# How does calculate the density of a compound material?

I have working on a particle detector question. There is a crystal that made of NaI. I need to calculate density of NaI. Here is my calculation way,

$\rho_{NaI} = \omega_{Na}*\rho_{Na}+\omega_{I}*\rho_{I}$

$\omega_i = \frac{A_i}{\Sigma A_i}$

To calculation ratios of the all component of compound, I am using atomical mass ratio. After that, I have obtained $\rho=4.322 g/cm^3$.

But Wikipedia says $\rho=3.67g/cm^3$

What mistake is I made in calculation ?

• There are equal numbers of both ions. And elemental densities are useless here. – Pieter Jan 5 '18 at 23:16
• Actually I did not understand what you mean. I think there is only importing thing is density. So, how can we calculate density of compound if we do not use these densities ? – agenel Jan 5 '18 at 23:25
• This is perhaps easiest to see with allotropes. Both diamond and graphite are made of carbon, yet they have different densities. You cannot calculate the density of compounds starting from elemental densities. – Jon Custer Jan 5 '18 at 23:33
• @JonCuster Good example to catching misunderstanding points. Thank you. – agenel Jan 5 '18 at 23:39