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4 LIQUIDS AND SOLIDS eLearn.Punjab

Figs (4.17 a, b) The unit cell of sodium chloride showing that four NaCl formula units are present in a unit cell.

The location of Na+ and Cl- is such that each Na+ is surrounded by six Cl- placed at the corners
of a regular octahedron Fig. (4.17 a). So the coordination number of each Na+ is six. Similarly,
each Cl- is also surrounded by six Na+. Na+ and C1- are not connected to one another by pairs
because all six Cl- ions are at the same distance away from one Na+. It has been observed that
independent molecules of NaCl do exist in the vapour phase. Anyhow, in solid NaCl there
are no independent molecules of NaCl. That is why NaCl is said to have formula unit of NaCl.
While looking at the Fig.(4.17 b), we see that there are eight Cl- at the comers of the cube, and
each is being shared amongst eight cubes. l/8th part of each Cl- ion is considered for this unit cell.
So, one complete Cl- is contributed by eight corners. Similarly,six chloride ions are present at the
face centres and each is being shared between t wo cells. Thus.per unit cell there are 8/8 + 6/2 =
4 Cl- ions. You can justify the presence of 4 Na+ , if you take a unit cell having 8Na+ at eight corners
and 6Na+ at faces. So, there are equal number of Na+ ions, and therefore 4 NaCl units are present
per unit cell. Fig (4.17b)

Lattice Energy

Solids are composed of atoms, ions or molecules. However, many solids of daily importance
are ionic in nature. As mentioned earlier these ions exist in a three dimensional array which is
called as lattice.

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