Why Do Metals Conduct Electricity?

Metals are generally excellent electrical conductors because of their metallic structure.

A metal consists of:

• Positive metal ions arranged in a giant lattice.
• Delocalized electrons that can move throughout the structure.

These mobile electrons allow electrical charge to move through the metal.

This gives us the basic relationship:

metallic structure → mobile delocalized electrons → electrical conductivity

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Delocalized Electrons

In metallic bonding, the outer electrons of metal atoms become delocalized.

A delocalized electron is not associated with one particular atom.

Instead, it can move throughout the metallic lattice.

The positive metal ions remain in relatively fixed positions while the electrons can move between them.

This is very different from substances in which electrons are tightly localized within particular bonds.

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Electrical Current

An electric current is a flow of electrical charge.

In a solid metal, this charge is carried by:

electrons

When there is no potential difference across the metal, the delocalized electrons are already moving, but their motion is random.

There is no overall movement of charge in one direction.

When a potential difference is applied, the electrons gain a small overall drift through the metal.

This produces an electric current.

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Electron Flow in a Metal

Imagine connecting a metal wire to a battery.

The battery creates a potential difference across the wire.

The electric field established in the wire causes the delocalized electrons to experience a force.

The electrons develop an overall movement toward the positive terminal.

Therefore:

electron flow is from negative toward positive

in the external metallic circuit.

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Conventional Current

By convention, electrical current is described as moving:

from positive to negative

This convention was established before scientists understood that electrons were the moving charge carriers in metals.

Therefore:

electron flow: negative → positive

conventional current: positive → negative

Both descriptions can be used, but they refer to opposite directions.

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Electrons Do Not Race Directly Through the Wire

A common misconception is that individual electrons travel extremely rapidly from a battery all the way through a circuit.

The actual average drift velocity of electrons can be quite small.

However, when a circuit is completed, an electric field is established through the conducting material very rapidly.

Electrons throughout the conductor respond to this field.

This is why a lamp can turn on almost immediately even though individual electrons are not racing from the battery to the lamp at the speed of light.

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Resistance in Metals

Electrons do not move through a metal completely without interference.

The metal ions in the lattice are vibrating.

As electrons move through the structure, they interact with the lattice and can be scattered.

This opposition to current is associated with:

electrical resistance

A good conductor has relatively low resistance.

A poor conductor has relatively high resistance.

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Temperature and Resistance

When a metal becomes hotter:

• Metal ions vibrate more strongly.
• Moving electrons experience more scattering.
• Electron flow becomes more difficult.

For most ordinary metals:

temperature increases → resistance increases

This is an important property of metallic conductors.

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Why Wires Can Become Hot

When electrons move through a conductor with resistance, electrical energy can be transferred to the lattice.

The ions vibrate more strongly.

The temperature of the metal increases.

This is called resistive heating or Joule heating.

It is useful in devices such as:

• Electric heaters.
• Toasters.
• Kettles.
• Hair dryers.

However, unwanted resistive heating in electrical cables represents an energy loss.

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Electrical Conductivity

Electrical conductivity describes how readily a material allows electric current to flow.

High conductivity means:

charge moves relatively easily

Low conductivity means:

charge movement is strongly restricted

Metals generally have high electrical conductivity because they contain mobile delocalized electrons.

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Good Metallic Conductors

Some metals conduct electricity better than others.

Silver is an exceptionally good electrical conductor.

Copper is also an excellent conductor and is widely used in electrical wiring.

Aluminum is another important conductor.

The material chosen for a particular application depends on more than conductivity.

Engineers may also consider:

• Cost.
• Density.
• Strength.
• Corrosion resistance.
• Flexibility.
• Availability.

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Why Copper Is Used for Wiring

Copper is widely used for electrical wiring because it combines several useful properties.

It has:

• High electrical conductivity.
• Good ductility.
• Good mechanical strength.
• Good durability.

Ductility allows copper to be drawn into long, thin wires.

Its conductivity allows electrical current to pass with relatively low resistance.

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Why Aluminum Is Used in Power Lines

Aluminum is also a good conductor.

It is not as electrically conductive as copper for the same cross-sectional area, but it has a major advantage:

low density

Aluminum is much lighter than copper.

For overhead power transmission, low mass can be extremely important.

Therefore, engineering decisions involve balancing several material properties rather than simply selecting the material with the highest conductivity.

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Thermal Conductivity

Metals are generally good thermal conductors as well as electrical conductors.

Thermal conductivity describes how readily thermal energy moves through a material.

There are two important contributors to thermal conduction in metals:

• Mobile delocalized electrons.
• Vibrations of the metallic lattice.

The mobile electrons are particularly effective at transferring energy rapidly through the metal.

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How Thermal Energy Moves Through a Metal

Suppose one end of a metal rod is heated.

Particles near the hot end gain energy.

The metal ions vibrate more strongly.

The delocalized electrons also gain energy and move throughout the structure.

These mobile electrons transfer energy through interactions with other electrons and the lattice.

As a result:

thermal energy spreads rapidly through the metal.

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Everyday Example: A Metal Spoon

Place a metal spoon in a hot drink.

After a short time, the handle becomes warm.

Thermal energy has traveled from the hot end of the spoon toward the cooler end.

This happens relatively quickly because the metal is a good thermal conductor.

A wooden or plastic spoon behaves differently because these materials are much poorer thermal conductors.

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Conductors and Insulators

Materials can be broadly classified according to their electrical conductivity.

Conductors

Allow electrical charge to move relatively easily.

Examples:

• Copper.
• Aluminum.
• Silver.
• Iron.

Insulators

Strongly resist the movement of electrical charge.

Examples:

• Rubber.
• Many plastics.
• Glass.
• Dry wood.

This is why electrical cables often contain:

metal conductor inside + insulating plastic outside

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Why Many Nonmetals Do Not Conduct

In many nonmetallic molecular substances, electrons are localized in bonds or around particular atoms.

They are not free to move throughout the entire material.

Therefore, these substances generally cannot carry electrical current easily.

For example, many plastics are good electrical insulators.

This makes them useful for coating electrical wires.

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Comparing Metals and Molecular Substances

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Comparing Metals and Ionic Compounds

Ionic compounds behave differently from metals.

A solid ionic compound contains charged particles, but the ions are locked into fixed lattice positions.

Therefore:

solid ionic compound → usually does not conduct

When the ionic compound is melted or dissolved in water, its ions may become mobile.

Then:

molten or aqueous ionic compound → can conduct

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Comparing Charge Carriers

Different conducting materials can use different charge carriers.

Metals

Charge carrier:

electrons

Molten ionic compounds

Charge carriers:

positive and negative ions

Ionic solutions

Charge carriers:

dissolved ions

This distinction is important.

Electrical conductivity does not always require moving electrons.

It requires mobile charged particles.

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Graphite: An Important Exception

Carbon is a nonmetal, but graphite can conduct electricity.

In graphite, each carbon atom forms bonds with three other carbon atoms.

Some electrons become delocalized across the layers.

These electrons can move and carry charge.

Therefore, graphite demonstrates that:

electrical conductivity depends on structure and mobile charge carriers, not simply whether an element is classified as a metal.

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Diamond: A Useful Comparison

Diamond is also made entirely from carbon.

However, diamond has a very different structure.

Each carbon atom forms four strong covalent bonds.

The electrons are localized in these bonds.

There are no comparable mobile delocalized electrons available to carry charge.

Therefore:

diamond does not normally conduct electricity.

Graphite and diamond demonstrate an important principle:

structure determines properties.

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Comparing Graphite and Diamond

The chemical composition is the same.

The structure is different.

Therefore, the physical properties are different.

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Conductivity and Metallic Structure

Metallic conductivity can be summarized as:

metal atoms contribute outer electrons

↓

electrons become delocalized

↓

electrons can move throughout the lattice

↓

electrons carry electrical charge

↓

metal conducts electricity

This is the key structure-property relationship.

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Thermal Conductivity and Metallic Structure

Similarly:

delocalized electrons gain energy

↓

electrons move through the lattice

↓

energy is transferred rapidly

↓

metal conducts thermal energy

The same structural feature therefore contributes to both:

electrical conductivity

and:

thermal conductivity

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Comparing Conductivity Experimentally

Conductivity can be investigated using a simple electrical circuit.

A sample can be placed into a circuit containing:

• A power source.
• Connecting wires.
• An indicator such as a lamp or suitable meter.

Different materials can then be tested.

A better quantitative investigation would measure current and potential difference rather than relying only on lamp brightness.

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Comparing Thermal Conductivity Experimentally

Different materials can also be compared for thermal conductivity.

For example, rods of different materials could have one end heated under similar conditions.

The rate at which thermal energy reaches another point on each rod can then be compared.

Important control variables could include:

• Rod length.
• Rod diameter.
• Starting temperature.
• Heating time.
• Distance from the heat source.

A fair comparison requires these variables to be controlled.

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Electrical Conductivity and Resistance

Conductivity and resistance are related but different concepts.

A material with high electrical conductivity allows charge to move readily.

A conductor with high resistance opposes current more strongly.

For a particular wire:

longer wire → greater resistance

greater cross-sectional area → lower resistance

Temperature and the material itself also affect resistance.

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Why Thin Wires Have Greater Resistance

Imagine electrons moving through a wire.

A thin wire provides a smaller cross-sectional area for charge flow than a thick wire made from the same material.

Therefore:

thinner wire → greater resistance

for wires of the same length and material.

This is important when choosing wires for electrical systems.

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Why Long Wires Have Greater Resistance

A longer wire gives moving electrons a longer path through the metallic lattice.

There are more opportunities for interactions that impede the flow of charge.

Therefore:

longer wire → greater resistance

for wires of the same material and cross-sectional area.

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Material Matters

Different metals have different electrical conductivities because their electronic structures and interactions between electrons and the lattice differ.

Therefore, two wires with identical:

• Length.
• Diameter.
• Temperature.

can still have different resistances if they are made from different materials.

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Real-World Connection: Electrical Cables

A typical electrical cable combines materials with very different properties.

Inside

Copper or aluminum.

Purpose:

conduct electricity

Outside

Plastic or rubber-like insulation.

Purpose:

prevent unwanted movement of charge and protect users

The cable works because each material is chosen for a different property.

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Real-World Connection: Cooking Pans

Cooking pans are often made from or incorporate metals because metals transfer thermal energy effectively.

However, handles may be made from:

• Plastic.
• Wood.
• Silicone.
• Other insulating materials.

This reduces thermal energy transfer to the user's hand.

A single product can therefore deliberately combine:

thermal conductors + thermal insulators

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Real-World Connection: Heat Sinks

Electronic devices can generate significant thermal energy.

Metal heat sinks are used to transfer this energy away from electronic components.

Metals such as aluminum and copper are useful because of their high thermal conductivity.

Fins increase the surface area available for transferring energy to the surroundings.

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Real-World Connection: Power Transmission

Electrical power must often travel over large distances.

Resistance in transmission wires causes energy to be transferred as thermal energy.

This is why conductor material and wire dimensions are important.

Engineers choose materials that balance:

• Conductivity.
• Mass.
• Strength.
• Cost.
• Durability.

Aluminum-based conductors are commonly useful for overhead transmission because they combine good conductivity with relatively low mass.

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Worked Example: Copper and Plastic

Question: Why does copper conduct electricity while plastic usually does not?

Copper

Copper has a metallic structure containing mobile delocalized electrons.

These electrons can move through the lattice and carry charge.

Plastic

Electrons in most plastics are localized within chemical bonds.

There are no comparable mobile charge carriers.

Conclusion

Copper conducts well, while plastic acts as an electrical insulator.

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Worked Example: Solid NaCl and Copper

Question: Both copper and sodium chloride contain charged particles. Why does copper conduct when solid while NaCl does not?

Copper

Contains mobile delocalized electrons.

Therefore:

conducts when solid

Solid NaCl

Contains Na⁺ and Cl⁻ ions locked into fixed lattice positions.

Therefore:

does not conduct when solid

When NaCl melts, the ions become mobile and the liquid can conduct.

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Worked Example: Graphite and Diamond

Both substances contain only carbon atoms.

However:

graphite has mobile delocalized electrons

while:

diamond does not

Therefore:

graphite conducts electricity

while:

diamond does not normally conduct

The difference results from their structures.

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Worked Example: Heating a Metal Rod

Suppose one end of a copper rod is heated.

Particles at that end gain energy.

The mobile electrons transfer energy rapidly through the structure, while lattice vibrations also transfer energy.

Therefore, the other parts of the rod become warmer.

This demonstrates:

thermal conductivity

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Common Mistakes

Saying Positive Metal Ions Flow Through a Wire

In a solid metal, the ions remain in lattice positions.

The mobile charge carriers are electrons.

Saying Electrons Are Stationary Until a Battery Is Connected

Delocalized electrons are already moving.

Applying a potential difference creates an overall drift.

Confusing Electron Flow with Conventional Current

Electron flow:

negative → positive

Conventional current:

positive → negative

Saying All Nonmetals Are Insulators

Graphite is an important exception because it contains delocalized electrons.

Saying Ionic Compounds Never Conduct

They generally do not conduct when solid, but they can conduct when molten or dissolved because their ions become mobile.

Assuming All Metals Conduct Equally Well

Different metals have different electrical conductivities.

Forgetting About Thermal Conductivity

Metallic structure helps explain both electrical and thermal conduction.

Assuming Conductivity Depends Only on the Material

For an actual wire, resistance also depends on dimensions and temperature.

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Check Your Understanding

1. Why do metals conduct electricity?

2. What is a delocalized electron?

3. What particles carry electrical charge through a solid metal?

4. What happens to electrons when a potential difference is applied across a metal?

5. State the direction of electron flow in an external circuit.

6. State the direction of conventional current.

7. Why can a lamp turn on quickly even though electron drift velocity is relatively small?

8. What causes electrical resistance in a metal?

9. Why does the resistance of most metals increase as temperature increases?

10. Explain resistive heating.

11. Why is copper widely used for electrical wiring?

12. Why is aluminum useful for overhead power lines?

13. Explain why metals are generally good thermal conductors.

14. Why does a metal spoon become warm when left in a hot drink?

15. Compare electrical conductors and insulators.

16. Why does solid NaCl not conduct electricity?

17. Why can molten NaCl conduct electricity?

18. What particles carry current through an ionic solution?

19. Why can graphite conduct electricity?

20. Why does diamond not normally conduct electricity?

21. How does increasing wire length affect resistance?

22. How does increasing wire thickness affect resistance?

23. Why are electrical cables made from both a metal and an insulating material?

24. Explain how metallic structure accounts for both electrical and thermal conductivity.

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Key Terms

• Electrical conductivity – ability of a material to allow electrical charge to move through it.
• Thermal conductivity – ability of a material to transfer thermal energy.
• Delocalized electron – electron able to move throughout a larger structure rather than belonging to one particular atom or bond.
• Electric current – rate of flow of electrical charge.
• Electron flow – movement of electrons through a conductor.
• Conventional current – conventional direction of positive charge flow, opposite to electron flow in metals.
• Drift velocity – average directed motion of charge carriers caused by an electric field.
• Resistance – opposition to electrical current.
• Conductor – material that allows electrical charge to move relatively easily.
• Insulator – material that strongly resists electrical charge movement.
• Charge carrier – mobile charged particle responsible for electrical conduction.
• Resistive heating – transfer of electrical energy into thermal energy due to resistance.
• Metallic lattice – giant structure of positive metal ions surrounded by delocalized electrons.

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Key Takeaways

• Metals conduct electricity because they contain mobile delocalized electrons.
• Positive metal ions remain in relatively fixed lattice positions.
• Delocalized electrons move throughout the metallic structure.
• Applying a potential difference produces an overall drift of electrons.
• Electron flow in an external metallic circuit is from negative toward positive.
• Conventional current is defined in the opposite direction.
• Electrical resistance results from interactions between moving electrons and the metallic structure.
• For most metals, increasing temperature increases resistance.
• Resistance can convert electrical energy into thermal energy.
• Different metals have different electrical conductivities.
• Copper combines high conductivity with excellent ductility and is widely used for wiring.
• Aluminum combines good conductivity with low density and is useful for power transmission.
• Metals are also generally good thermal conductors.
• Mobile electrons contribute strongly to energy transfer through metals.
• Many molecular substances are poor electrical conductors because they lack mobile charged particles.
• Solid ionic compounds generally do not conduct because their ions cannot move.
• Molten or dissolved ionic compounds can conduct because their ions are mobile.
• Graphite is a nonmetal that conducts because it contains delocalized electrons.
• Diamond does not normally conduct because its electrons are localized in covalent bonds.
• Conductivity depends fundamentally on the presence of mobile charge carriers.
• Metallic structure → delocalized electrons → mobile charge → electrical conductivity.