Dot-and-Cross Diagram for Hydrogen Chloride

Introduction

Hydrogen and chlorine follow different shell-counting patterns: hydrogen needs a duet, while chlorine commonly follows an octet. A single shared pair satisfies both local counts in HCl. This example also introduces unequal sharing, because a bond between different elements need not distribute its electron density equally between its two ends.

Core explanation

Hydrogen contributes one valence electron and chlorine contributes seven, giving eight available electrons. Choose a cross for hydrogen and dots for chlorine. Place the cross and one dot between H and Cl as the shared pair. Chlorine's remaining six dots form three lone pairs.

Hydrogen counts the two bonding electrons towards its first-shell duet. Chlorine counts the same two bonding electrons plus six lone-pair electrons, giving eight locally. The entire molecule contains eight valence electrons, not ten, because adding the local counts double-counts the shared pair.

Write H–Cl as the displayed formula: the single line represents one shared pair. The formula HCl gives one atom of each element. A complete dot-and-cross diagram also includes chlorine's three lone pairs, which a simple displayed formula normally omits.

Chlorine attracts the bonding electron density more strongly than hydrogen, so the bond is polar. Hydrogen has partial positive character, written δ+, and chlorine partial negative character, written δ−. These labels are not the same as separate H⁺ and Cl⁻ ion charges. The neutral molecule still contains shared electron density connecting the atoms.

The environment matters. Hydrogen chloride gas contains molecules, while dissolving it in water leads predominantly to hydrated proton species and chloride ions. At this level, that acid behaviour is a separate chemical change from drawing an isolated HCl molecule. Do not use the familiar aqueous name hydrochloric acid as evidence that every HCl molecule must already be a pair of fully separated ions.

Step-by-step reasoning

1. Count eight valence electrons from one hydrogen and one chlorine. 2. Place one electron from each atom in a shared pair between them. 3. Arrange the remaining six electrons as three lone pairs on chlorine. 4. Check hydrogen's duet, chlorine's octet and overall neutrality; add partial-charge labels only when discussing unequal sharing.

Visual explanation

Draw H beside Cl with a cross and dot between them. Place three dot pairs around chlorine. In a second version use Hδ+–Clδ−, explaining that δ denotes partial charge within the molecule, not a pair of separate ions.

Real-world analogy

Two people holding the same rope can pull with unequal strength without either person releasing it. This helps picture unequal sharing. Electrons do not behave as a literal rope, but the analogy distinguishes a biased shared connection from complete transfer.

Real-world example

An equation that labels HCl(g) refers to gaseous hydrogen chloride, while HCl(aq) conventionally refers to its aqueous acid solution. State symbols give important context for the particles present. The same element ratio can require different descriptions when the surrounding medium changes.

Why?

Why can one pair satisfy both atoms despite their different shell capacities? Hydrogen counts only its first shell and needs two electrons. Chlorine also has three lone pairs, so adding the shared pair completes its eight-electron outer count without adding extra electrons around hydrogen.

Common misconception

“Chlorine has three lone pairs, so hydrogen must have three too.” The atoms start with different valence inventories and have different shell capacities. Hydrogen supplies only one electron and counts a duet in its bond; it has no lone pair in HCl.

Worked example

A proposed HCl drawing shows one shared pair and two lone pairs on chlorine. Its total is six valence electrons, while the neutral atoms supply eight. Add a third chlorine lone pair. The corrected drawing uses all eight electrons, gives H a duet and Cl an octet, and requires no overall ionic charge.

Quick check

1. Which atom has the three lone pairs in the standard HCl dot-and-cross diagram? Answer: Chlorine; hydrogen has no lone pair and shares just one bonding pair.

Exam focus

Show the lone pairs when asked for outer-shell electrons. Distinguish δ charges in a polar covalent molecule from full ion charges in brackets, and use state information when discussing aqueous acid behaviour.

Advanced insight

A proton in water is associated with surrounding water molecules rather than existing as an isolated bare H⁺ particle. Writing H₃O⁺ is one useful introductory representation, though hydration involves a more extensive and changing hydrogen-bonded environment.

Summary

HCl has one shared pair and three lone pairs on chlorine. Hydrogen counts a duet, chlorine an octet, and the total valence inventory is eight. Unequal sharing makes the bond polar; its behaviour in water requires a separate description of hydrated ions.

Practice questions

1. How many valence electrons are supplied by neutral H and Cl together? Answer: Eight: one from hydrogen and seven from chlorine. 2. Does δ− on chlorine mean the isolated HCl molecule has overall charge −1? Answer: No. It marks partial negative character within an overall neutral polar molecule. 3. Why should a gaseous HCl diagram not automatically be replaced by separate ions because hydrochloric acid conducts? Answer: The gas and aqueous solution involve different environments and particle descriptions; ion formation in water is not the same as the isolated molecular diagram.