Cadmium Oxide - unit cells etc

[phyuk2008]

 

Question:

 

What is the coordination number of (a) Cadmium and (b) oxygen and what are the geometrical arangment around cadmium and oxygen?

 

Answer:

 

cd = 4

o = 4

 

Question:

 

determine the number of cadmiun and oxygen atoms in the structure, then determine the emperical forumla of the compound and the number of forumla units in the unit cell

 

Answer:

 

cd = 4

o = 4

 

emperical forumla - CdO

 

How is the last part worked out?

 

Question:

 

Calculate the density of cadmium oxide

 

Answer:

 

Molecular weight = 128.4104 g/mol

 

Avogadro's number = 6.0220x10^23

 

what is the unit cell dimesions? Is it Cd to Cd measurement?

[hermanntrude]

You have to remember that the unit cell isn't a discrete entity. it continues more or less indefinitely.

 

Look at the central oxygen atom. How many Cd atoms surround it? I'll give you a clue: it's more than 4. Also check your answer for Cd.

 

I'm a bit confused about the second question, since there are 13 oxygen atoms and 14 cadmium atoms, but you were correct in saying that the empirical formula is CdO. It might just be that 14:13 is so close to 1:1 that you're expected to round off to 1:1.

 

to be able to answer question 3 you will need to know something about the dimensions of the unit cell.

[John Cuthber]

Consider the Cd at the bottom right. It's not just part of the unit cell pictured- it's shared between 6 unit cells so there's only1/6 of it in this cell and there are 8 like it so that's 8/6 Cds per unit cell. Then there's the Cd in the middle of the square face- it's shared between 2 unit cells so it counts as 1/2

There are 6 of them so that's another 6/2 Cs atoms in the cell.

Do the same with all the types of Cd and O atoms in the cell and the ratio should come out as 1:1

(I think- it's a long time since I did this sort of thing)

[hermanntrude]

John is correct. Apologies for confusing the issue