Metallic and Network Bonding Revisited

Introduction

An elemental substance need not consist of isolated molecules. Copper, diamond and solid iodine illustrate three different ways to organise atoms. Their properties follow from the particles and connections that actually extend through the solid. A Lewis sketch of one bond is useful, but it cannot by itself describe an entire metal or covalent network.

Core explanation

In a metal, valence electrons occupy states spread through a large array of atoms. The simple classroom picture of positive metal cores in a mobile electron sea conveys electrical conduction and malleability, though actual metallic bonding is described more precisely with electronic bands. When layers of metal atoms slip, bonding is not restricted to one fixed pair of neighbours, so the material can deform without necessarily breaking into separate molecules. Metallic strength and melting points vary widely; one should not infer an exact value merely from the word metal.

Diamond is a covalent network. Each carbon is bonded approximately tetrahedrally to four other carbon atoms, with the pattern extending throughout the crystal. Melting or cutting the crystal requires disrupting numerous strong bonds within the network. There are no mobile charge carriers in pure diamond under ordinary conditions, so it is a poor electrical conductor. Graphite is also elemental carbon but has a different network: each carbon bonds to three neighbours in sheets. Delocalised electrons within sheets help graphite conduct, while relatively weak attractions between sheets permit easy sliding. Thus a material cannot be classified by elemental formula alone.

Solid iodine contains discrete I₂ molecules. Strong covalent I–I bonds hold each molecule together, but dispersion attractions hold the molecules to their neighbours. When iodine sublimates, intact I₂ molecules enter the gas; the process principally overcomes intermolecular attraction, not the covalent bond inside every molecule. Molecular solids often melt or sublime more readily than extended networks, but dispersion can become large for very large molecules, so the distinction should be framed structurally rather than as an absolute temperature rule.

Conductivity must specify physical state. Solid ionic sodium chloride is an extended electrostatic lattice, yet its ions cannot move freely; molten NaCl conducts because ions can move. This differs from a metal, whose electrons carry current in the solid state. Graphite's electrical behaviour also differs from diamond's despite both being carbon. Identify the mobile charge carrier before explaining an observed current.

Step-by-step reasoning

1. Decide whether the structural units are atoms in a metal, atoms in an extended covalent network, or individual molecules. 2. Identify what holds those units together across the whole sample. 3. Ask whether electrons or ions can move through the specified state. 4. Connect melting or deformation to the connections that must be disrupted. 5. Check for allotropes or structural exceptions before generalising from composition.

Visual explanation

Sketch copper as connected atom cores under a spread-out electron cloud, diamond as a three-dimensional bond framework, and iodine as separate paired atoms with dotted intermolecular attractions. Circle the actual unit that moves during melting or sublimation.

Real-world analogy

A metal resembles a crowded hall where a shared floor of activity extends around every person. Diamond resembles a rigid scaffold bolted together in all directions. Iodine resembles many separate two-person teams that merely stand near one another. The analogy concerns connectivity, not literal motion of electrons or atoms.

Real-world example

Electrical wiring uses copper because it carries charge through a solid and can be drawn into wire. Diamond is selected for abrasive cutting because of its hard network. Graphite in a pencil transfers thin layers to paper because its sheets can slide.

Why?

Why can graphite conduct while diamond does not? Graphite's carbon bonding leaves delocalised electron states within its sheets. Diamond's bonding electrons are more localised in a tetrahedral framework, so there are no comparably mobile carriers under ordinary conditions.

Common misconception

“Covalent substances never conduct.” Graphite is a covalent network with mobile electrons. Conversely, a covalent molecular substance usually lacks mobile charge carriers. The arrangement of bonds, not the adjective covalent alone, matters.

Worked example

Classify copper, diamond and iodine crystals. Copper is metallic: electrons spread through the atomic lattice, allowing solid-state conduction. Diamond is network-covalent: carbon–carbon bonds extend in three dimensions, giving rigidity and poor ordinary electrical conduction. Iodine is molecular: I₂ molecules are individually covalent but held together in the crystal mainly by dispersion. Heating iodine to sublime separates molecules rather than cleaving each I–I bond.

Quick check

1. Which carries charge in molten NaCl? Answer: Mobile Na⁺ and Cl⁻ ions, not electrons moving as in copper.

Exam focus

State the structural unit and mobile charge carrier. Do not explain graphite or diamond with a generic claim about all carbon. Distinguish breaking bonds within molecules from separating molecules.

Advanced insight

Electronic band theory describes allowed energy ranges in solids. Whether a band is partially filled, or whether a small energy gap separates occupied and empty states, helps explain metal, semiconductor and insulator behaviour more accurately than the electron-sea picture.

Summary

Metallic bonding, covalent networks and molecular solids differ in how bonding extends through the sample. Connectivity controls many mechanical and thermal trends; available moving electrons or ions control electrical conduction. Allotropes demonstrate why formula alone is inadequate.

Practice questions

1. Why is diamond rigid? Answer: Strong covalent bonds extend in three dimensions through its crystal. 2. What holds iodine molecules together in the solid? Answer: Primarily London dispersion attractions between separate I₂ molecules. 3. Why can solid copper conduct? Answer: Delocalised electronic states provide mobile charge carriers. 4. Does graphite's conductivity prove it is metallic? Answer: No. It is a covalent network with delocalised electrons in its sheets.