The different structure of caesium chloride If they start touching, you introduce repulsions into the crystal which makes it less stable. That represents the maximum number of chloride ions that you can fit around a central sodium ion before the chloride ions start touching each other. So why does each ion surround itself with 6 ions of the opposite charge? That means that to gain maximum stability, you need the maximum number of attractions. The more energy that is released, the more energetically stable the structure becomes. The more attraction there is between the positive and negative ions, the more energy is released. You should be able to draw a perfectly adequate free-hand sketch of this in under two minutes - less than one minute if you're not too fussy! It doesn't matter whether you end up with a sodium ion or a chloride ion in the centre of the cube - all that matters is that they alternate in all three dimensions. Use different colours or different sizes for the two different ions, and don't forget a key. Now all you have to do is put the ions in. To complete the process you will also have to join the mid point of each face (easily found once you've joined the edges) to the mid point of the opposite face. Now the tricky bit! Subdivide this big cube into 8 small cubes by joining the mid point of each edge to the mid point of the edge opposite it. Turn this into a perfect cube by joining the squares together: If you get it wrong, the ions get all tangled up with each other in your final diagram. You might have to practice a bit to get the placement of the two squares right. Now draw an identical square behind this one and offset a bit. The pattern repeats in this way over countless ions. You must remember that this diagram represents only a tiny part of the whole sodium chloride crystal. Sodium chloride is described as being 6:6-co-ordinated. By chance we might just as well have centred the diagram around a chloride ion - that, of course, would be touched by 6 sodium ions. The sodium ion in the centre is being touched by 6 chloride ions. Only those ions joined by lines are actually touching each other. We normally draw an "exploded" version which looks like this: This diagram is easy enough to draw with a computer, but extremely difficult to draw convincingly by hand. If you look at the diagram carefully, you will see that the sodium ions and chloride ions alternate with each other in each of the three dimensions. That is different from, say, a water molecule which always contains exactly 2 hydrogen atoms and one oxygen atom - never more and never less.Ī small representative bit of a sodium chloride lattice looks like this: There could be billions of sodium ions and chloride ions packed together, or trillions, or whatever - it simply depends how big the crystal is. It means that you can't state exactly how many ions there are. You should be clear that giant in this context doesn't just mean very large. So sodium chloride (and any other ionic compound) is described as having a giant ionic structure. Compounds like this consist of a giant (endlessly repeating) lattice of ions. Sodium chloride is taken as a typical ionic compound. How the ions are arranged in sodium chloride The structure of a typical ionic solid - sodium chloride It isn't important for understanding this page, however. Note: If you need to revise how ionic bonding arises, then you might like to follow this link. It also explains why caesium chloride has a different structure from sodium chloride even though sodium and caesium are both in Group 1 of the Periodic Table. Where the parameters A and n are chosen to give predictions consistent with experimental data.This page explains the relationship between the arrangement of the ions in a typical ionic solid like sodium chloride and its physical properties - melting point, boiling point, brittleness, solubility and electrical behaviour. Fortunately, however, this energy can be described accurately by a simple formula that contains adjustable parameters: Calculating this repulsive potential requires powerful computers.
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