Dot-and-Cross Diagram for Ethene

Introduction

Ethene contains two carbons and four hydrogens. A single carbon–carbon connection plus two hydrogens on each carbon would leave each carbon short of its usual octet. A double bond completes the structure. This example extends dot-and-cross counting beyond small diatomic molecules and introduces a central feature of unsaturated organic compounds.

Core explanation

The two carbon atoms contribute eight valence electrons, and four hydrogens contribute four, giving twelve total. Draw the connectivity with two carbons joined and two hydrogens attached to each carbon. Every hydrogen must have one ordinary single bond.

The four C–H bonds use eight electrons. Four electrons remain for the connection between the carbons, forming two shared pairs and therefore a C=C double bond. Each carbon counts four electrons in that double bond plus four in its two C–H bonds, giving an octet. Each hydrogen counts its own bonding pair and has a duet.

In an origin-labelled diagram, each carbon contributes two electrons to the double bond and one to each of its two C–H bonds. Hydrogens contribute one each. Different symbols can distinguish the carbon atoms, with an additional clear symbol for hydrogen contributions if necessary. The legend must make the bookkeeping unambiguous; symbol choice itself has no physical meaning.

The displayed formula is H₂C=CH₂. There are no lone pairs on carbon or hydrogen in the standard neutral ethene Lewis structure. Each carbon has three neighbouring atoms but a bond-order total of four. Confusing neighbour count with bond count would make the correct structure appear to violate carbon's familiar valency.

Around each carbon there are three electron regions, so the local arrangement is approximately trigonal planar. Ethene's six atomic centres are planar in its equilibrium structure. The double bond also restricts rotation: twisting one end relative to the other disrupts part of the bonding interaction. A double line therefore indicates more than an additional mark in a formula; it has consequences for geometry and reactivity.

Step-by-step reasoning

1. Count twelve valence electrons from two carbons and four hydrogens. 2. Draw four C–H single bonds, using eight electrons. 3. Allocate the remaining four electrons to a C=C double bond. 4. Check each carbon's octet, all hydrogen duets, the absence of lone pairs and the three electron regions around each carbon.

Visual explanation

Draw H₂C=CH₂ with the hydrogens angled above and below the C=C line. Replace each C–H line with a shared pair and the double line with two shared pairs. Label the structure planar and count three neighbours around each carbon.

Real-world analogy

A door attached by two aligned supports cannot twist as freely as an object hanging from a loose swivel. This suggests restricted rotation, though a real double bond's restriction comes from orbital overlap rather than mechanical hinges or literal supporting bars.

Real-world example

Ethene is a feedstock for making poly(ethene). In addition polymerisation, its double-bond electronic arrangement is reorganised as monomers become connected into chains. Carbon atoms are conserved; the process does not simply add extra atoms to an unchanged C=C bond while preserving every original bond order.

Why?

Why does C₂H₄ need a double bond in this standard connectivity? Four C–H bonds account for eight of its twelve valence electrons. The remaining four form two shared pairs between carbons and complete both octets without adding non-existent hydrogens or charges.

Common misconception

“A carbon with only three neighbours must have only three bonds.” In ethene, each carbon has two hydrogen neighbours and one carbon neighbour, but the carbon neighbour is connected by a double bond. Its bond-order total is therefore four.

Worked example

A drawing of C₂H₄ shows only a single C–C bond and four C–H bonds, with no lone or unpaired electrons. It contains five pairs, or ten electrons, rather than the required twelve. Add a second shared pair between the carbons. The corrected six pairs account for all electrons and give both carbons octets.

Quick check

1. How many electrons are shared directly between the two carbons in ethene's double bond? Answer: Four electrons, forming two shared pairs.

Exam focus

Include four C–H bonds and one C=C double bond. Check local valency by bond order, and do not draw an extra lone pair on either carbon after all twelve electrons are assigned.

Advanced insight

A carbon-carbon double bond is often described as one σ interaction and one π interaction. Sideways orbital overlap in the π component helps explain planarity and restricted rotation. The two components are not identical copies of a single bond and do not contribute equal energies.

Summary

Ethene uses twelve valence electrons in four C–H single bonds and one C=C double bond. Each carbon has three neighbours but four shared pairs locally. The planar structure and restricted rotation follow from the double bond's electronic arrangement.

Practice questions

1. How many bonding pairs occur in the entire ethene Lewis structure? Answer: Six: four in C–H bonds and two in the C=C bond. 2. How many lone pairs are drawn on carbon in neutral ethene? Answer: None in the standard structure. 3. Why is adding two more hydrogens not a correction to a question specifically asking for C₂H₄? Answer: It changes the molecular formula to a different compound instead of constructing the requested electron arrangement.