Fancy Square

October 16, 2008 by  
Filed under Die Cutting Machines and Supplies

At Die Cut Machines your source for Die Cutting Machines and Crafting Supplies we hope the Fancy Square products and information here meets your needs.

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Can you suggest a nice restaurant near Times Square?

Going to NYC (Times Square area) for Christmas and I want to take my parents out to a nice dinner. So far, I've heard of the 21 club and Carmines in the Theater District. It's our Christmas Eve dinner, so I'm looking for something a little fancy. Any suggestions?

joe allen's
take a cab and go to patsy's
need reservations!

Understanding Molecular Geometry

Suppose we are in outer-space and there is no gravity.  Now let us imagine that we have a big ball that can be attached to a bunch of little magnets with some string.  These little magnets are really powerful and they don't want to be anywhere close to each other. If we attach only one magnet to our ball, there's no problem, it can go anywhere around the ball it wants.  But then we attach two magnets - all of a sudden they are on exact opposite sites depending on how hard the magnetic forces are pushing on each other.  If we were to attach 3 magnets, they will push each other into a triangle so that they can stay as far away from each other as possible.  Same thing happens when we add 4, 5, 6, 7, 8 or even more magnets to our ball.  Their magnetic forces push them as far apart as they can be while still attached to our ball.  This is the basic concept of molecular geometry, only with the difference that an atom’s nucleus is the ball in outer space, and electrons are Our Little magnets that don't like being close together.  The angles between the electrons tell us how far apart they are, with larger angles meaning farther apart and smaller angles meaning closer together.

 It is important to realize that electron-geometry and molecular-geometry are not the same thing, but are related.  The absolute simplest electron geometry is the one where we only have 2 magnets and they push each other to the opposite sides of the ball.  If we measured the angle between the magnets, it would equal 180°.  This is known as linear electron geometry. For all of the examples, let us consider that X will stand for the central atom, and Y will stand for the electrons.  We may use the conventional wedges and dashed lines used in chemistry to indicate bonds and bond angles. When we have 3 magnets, they push each other into a triangle where the measured angle is equal to 120°. This is known as trigonal planar electron geometry. When we have 4 magnets, things get more complicated because the magnets push each other so that they form a strange shape called a tetrahedral, with angles measuring 109.5°

Some people can imagine a tetrahedral geometry better by pretending to look straight down one of the lines, this makes the other 3 lines appear to be in a triangle. When we get up to 5 magnets, something interesting happens – we combine the linear and the trigonal planar geometries.  The linear geometry goes up and down (vertical plane), while the trigonal planar geometry goes left, right, forward and back (horizontal plane).  The angle between the magnets in the linear geometry is 180°.  The angle between the magnets in the trigonal planar geometry is 120°. The angle between the magnets in the linear and the trigonal planar geometry is 90°. 

This is known as trigonal bipyramidal.  If we pretend that the bottom magnet isn't there, we can see how everything else forms a pyramid shape.  Then if we pretend the top magnet isn't there, we can see another pyramid.  Hence two triangular pyramids = trigonal bipyramidal. The last electron geometry we're going to talk about is actually one of the simplest.  When we have 6 magnets, they actually push each other into a completely symmetrical shape, a combination of 3 linear geometries where the angle between any of the magnets is 90°. This is known as an octahedral.  Here we can see where the octa (8) part comes from if we imagine stacking 8 blocks together to make one big block - the middle of each side of that big block is where a magnet would be if our ball was in the middle.

These are the 5 Basic Shapes that make up all of electron geometry.  If we can remember those 5 shapes, we can figure out any shape in molecular geometry.  We have already come across things like a "steric number" and "hybridization".  For us, the steric number is just the number of magnets that we have and the hybridization is just a fancy way of writing the steric number so that it applies to a more advanced chemical theory called molecular orbital (or MO) theory. In case of molecular geometry we just need to apply our knowledge of all the grunt work out of the way learning electron geometry.  We just need to know a couple of following things to do this.

1 - In all those cases we referred to Y as a magnet.  It is actually either a bond between atoms also known as bond pair, or a lone pair of electrons.

2 - When we name molecular geometries, we ignore lone pairs of electrons, even though they are still there and still push away other electrons like the little magnets that they are.

3 - Lone pairs of electrons are stronger magnets than bond pairs are, so they will push harder.

4 - When every Y stands for a bond pair, the molecular geometry is the same as the electron geometry.

Like before we may start with the linear electron geometry.  The good news is that it does not matter if Y is a bond pair or a lone pair as they still both form a linear molecular geometry.  Even though lone pairs push harder than bonds, it is impossible to get farther away than 180°.  Next we have the trigonal planar electron geometry.  If all of our Y's are bonds, the molecular geometry will again be trigonal planar.  However in this case, we can replace one Y with a lone pair of electrons to get a "bent" molecular geometry.  As  rule 3 says, lone pairs push harder than bonds do, so while the angle between bonds is 120° for a trigonal planar molecule, it is actually <120° for a bent molecule, all because that lone pair is pushing the bonds farther away.

It starts to get fun here with the tetrahedral electron geometry.  Again, if all the Y's are bonds, the molecular geometry is just tetrahedral again with angles equal to 109.5°.  Now if we replace just one of the bonds with a lone pair, we get what is called trigonal pyramidal - it looks like a pyramid with 3 sides - and because the lone pair is pushing harder than the bonds, the angle is <109.5°.  Finally, with a tetrahedral, we can replace 2 of the Y's with lone pairs (i.e. water - 2 bonds and 2 lone pairs) and we get a familiar looking "bent" shape, only the angle between the bonds is now <109.5°.

Now we have to look at the trigonal bipyramidal electron geometry.  We know that this is a combination of linear and trigonal planar geometries.  This becomes important when we start adding lone pairs.  Since the lone pairs push harder than bonds or bond pairs do, they want to be farther away from everything.  That means that when we add them, they must be added to the horizontal plane (trigonal planar geometry).  This puts the lone pairs as far away as possible (120° angle is farther away than a 90° angle). 

So we begin, as always, when all Y's are bonds, the molecular geometry is the same as always - trigonal bipyramidal - with angles equal to 120° and 90°.  When we replace one bond with a lone pair (in the horizontal plane) we get what looks like a see saw if we turn it sideways so we call that a seesaw molecular geometry with angles <120° and <90°.  Replacing two bonds (in the horizontal plane) with lone pairs of electrons gives us what looks like a T if we turn it sideways, so naturally we call that T-shaped molecular geometry, with angles <90°.  Finally, we can replace all three of the bonds in the horizontal plane with lone pairs, and we're just left with our linear geometry (vertical plane) so naturally we call that a linear molecular geometry with an angle of 180°.

Last, but not least, we have the octahedral electron geometry.  As always, if all Y's are bonds, then the molecular geometry will be octahedral with angles of 90°.  When we replace one bond with a lone pair of electrons, we get another pyramid shape, but this time with 4 sides.  We call this molecular geometry square pyramidal with angles <90°.  Finally, if we replace two bonds with lone pairs of electrons, the lone pairs must be added on opposite sides because of their more powerful magnetic force - they push harder.  This gives us a molecular geometry where we have 4 bonds in a single plane, so we call it square planar where the angles all equal 90°. At this point, one should be able to tell the difference between electron geometry and molecular geometry, as well as how they are related.

About the Author

Dr. Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, India.


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