Expert: Henry Boyter - 3/7/2006

Question
Hiya sir i need some help in working out the unit cell length of KBr , assuming the atomic radius of a bromide ion is 1.95 * 10^-10m
This is how i done it i am not sure whether i am correct.

Because the KBr lattice is a face centred cubic. - the formula to use is 2*sqrt(2r)
i converted m into pm = 195 pm

2*sqrt(2*195) = 552 pm

I am kind of confused on how do i work out the number of units? Like 1/8 edge and 1/4 corner etc . I read it online i am still confused. It would be highly appreciated if you could help me .Thank you  

Answer
Try this, especially if you have a small brother or sister.  Steal their blocks and start stacking them (or use anything similar).  When the innermost block is covered, you will see that if an atom sits on a corner of that block, it is also part of the corner of seven other blocks, three on the same level and four above or below.  Thus each block has 1/8 of that atom.  Remember, it is the center of the atom that is on the corner.

Then look at the middle of an edge.  That atom is part of the edge of three other blocks, thus 1/4.  The others you can work out from there for yourself.