Chemistry Reference
In-Depth Information
(c) Stack up three layers so that the sequence is ABA. The coordination
number of the small balls has to be six, of the big balls has to be four.
What is the ratio of the balls?
Information: The O
2
ions form a hexagonal close-packing in aluminium
oxide crystals, only 2/3 of octahedral gaps are occupied by Al
3+
ions. The
coordination is 6/4, the ratio of ions is 2:3. Therefore the models have to be
abbreviated to formulae like {(Al
3+
)
2
(O
2
)
3
}orAl
2
O
3
.
M 6.15: Form a few small balls with modeling clay that fit into the tetrahedral gaps
of big spheres. With the small and the big spheres build an elementary
cube
(a) For the zinc sulfide structure (see picture).
(b) For the lithium oxide structure (see picture).
Information: zinc sulfide can be described as a cubic close-packing of S
2
ions, half of the tetrahedral gaps are occupied by Zn
2+
ions. The coordina-
tion is 4/4, the formula for the unit cell is {(Zn
2+
)
4
(S
2
)
4
}, the empirical
formula ZnS.
Lithium oxide can be shown as a cubic close-packing of O
2
ions, all of
the tetrahedron gaps are occupied by Li
+
ions. The coordination is 4/8, the
formula for the unit cell is {(Li
+
)
8
(O
2
)
4
}, the empirical formula Li
2
O.
On the following two pages you will find the expected model drawings for the
models M6.13 to M6.15. Compare your own drawings with these.
