Ethylene (C₂H₄), also known as ethene, is a simple alkene with a significant presence in the chemical industry. Understanding its Lewis dot structure is crucial for grasping its bonding and reactivity. This guide will walk you through drawing the Lewis structure for C₂H₄ step-by-step, explaining the underlying principles along the way.
Counting Valence Electrons
The first step in drawing any Lewis structure is determining the total number of valence electrons. Carbon (C) has four valence electrons, and hydrogen (H) has one. Therefore, for C₂H₄:
- 2 Carbon atoms × 4 valence electrons/atom = 8 electrons
- 4 Hydrogen atoms × 1 valence electron/atom = 4 electrons
- Total valence electrons = 12
Arranging Atoms
Next, we arrange the atoms. In ethylene, the two carbon atoms form a central backbone, with the hydrogen atoms bonded to them. A common arrangement is to place the carbons next to each other: C-C. The four hydrogens are then added, typically one hydrogen on each carbon:
H H
| |
C---C
Forming Single Bonds
We now connect the atoms with single bonds (each represented by a pair of electrons). Each single bond accounts for two valence electrons. Connecting the carbon atoms and each carbon to two hydrogens uses:
- 1 C-C bond = 2 electrons
- 4 C-H bonds = 8 electrons
- Total used = 10 electrons
Distributing Remaining Electrons
We started with 12 valence electrons and have used 10. This leaves 2 electrons unaccounted for. These electrons are placed as a bond between the two carbon atoms, creating a double bond:
H H
| |
C=C
Completing the Octet Rule
The octet rule states that atoms (except hydrogen and helium) tend to bond in such a way that they have eight electrons in their valence shell. In this structure:
- Each carbon atom has four bonds (two single bonds to hydrogen and one double bond to the other carbon). This satisfies the octet rule for carbon (8 electrons).
- Each hydrogen atom has one bond (a single bond to a carbon atom), satisfying its duet rule (2 electrons).
The Final Lewis Dot Structure for C₂H₄
The final Lewis dot structure for ethylene (C₂H₄) is:
H H
| |
:C=C:
Each line represents a shared pair of electrons, and the colons represent lone pairs (in this case, there are none). This structure shows the double bond between the carbon atoms, which is responsible for the molecule's planarity and reactivity.
Understanding the Implications of the Double Bond
The presence of the double bond in ethylene is crucial. It explains the molecule's rigidity (compared to ethane, which only has single bonds) and its propensity for addition reactions, where the double bond breaks to form two new single bonds. This understanding is fundamental in organic chemistry.
