The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

Packing fraction of salt (NaCl), HELP!

ImageUploadedByStudent Room1407924030.085619.jpg
I have the above question.
When I see something like this I first visualise what it will look like in order to let me find how my Na and Cl atoms there are.

Usually I would end up drawing an image like this:
ImageUploadedByStudent Room1407924111.295156.jpg

Then, after knowing the number of atoms , I can find out the total volume of these atoms.

In order to find out the packing fraction I now need the length "a" of the unit cell. I found this out by looking at a single line along the unit cell and enlarge the atoms there till they touch. E.g the top line at the back has 2 chlorine atoms in the corner and one sodium atom in between. Adding them up gives me 2x1.81+ 2x0.99= 5.6 angstroms

My method of answering this relies heavily on drawing the image correctly and making sure all the atoms are there. I would draw an FCC with Cl atoms then use the basis vectors on each chlorine atom to find the position of the sodium atoms. I also draw unit cells next to my main unit cell, and use the nearest chlorine atoms of these cells in order to find other sodium atoms that might be in the MAIN unit cell (after 10 minutes, this will give me the image above)

The answer is shown here:
ImageUploadedByStudent Room1407925028.308082.jpg

The answer looks like it takes 2 minutes to write down and didn't even require an image. I don't quite understand it so can someone help? The calculations I understand, but not the words

My assumption is "rock salt structure" means you have an FCC of a single atom, and then you add the second atom to the middle of every line, and put on in the centre of the unit cell. This means on any length you choose, you will always get the diameter of the two atoms (even if they are semispheres). That essentially means I just need to add the radii of both atoms and double the answer (I.e add their diameters).

I don't quite understand what they mean by 4 lattice points, where are these lattice points? If it's an FCC unit cell, a conventional one would have 12 points. Does it mean if I add all the halves/corners, then I will have 4 atoms worth? (I.e these lattice points=full atoms inside the atom after adding everything up)

If so why do you "attach a basis of 2 atoms to each [lattice point]"?


Posted from TSR Mobile
(edited 9 years ago)
ImageUploadedByStudent Room1407924016.556351.jpg6.0KB
Hi there,

While you're waiting for an answer, did you know we have 300,000 study resources that could answer your question in TSR's Learn together section?

We have everything from Teacher Marked Essays to Mindmaps and Quizzes to help you with your work. Take a look around.

If you're stuck on how to get started, try creating some resources. It's free to do and can help breakdown tough topics into manageable chunks. Get creating now.

Thanks!

Not sure what all of this is about? Head here to find out more.

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you