Homework Help: chem

Posted by Dr Bob plz help on Saturday, January 24, 2009 at 3:35pm.

The crystal structure of lithium sulfide (Li2S) is pictured here. (Intro 1 figure) The edge length of the unit cell is 5.88*10^2^2 pm.

i guess it is FCC ( face cenred cubic)

1--

What are the coordination numbers of and Li^+ and S^2- in the lithium sulfide crystal shown? Enter the values in order of Li^+ and S^2-, respectively.

Ans... i though it shld be 4,4 .. but it says " Face ions are shared between two unit cells."...

plz someone

2- How many formula units are in the unit cell shown for Li2S ?

Ans- i gues it shld be 1 but it says -
The formula represents one formula unit. How many sets of are in the unit cell?

chem - DrBob222, Saturday, January 24, 2009 at 4:21pm
I don't have the book and can't see the picture. I can't find anything on the Internet so I'm passing on this one. I've also looked in all the references I have at home and can't find anything on Li2S.
chem - Mrs C, Sunday, January 25, 2009 at 9:24pm
The Number of Formula Units is 4, but I don't know why, I just happened to guess it and it marked it as correct for me.
As for part 1, I don't know :(
chem - Dash, Monday, January 26, 2009 at 11:14pm
It's 4,8. Easy, you just count the lines, but note that the S(2-) is on the surface, so it's surrounded by the Li+ of the adjacent unit cell as well as its own.
chem - Anonymous, Tuesday, January 27, 2009 at 12:03am
What is the density of Li2S in grams per cubic centimeter? this was the 3rd part of this question :/
chem - friend, Tuesday, January 27, 2009 at 10:49pm
The density of lithium sulfide is 1.5g/cm^3
chem - SJ, Friday, January 27, 2012 at 2:42pm
the number of formula units is 4: there are 4 Sulfur atoms in the unit cell (8* 1/8 + 6* 1/2) and 8 Lithium atoms (all contained within the unit cell)

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