Subject: Why do some substances have a greater index of refraction than others?

Hi!

Thank you taking the time to read this.  

Specifically, my question is   

I've been through many websites and they say that "As mass density increases, 
GENERALLY, the index of refraction increases"... 

but I would like to know why some substances don't follow this.  For example, 
isopropyl alcohol(commonly known as rubbing alcohol) has a density of 0.785g/mL 
while water has a universal density of about 0.99g/mL.  Following the rule 
above (As density increases, GENERALLY, the index of refraction increases), 
water should have a greater index of refraction than alcohol but instead, it is 
the opposite.  

This phenomenon also occurs with canola oil and water.    

(water's index of refraction = 1.333 // alcohol's index of refraction= 1.375) 
index of refraction: alcohol>water | mass density: water>alcohol
 
(water's index of refraction = 1.333 // canola oil's refractive index= 1.466)
index of refraction: oil>water | mass density: water>oil

I vaguely believe that it has something to do with the atoms inside the 
substance and 'optical density' but I can't get a hold of any websites  
explaining to me why this phenomenon occurs.  

I would appriciate any information regarding this occurance.  Thank you for 
help, I am fully thankful for your effort.

Francis  

Re: Why do some substances have a greater index of refraction than others?