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
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.
- (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.
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.
- 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.
- 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.
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 — 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.