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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: deutdeut on August 23, 2006, 10:37:44 PM

Title: Coordination number
Post by: deutdeut on August 23, 2006, 10:37:44 PM

In an ionic crystal, if we want to find the coordination number of a particular ion, we have to find out the number of the ions surrounding it. But there are so many ions present in the ionic structure. How can I know which ions are surrounding it and which one are not? I always count the ions wrongly. Can someone offer some help?

Title: Re: Coordination number
Post by: Borek on August 24, 2006, 03:09:15 AM

Start identifying unit cell.

Title: Re: Coordination number
Post by: deutdeut on August 24, 2006, 03:50:15 AM

But which ion should I start with?

Title: Re: Coordination number
Post by: Borek on August 24, 2006, 04:11:24 AM

Cell unit has nothing to do with any particular ion, it is defined by crystal lattice.

Title: Re: Coordination number
Post by: deutdeut on August 24, 2006, 04:40:18 AM

So, how can I know which ions are surrounding the ion that I want to find its coordination number?

Title: Re: Coordination number
Post by: jennielynn_1980 on August 24, 2006, 07:35:35 AM

you have to figure out which kind of lattice it is and what kind of packing the crystal has.  For example there is body centred packing, hexagonal closest packing, cubic closest packing, etc.  Each type of packing has a unit cell which has a certain number of atoms or particles.  If you can find out the type of packing, then you will be able to figure out the number of atoms in a unit cell and then you will be able to figure out the coordination number.

Good luck.

Title: Re: Coordination number
Post by: deutdeut on August 24, 2006, 07:51:41 AM

The crystal structure is huge, if the unit cells are packed together, how can I determine the number of ions surrounding the given ion?

Title: Re: Coordination number
Post by: jennielynn_1980 on August 24, 2006, 08:11:31 AM

A unit cell is the base of the huge crystal structure.  Are you looking at a particular crystal?

Title: Re: Coordination number
Post by: deutdeut on August 24, 2006, 09:58:14 PM

So, what approach should be taken when looking at the crystal structure?

Title: Re: Coordination number
Post by: AWK on August 25, 2006, 08:08:39 AM

You shoulld calculate contacts between ions in the crystal structure. Take the smallest equal contacts.

Title: Re: Coordination number
Post by: Donaldson Tan on August 25, 2006, 07:50:49 PM

the ratio of cationic radius to anionic radius is a good gauge to decide the type of crystal packing. Once you have identified the type of crystal packing, you can deduce the coordination number.