What Went Wrong in My NaCl Density Calculation?

[Iyafrady]

I want to find the ensity of NaCl given the atomic weight of Na is 23 and that of Cl is 35.5.The ionic radius are 1A and 1.8A respectively.I foundthevolume of each element to be Vna=4.2A^3 and Vcl=24.4A^3.

V(NaCl)=28.6E-23
MASS(NaCl)=[(23+35.5)g/mole]*(1/avogadros number)=9.7E-23
So D=m/v=(9.7E^-23)/(28.6E-23)=3.39

but this is way off from the listed density of 2.17g/cm^3...dont know what I am doing wrong.

 

[Borek]

There is a lot of space between those spheres, you know...

You have to calculate volume of the unit cell.