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Can anyone explain the shapes of ionic compounds? What does it mean? What does the coordinate number mean? I know that in $\ce{NaCl}$, one $\ce{Na}$ atom is surrounded by 6 $\ce{Cl}$ atoms, and vice versa, but can it be explained with the help of coordinate numbers?

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  • $\begingroup$ Another more complicated case that can occur is when the ions interact with molecules or when there are resonance hybrids between ionic bonds and covalent bonds. Than one needs both VESPR or MO and methods for dealing with ionic compounds. $\endgroup$ – Steven Stewart-Gallus Jul 14 '16 at 23:57
  • $\begingroup$ The question is ambiguous. Do you mean crystals of 'inorganic' compounds, say NaCl or k$_4$Fe(CN)$_6$ etc or do you mean the structure of ions in solution or these ions in a crystal e.g. Fe(CN)$_6^{4-}$. What about ions of organo-metallic compounds, say Ruthenium tris(2,2'-bipyridyl) chloride. $\endgroup$ – porphyrin Jul 21 '16 at 18:21
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Only for solids that have regular and repeating pattern of constituent particles we define shape and geometry.

So, on the basis of order of arrangement of constituent particles solids are classified into crystalline and amorphous.The former has regular pattern of arrangement of particles which repeat itself periodically over the entire crystal (as shown below is the two dimensional structure of quartz)

quartz

Amorphous solids consist of particles of irregular shape(as shown below is 2d structure of quarts glass.

quarts glass

If the three dimensional arrangement of constituent particles of crystalline solid is represented diagramatically in which each particle is depicted as a point, the arrangement is called crystal lattice

Unit cell is the smallest portion of crystal lattice which when repeated in all directions generate the entire crystal (see below). There are only 14 possible crystal lattices called Bravais Lattices( after a French mathematician who first described them.

unit cell

Parameters of unit cell

On the basis of unit cell parameters (axial distances and axial angles) we get the following seven primitive unit cell which defines the shape of solid.

Bravais Lattices

COORDINATION NUMBER this term is used in crystallography in a somewhat different sense than it is in coordination compounds.

The number of nearest neighbors (or touching particles) that a particle has in a crystal is called its coordination number. All atoms in bravais lattice have the same coordination number.

In a simple or primitive unit cell, around any given atom there would be six equally spaced nearest neighbours.Thus the coordination no=6.

common salt crystal

Any other particle in crystal lattice will be at a greater distance from the particle under consideration. This also apply to your NaCl crystal.

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