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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: cwolf on April 02, 2007, 08:26:42 PM

Title: Why are Crystalline Structures named the way they are?
Post by: cwolf on April 02, 2007, 08:26:42 PM
Can someone explain..

1. Why are Crystalline Structures called "hexagonal", "face-centred" and "body centred" packed structures? 

I think its the way the central atom and the surrounding atoms are arranged, but I know theres much more than that...
Title: Re: Why are Crystalline Structures named the way they are?
Post by: Borek on April 03, 2007, 02:49:46 AM
Quote from: cwolf on April 02, 2007, 08:26:42 PM
I think its the way the central atom and the surrounding atoms are arranged, but I know theres much more than that...

Not necesarily. These are just names. Could be they have some interesting story behind (names sometimes do) but I have never heard one.
Title: Re: Why are Crystalline Structures named the way they are?
Post by: P on April 03, 2007, 04:09:13 AM
The molecules are arranged Hexagonally in an Hexagonally close packed arrangement. With the Face Centred Cubic packing - the central molecule is in in the middle of the 'face' of the unit cell. with Body Centred - it is in the middle of the cell (in the 'body' of it).
Title: Re: Why are Crystalline Structures named the way they are?
Post by: Sam (NG) on April 03, 2007, 04:11:41 AM
It is merely a description of how the unit cell appears in the Bravais Lattice of each compound.  Each point in the diagram represents a lattice point in the crystal structure.

Cubic Close Packing
Primitive Cubic:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F5%2F55%2FCubic.svg%2F80px-Cubic.svg.png&hash=2a6cf484b376a64fd596cba35362b93059aa87b6)

Body Centred Cubic:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2Fa%2Fa3%2FCubic-body-centered.svg%2F80px-Cubic-body-centered.svg.png&hash=391503d29319228b521582316dec7341b4e1aa0b)

Face Centred Cubic:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2Fc%2Fc9%2FCubic-face-centered.svg%2F80px-Cubic-face-centered.svg.png&hash=512be80f2358c733694eafebe50aad11d73ec688)

Hexagonal Close Packing
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2Ff%2Ff6%2FHexagonal_lattice.svg%2F80px-Hexagonal_lattice.svg.png&hash=95c7588448b3bc1e74a4a371352fc950b5490123)

Quote from: P on April 03, 2007, 04:09:13 AM
The molecules are arranged Hexagonally in an Hexagonally close packed arrangement. With the Face Centred Cubic packing - the central molecule is in in the middle of the 'face' of the unit cell. with Body Centred - it is in the middle of the cell (in the 'body' of it).

In most cases, it is not a molecule that resides on each lattice point in the unit cell.  Take NaCl fcc arrangement for example, If you put Cl on the lattice points then Na occupies the octahedral holes between the lattice points:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2Fd%2Fdb%2FNaCl-estructura_cristalina.svg%2F200px-NaCl-estructura_cristalina.svg.png&hash=1c3905fa07453b402ee5179e10a718b7995910ed)
Title: Re: Why are Crystalline Structures named the way they are?
Post by: AWK on April 03, 2007, 09:58:52 AM
Quote from: cwolf on April 02, 2007, 08:26:42 PM
Can someone explain..

1. Why are Crystalline Structures called "hexagonal", "face-centred" and "body centred" packed structures? 

I think its the way the central atom and the surrounding atoms are arranged, but I know theres much more than that...

These names are simply a classification of ctystal structures according to symmetry and translations.
Crystals are classified according to symmetry of the unit cell as: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hrxagonal and cubic.. Moreover some of thiese types can showe more complex translational lattice then primitive onenamed body centered, face centered orside centered.