[hsebast1]

i was told by my instructor that LiBr has a density of 3.464 g/cm3 and it also the same structure of NaCl (unit cell). he told us to calculate the intermolecular distance.
how can i go about answering this question. the best way that i see is right for this is to figure out the lenght of the side of the unit cell and cut it in half, hence your answer. formula for density is mass/volume. how can i calulate the mass, therefore i can get the volume and then lenght of the side on the unight cell..is this a right method....

[cth]

1) If you know NaCl structure, you can determine how many Na⁺ and Cl^- you have in each unit cell.
2) You can calculate as well the weight of one atom of Na and one atom of Cl from their molecular weight and the Avogadro number.
3) From these two information, you can calculated the size of the unit cell and its volume.
4) Then, knowing how ions are arranged into NaCl, you can determine the distance Li-Br.

[hsebast1]

and how is this done

[cth]

1) number of Na and Cl
Do you know how NaCl is arranged? You can have a look there http://www.greatneck.k12.ny.us/GNPS/SHS/dept/science/Blumberg/index.htm for a nice looking picture.  You see the green and purple spheres? You simply need to count them. Easy.  But carefull, there are a few pitfalls you need to avoid to get it right. 
The unit cell is cubic.
Each atom that is sitting on one corner of the cube should be counted for 1/8 of an atom only. Because that atom is shared equally between 8 adjacent unit cells.
Each atom that is sitting on one edge of the cube should be counted for only 1/4 of an atom, because it is shared between four adjacent unit cells.
Each atom sitting on one face of the cube should be counted for 1/2, because that face of the cube is shared with the neighboring cube equally.
Each atom inside the cube count for 1.

After adding all these atoms and fractions of atoms, you should find integer numbers. And as the formula NaCl indicates, there is the same number for Na and Cl (which is different from 1).

2) atomic weight
You know the density of LiBr, which you'll need to transform into g/ų btw. So, if you know the weight in grams of all the atoms inside the cell, then you can calculate the cell volume in ų.

The molecular weight of Na is 22.99g/mol. Which means one mole of sodium weights 22.99g. And how many atoms are there in a mole of Na? Remember how to use the Avogadro constant http://en.wikipedia.org/wiki/Avogadro_constant?

3) When you have the cell volume, you can calculated its size in Å easily because it is a cube.

4) When you have the size, you need to go back to the picture of the structure. In this cube, how are the ions touching each other? Is it through the cube diagonal, face diagonal, cube edge? This gives you a relation between the cube size and the radius of Na⁺+Cl^-.

Job's done