which means they form a trigonal planar arrangement with a bond angle of approximately 120 degrees around the carbon atoms.

Key concepts

Electron Pair Sharing

In chemistry, electron pair sharing is a fundamental concept that helps us understand how atoms bond with each other to form compounds. When two atoms come together to form a covalent bond, they share electrons so that each atom can achieve a more stable electron configuration, often resembling that of the nearest noble gas.
In the case of a carbon-carbon double bond, each carbon atom contributes two electrons towards the bond. This results in the sharing of two electron pairs, forming a stronger connection compared to a single bond.
One practical advantage of this double bond formation is its strength and the ability to impart specific properties to the molecule, such as rigidity and decreased rotation around the bond. These characteristics are particularly important in organic compounds, which frequently contain carbon-carbon double bonds.

VSEPR Theory

VSEPR, or Valence Shell Electron Pair Repulsion theory, is a useful model for predicting the shape of molecules. It explains how electron pairs around a central atom will arrange themselves to minimize electron repulsion. This arrangement affects the geometry of the molecule.
In the case of a carbon-carbon double bond, the two shared electron pairs form regions of electron density around the carbon atoms. As these electron pairs repel each other, they arrange themselves in a linear orientation to minimize repulsion. Consequently, this leads to a bond angle of 180 degrees between the carbon atoms when only the double bond is considered.
However, when forming single bonds with hydrogen atoms, as in ethene, the spatial arrangement becomes trigonal planar around each carbon due to hybridization, creating bond angles of approximately 120 degrees. Understanding VSEPR theory is crucial as it provides insights into molecular shapes, physical and chemical properties, as well as reactivity.

Lewis Structure

Lewis structures are a very helpful way to represent molecules visually and understand how atoms share electrons. When drawing a Lewis structure, start by calculating the total number of valence electrons in the molecule.
For example, in ethene ethene (C\(_{2}\)H\(_{4}\)) is composed of two carbon atoms and four hydrogen atoms. Each carbon atom has 4 valence electrons, and each hydrogen atom has 1, giving a total of 12 electrons.

• Begin by positioning the carbon atoms in the center, drawing a double bond between them to represent the shared electron pairs.
• Next, distribute the remaining electrons by forming single bonds between each hydrogen atom and the carbon atoms.

This forms a complete and stable configuration, illustrating the covalent bonding and helping to visualize the 120-degree angles caused by the trigonal planar arrangement around the carbons.
The Lewis structure tool simplifies understanding molecular composition and the sharing of electron pairs, fostering comprehension of chemical bonding intricacies.