# Why does the colour of gold sol change with particle size? [duplicate]

Regarding the colour of gold sol, my book says

Finest gold sol is red in colour; as the size of particles increases, it appears purple, then blue and finally golden.

This is the image I found on Wikipedia which portrays the same phenomenon.

The same page also discusses the effect of size etc. on the colour, but I don't understand exactly what's happening.

Why shouldn't the colour be the same for all sizes? Isn't it characteristic of the molecules which make the sol as the light absorbed and reflected depends on them?

Does this mean that when we break down any other thing to very small particles, it's sol will change in colour?

• Generally, the electronic structure of a small particle is a function of size - the high fraction of atoms that are on the surface alters the overall bonding. So, yes, changing size can change the color you see since the electronic transitions in the small volume are different. – Jon Custer Mar 10 '19 at 0:47
• Yes, the color of colloids is dependent on size. Besides absorption, there is scattering as well. @Natasha, read about the term plasmon, which is collective oscillation of charge density in an electrical conductor. This is the beauty of nanochemistry although Faraday the great knew about this property long time ago. – M. Farooq Mar 10 '19 at 1:18