Homework Help: Calculating radius of Rubidium in unit cellHelp.

1. Oct 31, 2012

christian0710

1. The problem statement, all variables and given/known data

Calculate the Radius of Rubidium ion (which adopts sodium chloride structure.), if the density of Rubidium chloride is 2.76g*cm^(-3) and if we assume that the ions touch each other along the sides of the edge of the unit cell.

I'm a bit lost here. I assume the density is the density of the whole unit cell. But how can I find a radius without knowing any length at all?

2. Oct 31, 2012

Staff: Mentor

How many moles of RbCl per one cubic centimeter of the crystal? How many molecules it is? Unit cells?

3. Oct 31, 2012

christian0710

In one unit cell there must be 4 moles of sodium and 4 chloride units even thought the cation sodium is much smaller than Cl.

4. Oct 31, 2012

Staff: Mentor

Atoms, not moles. And mole is known number of atoms (molecules, ions, whatever).

5. Oct 31, 2012

christian0710

Ahh yes, so perhaps if we know the radii ratio? It's not given in the problem, but i could imagine we would have to know how many times one atom is bigger than another?

6. Nov 1, 2012

Staff: Mentor

On the second thought, while apparently mole step is necessary to calculate volume of the unit cell, I don't see how to calculate radius. Sum of radii of the cation and the anion is obvious, but separating them is not, unless I am missing something.

It happens that the radii are almost identical, but it is not said in the question.