Question

Draw the Lewis structure for ethylene, \(\mathrm{C}_{2} \mathrm{H}_{4}\)

Short answer

The Lewis structure for ethylene (C2H4) can be drawn by first counting the total valence electrons (12), then arranging the atoms with the two carbon atoms bonded together in the center and each carbon bonded to two hydrogen atoms. Place bond electrons between each atom, completing the octet rule for carbon by adding a double bond between the carbon atoms. The final Lewis structure is: \(H_{2}C=CH_{2}\).

Step-by-step solution

1. Count the valence electrons

All atoms in the molecule have valence electrons that they can share with other atoms to form a bond. Carbon has 4 valence electrons, while hydrogen has 1 valence electron. In ethylene (C2H4), there are 2 carbon atoms and 4 hydrogen atoms. Count the total number of valence electrons for the molecule: - Carbon: 2 atoms × 4 electrons = 8 - Hydrogen: 4 atoms × 1 electron = 4 Total valence electrons: 8 (from carbon) + 4 (from hydrogen) = 12

2. Arrange the atoms

In ethylene, the two carbon atoms are bonded to each other to form the backbone. Each carbon atom is also bonded to two hydrogen atoms. The molecule appears like this: H2C=CH2.

3. Place bond electrons

Now, place a pair of electrons between each carbon and hydrogen atom, as well as between the two carbon atoms. These are the shared electrons or bond electrons that hold the atoms together. - Carbon-carbon bond: 1 pair (2 electrons) - Carbon-hydrogen bond: 4 pairs (8 electrons) This accounts for all 12 valence electrons.

4. Complete the octet rule for carbon

Carbon requires 8 electrons in its valence shell to satisfy the octet rule. Check if each carbon atom has 8 electrons around it: - The left carbon atom has 2 electrons from the C-H bond, 2 electrons from the other C-H bond, and 2 electrons from the C-C bond. It currently has a total of 6 electrons. - The right carbon atom has the same situation as the left carbon atom with a total of 6 electrons. To complete the octet, an additional bond can be formed between the two carbon atoms, which will share 2 more electrons. Add a second bond between the carbon atoms.

5. Draw the Lewis structure

With all bond electrons accounted for and the octet rule satisfied, the final Lewis structure for ethylene (C2H4) looks like this: H2C=CH2 Remember, the double bond between the carbon atoms is represented by a double line.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how the atom will interact with others. These electrons can participate in forming chemical bonds with other atoms, as they are the ones most loosely held by the nucleus. In the context of ethylene ((C_{2}H_{4})), each carbon atom contributes four valence electrons and each hydrogen atom contributes one.

By adding up the valence electrons from all the atoms in ethylene, we come to a total of 12 valence electrons that are available for bonding. Understanding the count of valence electrons is the first step in predicting the chemical properties and bond patterns of a molecule, which eventually lead to the creation of its Lewis structure.

Chemical Bonding

Chemical bonding is the force that holds atoms together in a molecule. There are primarily three types of chemical bonds: ionic, covalent, and metallic. Ethylene, a simple hydrocarbon, is an example of a molecule that forms covalent bonds, which involve the sharing of pairs of valence electrons between atoms.

In the Lewis structure for ethylene, each hydrogen atom shares its single electron with a carbon atom, forming four simple covalent bonds. Moreover, the carbon atoms share a pair of electrons forming a single bond, and they also share another pair to fulfill the carbon atoms' need for more electrons, resulting in a double bond C=C. Through these shared pairs, or bonding pairs, all atoms achieve greater stability resembling noble gas configurations.

Octet Rule

The octet rule is a chemical rule of thumb that reflects the observation that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electron configuration as a noble gas.

The exception to this rule is hydrogen, which can reach a stable configuration with only two electrons. For the carbon atoms in ethylene, forming four single covalent bonds does not suffice to complete each carbon's octet. By forming a double bond, where two pairs of electrons are shared, each carbon ends up surrounded by an octet of electrons, adhering to the octet rule. This is reflected in the final Lewis structure of ethylene, (H_{2}C=CH_{2}), where each line between the atoms represents a pair of shared electrons.