For the molecule ethene (C2H4), which description correctly represents its electron dot and cross (Lewis) structure and the bonding around each carbon atom?

Introduction / Context:
This question focuses on the electron dot and cross structure, also called the Lewis structure, of the organic molecule ethene with formula C2H4. Ethene is a simple unsaturated hydrocarbon containing a carbon carbon double bond. Understanding its bonding helps students visualise sigma and pi bonds, valence electrons, and the octet rule as applied to carbon atoms in organic chemistry.

Given Data / Assumptions:

• Molecule: ethene, molecular formula C2H4.
• Each carbon atom has four valence electrons.
• Each hydrogen atom has one valence electron and forms one covalent bond.
• The electron dot and cross structure must show how electrons are shared to satisfy the octet rule for carbon.

Concept / Approach:
In ethene, the two carbon atoms are bonded by a double bond. A double bond consists of one sigma bond and one pi bond. Each carbon atom in ethene is sp2 hybridised, forms three sigma bonds, and uses remaining unhybridised p orbitals to form a shared pi bond. The electron dot and cross representation must show that each carbon is bonded to two hydrogens by single covalent bonds and to the other carbon by a double covalent bond, giving each carbon a total of eight shared electrons in its valence shell.

Step-by-Step Solution:

Verification / Alternative check:
An alternative way to confirm the structure is to use valence bond theory and hybridisation. Each carbon in ethene is sp2 hybridised, which predicts three sigma bonds arranged in a trigonal planar geometry and one unhybridised p orbital forming a pi bond. This exactly matches the description that each carbon forms three sigma bonds and one pi bond. The electron dot and cross diagram that reflects this shows two shared pairs between the carbons and one shared pair between each carbon and each hydrogen.

Why Other Options Are Wrong:

• Option B: Four single bonds to hydrogen and no C C double bond would correspond to ethane (C2H6), not ethene, and would use the wrong number of hydrogen atoms.
• Option C: A single bond between the carbons leaves each carbon short of an octet and does not match the known double bond in ethene.
• Option D: A triple bond with only one hydrogen on one carbon describes a different molecule with formula C2H2, similar to ethyne, not C2H4.
• Option E: Delocalised electrons over all six atoms would suggest an aromatic system like benzene, which is not the case for ethene.

Common Pitfalls:
Students sometimes confuse ethene with ethane or ethyne and assign the wrong number of bonds between carbons. Another common mistake is to forget that carbon must have four bonds in total and that ethene must contain a carbon carbon double bond, not a single or triple bond. Learners may also misuse the electron dot notation and fail to count total valence electrons correctly. Careful counting and applying the octet rule helps avoid these errors.

Final Answer:
The correct description is that each carbon atom forms three sigma bonds (two with hydrogen atoms and one with the other carbon atom) and one pi bond between the two carbons, giving each carbon a complete octet.