Which one of the following is primarily responsible for conduction of current in a metal?
- (a)Bound electrons
- (b)Free electrons
- (c)Both bound and free electrons
- (d)Ions
Answer
Why
Correct — B, free electrons. Metals are held together by metallic bonding, in which each atom releases its loosely held valence electrons into a shared 'sea' of delocalised electrons around a lattice of positive ions. These free electrons are not tied to any one atom, so when a potential difference is applied they drift through the metal, and that drift is the electric current. This is why metals are good conductors even in the solid state.
Why the others are wrong
- (a)Bound electrons — Bound electrons are held tightly in the inner shells or in fixed bonds and cannot move through the lattice, so they do not carry current; conduction in a metal is by the delocalised free electrons.
- (c)Both bound and free electrons — Only the free (delocalised) electrons drift and conduct. The bound electrons remain localised on their atoms and take no part in carrying the current.
- (d)Ions — The positive metal ions form the fixed lattice and merely vibrate about their sites in a solid metal; they do not migrate to carry current. Ions carry current in electrolytes and molten salts, not in a solid metal.
Concept
A metal can be pictured as a regular lattice of positive ions immersed in a 'sea' of delocalised valence electrons. Because these electrons belong to the metal as a whole rather than to individual atoms, they respond to an applied electric field by drifting collectively, producing an electric current. This electron-sea model explains why metals conduct electricity and heat well.
The trap is to think that either the atoms' inner (bound) electrons or the metal ions do the conducting. In a solid metal the ions are locked in place and only vibrate, while bound electrons stay on their atoms; it is the mobile free electrons that carry charge. Ions become the current carriers only in a different situation, such as a molten salt or an electrolyte solution.
Key facts
- Metallic bonding gives a lattice of positive ions in a sea of delocalised (free) valence electrons.
- An applied voltage makes these free electrons drift, and that drift constitutes the electric current.
- The electrical conductivity of a pure metal decreases as temperature rises, because stronger lattice vibrations scatter the drifting electrons.
- In electrolytes and molten ionic compounds it is ions, not free electrons, that carry the current.
Study next
Common traps
- Thinking ions carry the current in a solid metal — in a solid metal the ions are fixed and only free electrons move.
- Confusing the bound valence electrons of an insulator with the delocalised free electrons of a metal.
A direct concept question on electrical conduction in metals — the current carriers are the delocalised free electrons.
Related PYQs
Which one of the following non-metals is NOT a poor conductor of electricity?
- (a) Sulphur
- (b) Selenium
- (c) Bromide
- (d) Phosphorus
Answer(b) Selenium — it behaves as a semiconductor whose conductivity rises on heating or illumination, unlike sulphur, bromine and phosphorus, which are poor conductors.
Tests the same idea of what allows a material to conduct electricity — mobile charge carriers — contrasting good conductors and semiconductors with poor conductors, the electron-availability principle behind metallic conduction.
Practice
- practice — not a real PYQ
In an electrolyte such as copper sulphate solution, the electric current is carried mainly by
- (a)free electrons
- (b)ions
- (c)protons
- (d)neutrons
Answer(b) ions — in electrolytes and molten salts it is the moving ions, not free electrons, that carry the current. - practice — not a real PYQ
As the temperature of a pure metallic conductor is raised, its electrical resistance generally
- (a)increases
- (b)decreases
- (c)stays constant
- (d)falls to zero
Answer(a) increases — stronger lattice vibrations scatter the drifting free electrons, raising the resistance.