Charge carriers in metals — what carries current in a metallic conductor? In an ordinary metallic conductor, the electrical current is primarily due to:

Introduction / Context:
Identifying the dominant charge carriers in different classes of materials is a fundamental concept in electrical engineering and solid-state physics. Metals, semiconductors, and electrolytes each have different mobile species responsible for conduction.

Given Data / Assumptions:

• Solid metallic conductor at room temperature.
• No electrochemical reactions; purely electronic conduction.
• Crystal lattice ions are fixed in position.

Concept / Approach:
In metals, valence electrons are delocalized, forming a conduction band. Under an applied electric field, these electrons acquire a drift velocity and carry current. Positive “holes” are not the dominant carriers in metals (though metals can be described by electron-like or hole-like quasiparticles in band theory, the physical mobile charges are electrons). Ions in the solid lattice are not mobile over electronic timescales and do not contribute to ordinary conduction.

Step-by-Step Solution:
Apply Drude model: current density J = n * e * v_d where v_d is electron drift velocity.Recognize ions are fixed at lattice sites and cannot drift under small fields.Conclude electrons are the primary carriers in metals.

Verification / Alternative check:
Hall effect measurements in metals confirm electron charge sign in most cases (negative Hall coefficient), consistent with electron conduction.

Why Other Options Are Wrong:
Holes are majority carriers in p-type semiconductors, not in metals. Lattice ions are not mobile under normal conditions. Photons are energy quanta, not charge carriers in conductors.

Common Pitfalls:

• Transferring semiconductor intuition (holes) to metals.
• Confusing electrolytic conduction (ions in solution) with metallic conduction.

Final Answer:
Electrons