Question

Gold is plated with rhodium to give a base for mounting diamonds in modern jewellery. The rhodium-gold alloy consists of gold atoms in fcc structure with half the face centers being replaced by rhodium atoms. Formula of this alloy is :

• A. $A{u}_{5}R{h}_3$
• B. $AuR{h}_3$
• C. $A{u}_{3}R{h}_5$
• D. $A{u}_{3}Rh$

Answer 1

The correct option is A $A{u}_{5}R{h}_3$

In an fcc unit cell:
Number of atoms at corners $\left(Au\right)=\frac{1}{8}\times 8=1$
Number of face center atoms $=\frac{1}{2}\times 6=3$
Half of face centre atoms is $Rh$, other half is $Au$.
$\therefore Au=1+\frac{3}{2}=\frac{5}{2};Rh=\frac{3}{2}$
$\therefore A{u}_{5}R{h}_3$ is the formula of alloy.