Question

Explain the Hume-Rothery Rule. Which parameters are more critical than others? What happens if two metals satisfy/don’t satisfy the rule?

Answer 1

Hume-Rothery rule explains the conditions need to be satisfied in order to form a solid solution between two metals.

According to Hume-Rothery rule, there are four parameters which determines the degree to which a solute metal dissolves in a solvent metal. These are given as follows:

1. Atomic size

2. Crystal Structure

3. Electronegativity

4. Valency

Atomic sizes of the solute and solvent atoms do not differ by more than 15%. Otherwise, the solute atoms create substantial lattice distortions.

Crystal structure of both metals must be the same for extensive solid solubility.

Electronegativity difference between the elements should be small. Otherwise they will form an intermetallic compound.

Valency of the metals should be equal. Or else, a metal has more tendency to dissolve another metal with higher valency.

All these four parameters are required to see the solubility of a metal in another metal. Even though we can say, Crystal structure and Electronegativity are the critical parameters which determines the formation of a complete solid solution; means two elements are completely soluble in one another at all proportions.

Any two metals satisfying Hume-Rothery rule will form a complete solid solution like copper and nickel.

If metals violates Hume-Rothery rule, then they may form intermetallic compounds or electron compounds like that.

In a nutshell, Hume-Rothery rule governs the formation of substitution solid solutions.