# 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.
– user137289
Jan 5, 2018 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 ? Jan 5, 2018 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. Jan 5, 2018 at 23:33
• @JonCuster Good example to catching misunderstanding points. Thank you. Jan 5, 2018 at 23:39