Same element — the structure is doing all the work
The chemistry
DIAMOND — every carbon bonded to 4 others in a rigid 3D network.
Nothing can slide, and every outer electron is used in a bond. Result: the hardest natural substance known,
and a complete electrical insulator.
GRAPHITE — every carbon bonded to 3 others in flat sheets, with only
weak IMFs between the sheets and one spare delocalized electron per atom. Result: soft, slippery, and electrically
conducting.
The structure behind it: the number of covalent bonds each atom forms decides everything. 4 bonds = rigid,
insulating, hard. 3 bonds = layered, conducting, soft. Nothing about the element carbon itself changes.
Applications of graphite — soft and conducting
Pencils — Layers Left on Paper
Soft & slippery
A pencil "lead" is graphite mixed with clay. Drag it across paper and the weak IMFs between the sheets give way, so entire layers of carbon slide off and stick to the fibres. Harder pencils (2H) just have more clay to hold the layers back.
Motor Brushes & Sliding Contacts
Soft + conducting
An electric motor needs to feed current into a spinning shaft. Graphite is the only common material that is both a good conductor and soft enough to slide against metal without grinding it away — so every motor and generator uses graphite brushes.
Dry Lubricant
Layers slide
Oil boils away or gums up at high temperature. Graphite powder does not — the sheets keep sliding over each other at hundreds of degrees, so it lubricates locks, furnace parts and machinery where a liquid would fail.
Electrodes for Electrolysis
Inert conductor
Electrolysis needs an electrode that conducts but does not react with the molten or dissolved compound. Graphite conducts through its free electrons and is chemically inert — which is why aluminium smelters consume graphite anodes by the tonne.
Diamond Cutting & Drilling
Extreme hardness
Because every carbon is locked to four others in three dimensions, nothing in a diamond can shift. Diamond-tipped saws and drill bits cut stone, concrete and even other hard ceramics — and dentists' burs are diamond-coated for the same reason.
Diamond Heat Spreaders
Insulator, but conducts heat
Diamond is the best thermal conductor known — the rigid lattice passes vibrations along superbly — while being an electrical insulator. That rare combination makes synthetic diamond ideal for drawing heat out of high-power laser and radar chips without shorting them.
Side by side
Property
Diamond
Graphite
The structural reason
Bonds per carbon
4
3
The single fact everything else follows from
Hardness
Hardest known
Soft, slippery
Rigid 3D web vs. layers held by weak IMFs
Conducts electricity
No
Yes
All 4 electrons bonded vs. 1 delocalized per atom
Melting point
Very high
Very high
Both need strong covalent bonds broken
Typical use
Cutting, drilling, abrasives
Pencils, lubricant, electrodes
Hardness vs. softness + conductivity
This pair is the single best exam answer in the unit for "explain why structure determines properties". Two
substances, identical composition, opposite behaviour — and the whole difference is 4 bonds versus 3.
Alloys in Everyday Life
Distort the layers on purpose to get the property you want
Why anyone bothers
A pure metal has perfectly regular layers of identical ions, and those layers slide easily — which makes pure
metals soft. Add atoms of a different size and the layers become distorted, so they can no longer slip past
each other. The result is nearly always harder and stronger, and often better-looking or more corrosion
resistant as well.
Substitutional — similar-sized atoms take the place
of base-metal atoms. Brass, bronze, sterling silver, rose gold. Comparable radii ⇒ less variation in properties.
Interstitial — much smaller atoms wedge into the
gaps. Steel, cast iron. A tiny amount changes the properties enormously.
The structure behind it: metallic bonding is non-directional, so the cations can be rearranged without breaking the
bonding — which is why alloying works at all. Try the same trick on an ionic crystal and it just shatters.
Six alloys you have touched today
Stainless Steel — Cutlery & Appliances
Sub + interstitial
Iron with ~18% chromium substituted in and carbon in the interstices. The chromium reacts with air to form an invisible, self-repairing oxide film — scratch it and it seals again in seconds. That is why a fork survives a dishwasher and a plain iron nail does not.
Bronze — Statues, Bells & Bearings
Cu 78–95% · Sn 5–22%
Tin substitutes directly into the copper lattice, making it far harder than copper while staying easy to cast into moulds. It also resists seawater corrosion — which is why ship propellers and harbour fittings are bronze.
Brass — Instruments & Fittings
Cu 60–90% · Zn 10–35%
Zinc atoms are almost exactly copper-sized, so brass is a textbook substitutional alloy. It machines cleanly, resists corrosion, and is hard enough to hold a precise shape — everything a trumpet valve or a tap needs.
Rose Gold & 18K Gold — Jewellery
Au 75% · Cu · Ag
Pure gold is so soft it deforms in your fingers. 18K gold is 75% gold with copper and silver substituted in for hardness. Push the copper fraction higher and you get rose gold — an alloy chosen for the colour the copper contributes.
Sterling Silver — 92.5% Silver
Ag 92.5% · Cu 7.5%
Just 7.5% copper turns a metal too soft to use into cutlery and jewellery that survives daily handling. The trade-off is tarnish: the copper reacts with sulfur compounds in air, which is why silverware needs polishing.
Cast Iron — Pans & Engine Blocks
Fe 96–98% · C 2–4%
Small carbon atoms wedged into the gaps make cast iron very hard and rigid, and give it the thermal mass that keeps a steak searing. The same rigidity makes it brittle — a dropped cast iron pan can crack, where a steel one would only dent.
Alloy hardness explorer
Add an alloying element to a pure metal and watch what happens to the ability of the layers to slide.
0%
Hardness
Low
Malleability
Very high
Layers slide?
Easily
The named alloys, in one table
Alloy
Composition
Type
Property gained
Bronze
Copper 78–95%, tin 5–22%
Substitutional
Harder than copper, resists seawater
Brass
Copper 60–90%, zinc 10–35%
Substitutional
Hard, machinable, corrosion resistant
18K gold
Gold 75%, copper 12.5%, silver 12.5%
Substitutional
Hard enough to wear daily
Rose gold
Gold + a higher fraction of copper
Substitutional
Hardness — and a pink colour
Sterling silver
Silver 92.5%, copper 7.5%
Substitutional
Usable strength for cutlery and jewellery
Cast iron
Iron 96–98%, carbon 2–4%
Interstitial
Very hard and rigid (but brittle)
Steel
Iron + under ~1% carbon
Interstitial
Hard and strong, still tough
Stainless steel
Iron + ~18% chromium + carbon
Both
Hardness + a self-repairing rustproof layer
Ions on the Move
Electrolytes, sports drinks, IV fluids and batteries
The chemistry
An ionic compound only conducts when its ions are free to move — molten or dissolved. A solution that
conducts because of dissolved ions is called an electrolyte, and essentially every device or biological system
that moves charge through a liquid depends on one.
The structure behind it: ionic solids are made of charged particles held rigidly in a lattice. Dissolving supplies the
mobility that the charge already had — which is why salt water conducts and a salt crystal does not.
Where mobile ions matter
Sports Drinks — Replacing What You Sweat
Na⁺ · K⁺ · Cl⁻
Sweat is not just water — it carries dissolved Na⁺, K⁺ and Cl⁻ out of your body. Your nerves fire by moving those ions across cell membranes, so losing too many causes cramp and dizziness. A sports drink is essentially a dilute, flavoured ionic solution.
IV Saline — 0.9% NaCl
Medicine
A saline drip is sodium chloride dissolved to match the ion concentration of blood. Pure water would be catastrophic — with no dissolved ions outside them, red blood cells would swell and burst. The concentration matters as much as the compound.
Nerve Signals
Biology
Every thought you have is Na⁺ and K⁺ ions moving across a membrane. Your nervous system is, quite literally, an ionic conductor — and it stops working if the dissolved-ion balance drifts too far.
Batteries — Ions Inside, Electrons Outside
Electrochemistry
A battery drives electrons through your device's wiring, but inside the cell the charge is carried by ions moving through an electrolyte. No mobile ions, no current — which is why a battery left to dry out simply stops working.
Molten Salt Electrolysis
Industry · 950 °C
Aluminium is extracted by melting its ore in molten cryolite at around 950 °C and passing an enormous current through it. The whole industry exists because molten ionic compounds conduct — an unaffordable amount of heat, but there is no other route.
Why Water Near Electricity Is Dangerous
Safety
Pure H₂O is a poor conductor — it is a simple molecular substance. Tap water, bath water and sweat all contain dissolved ionic compounds, and those free ions are what carry a lethal current. The danger comes from the solute, not the solvent.
Conductivity meter
Choose what is in the beaker and how much of it, and watch the bulb. The only thing that matters is how many
free, charged particles there are.
60%
Dissolved particles
Ions
Charged?
Yes
Bulb
Bright
The comparison that catches people out
In the beaker
Dissolved particles
Conducts?
Why
Salt water
Na⁺ and Cl⁻ ions
Yes
Charged and free to move
Sugar water
Whole glucose molecules
No
Glucose is polar so it dissolves — but the molecules are neutral
Pure water
Essentially nothing
Barely
A simple molecular substance with no free charges
Solid salt
Ions, but locked in a lattice
No
Charged, but not free to move
Dissolving is not the same as conducting. Sugar dissolves beautifully in water and conducts nothing at all. What
makes a solution an electrolyte is not that something dissolved, but that what dissolved was charged.
Wiring, Cookware and Everything Shaped
One electron sea, four different jobs
The chemistry
Every property that makes metals useful comes from the same structural fact: positive ions in a mobile sea of
delocalized electrons. Those electrons carry electricity; they also carry heat; and because
the bonding is non-directional, layers of ions can slide, making metals malleable and ductile.
The structure behind it: in a covalent or ionic solid, every electron belongs to a specific bond or ion. In a metal
they belong to the whole crystal — which is exactly what "delocalized" means, and why nothing else behaves this way.
Applications
Copper Wiring
Conductive + ductile
Copper is chosen for two structural reasons at once: its electron sea conducts superbly, and its lattice is ductile enough to be drawn down into a hair-thin wire without snapping. Silver conducts slightly better but costs about a hundred times more.
Pans & Heat Conduction
Thermal conductivity
The free electrons pick up kinetic energy at the hot base and carry it through the whole pan in seconds, so food cooks evenly instead of burning in one spot. Copper-bottomed and aluminium pans exist for exactly this reason.
Aluminium Foil — Malleability
Malleable
Foil is rolled to about a hundredth of a millimetre without ever cracking, because each pass just slides layers of ions over each other while the electron sea holds everything together. Do the same to a salt crystal and it splits on the first pass.
Why Handles Are Plastic
Simple covalent
Plastic is a covalent molecular material with no free electrons, so heat crawls through it. The same structural rule that makes metal an excellent conductor makes the plastic handle an excellent insulator — and saves your hand.
Power Lines — Aluminium, Not Copper
Engineering trade-off
Overhead cables use aluminium with a steel core. Aluminium conducts a little worse than copper but weighs a third as much, so the pylons can be further apart — a reminder that real engineering weighs several properties at once.
Gold Contacts in Electronics
Unreactive metal
Gold is not the best conductor, but it is at the bottom of the activity series — it reacts with neither water nor acids. A gold-plated connector still conducts perfectly after years, while a copper one would have grown an insulating oxide layer.
Reactivity decides where a metal is allowed to go
1Very active (Na, K, Ca) — react with water or steam, so they are stored under oil and never used structurally.
2Somewhat active (Mg, Zn, Fe) — react with acids to give hydrogen: Mg + 2 HCl → H2(g) + MgCl2. Useful, but they need protecting from corrosion.
3Unreactive (Ag, Au, Pt) — react with neither water nor acid, which is precisely why they are used for jewellery, contacts and dental work.
✓Physical properties say what a metal can do; the activity series says whether it will survive doing it.
Quartz, Glass and Silicon Chips
What a giant covalent network is good for
The chemistry
Silica, SiO2, has essentially the same structure as diamond: every silicon atom covalently bonded to
four oxygen atoms in a continuous network. It is the main substance in rocks, and its pure
crystalline form is quartz. Being a giant covalent network gives it a very high melting point, real hardness,
and no free charges at all — so it is an electrical insulator.
The structure behind it: glass is made by melting silica and cooling it too fast for the network to crystallise. The
bonds are still covalent and still everywhere — which is why glass is hard and transparent, but also why it is brittle.
Applications
Glass-Making
≈ 1700 °C
Melting silica takes about 1700 °C — a direct consequence of having to break covalent bonds throughout a network. Adding sodium carbonate lowers that dramatically, which is the only reason window glass is affordable.
Borosilicate Labware
Thermal shock
Beakers are borosilicate glass: boron in the network makes it expand less when heated, so a hot beaker does not crack when it meets cold water. The structure is tuned by changing which atoms are in the network.
Silicon Chips
Semiconductor
Pure silicon has diamond's network structure and barely conducts. Add a few atoms of phosphorus or boron per million — doping, an interstitial-style trick — and its conductivity is transformed. Every processor you own is built on that control.
Optical Fibre
Transparent insulator
Ultra-pure silica carries light for tens of kilometres with almost no loss, and being an insulator it is completely immune to electrical interference. The world's internet traffic runs through a giant covalent network.
Abrasives — Sand & Carborundum
Hardness
Sandpaper works because silica is harder than wood, and silicon carbide (SiC) — another network solid — is harder still. Grinding wheels, sandblasting and cutting discs all exploit the same rigid 3D bonding.
Ceramic Insulators on Pylons
No free charges
Those grey discs holding cables away from a pylon are silica-based ceramics. With no ions free to move and no delocalized electrons, they hold off hundreds of thousands of volts — and survive rain, ice and decades of sunlight.
Every property traced back
Property of silica
Structural cause
What it is used for
Very high melting point
Covalent bonds throughout the network must be broken
Furnace linings, glass-making, crucibles
Hard
Every atom locked to 4 neighbours in 3D
Sandpaper, grinding wheels, sandblasting
Electrical insulator
No ions and no delocalized electrons
Pylon insulators, chip substrates
Insoluble in water
Water cannot break covalent bonds
Rocks and sand survive rivers and oceans
Transparent when pure
No free electrons to absorb visible light
Windows, lenses, optical fibre
Polarity in Practice
Like dissolves like — with real consequences
The chemistry
Water is a bent, polar molecule. It surrounds and separates other polar molecules and many ions — but it has
nothing to grip on a non-polar molecule, so those simply refuse to mix. That single rule, "like dissolves
like", explains salad dressing, oil spills, soap, and how medicines are designed.
The structure behind it: polarity is decided by the VSEPR shape, not just by the bonds. CO2 and
CCl4 both contain polar bonds and are both non-polar molecules, because their symmetry cancels the dipoles exactly.
Applications
Why Oil and Water Don't Mix
Like dissolves like
Oils are long non-polar hydrocarbon chains. Water molecules attract each other far more strongly than they attract oil, so they close ranks and squeeze the oil out into a separate layer. Nothing repels the oil — the water simply prefers itself.
Soap — Both Ends at Once
Amphipathic
A soap molecule has an ionic head that loves water and a long non-polar tail that loves grease. It surrounds a grease droplet tail-first and presents a charged outer surface, so the whole bundle washes away. Chemistry's neatest workaround.
Oil Spills Float
Environmental
Crude oil is non-polar and less dense than water, so it spreads into a thin surface film rather than dissolving. That is what makes booms and skimmers possible — and what makes the film so deadly to seabirds and to gas exchange at the surface.
Designing Medicines
Pharmacology
A drug must be polar enough to dissolve in blood but non-polar enough to cross a fatty cell membrane. Chemists tune that balance by adding or removing polar groups — and often supply a drug as its ionic salt form purely to make it dissolve.
Vitamins — Water vs Fat Soluble
Nutrition
Vitamin C is polar, so excess simply leaves in your urine. Vitamins A, D, E and K are non-polar and accumulate in body fat instead — which is exactly why you can overdose on those but not on vitamin C.
Dry Cleaning
Non-polar solvent
Greasy stains are non-polar, so water cannot shift them. Dry cleaning uses a non-polar solvent instead — like dissolves like, run in reverse. CCl₄ was the original choice, chosen for the tetrahedral symmetry that makes it non-polar.
Mixing tester
Pick two substances and see whether they mix — and, more importantly, why.
Solvent
Solute
Summary
Every application on this page, traced back to a structure
One table, the whole page
Application
Structure
Property used
Pencils, dry lubricant, motor brushes
Graphite
Layers slide (weak IMFs) + free electrons conduct
Drill bits, saws, heat spreaders
Giant covalent
Rigid 3D network ⇒ extreme hardness
Cutlery, statues, instruments, jewellery, pans
Metallic (alloys)
Distorted layers ⇒ harder than the pure metal
Sports drinks, IV saline, nerves, batteries
Ionic
Dissolved ions are charged AND mobile
Wiring, cookware, foil, power lines
Metallic
Delocalized electrons + sliding layers
Glass, optical fibre, chip substrates, abrasives
Giant covalent
High MP, hard, insulating, insoluble
Oil and water, soap, dry cleaning, drug design
Simple covalent
Molecular polarity — like dissolves like
The one sentence to take away
Structure is not a detail — it is the explanation. Two lumps of pure carbon end up in a pencil and in a drill
bit; two pieces of iron end up as a brittle pan and as flexible steel; the same sodium chloride is an insulator in the salt
cellar and a conductor in your bloodstream. Nothing about the ingredients changed in any of those pairs. What
changed was how the particles are arranged and what is free to move.
Where to next
Go back to the interactive tools for the lattice
viewer, the conductivity simulator and the alloy mixer — or test yourself with the
Structures Kahoot.