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VSEPR

Need help understanding molecular geometry?  You found the right site.  This will take you step by step through how to determine molecular geometry.  This site will take you through the basics of VSEPR theory as it applies to determining molecular geometry.  Keep in mind that this is not the only bonding theory.  This page will take you through Electron Geometry, you can click here to go straight to molecular geometry.

The Concept

Imagine that you are in outer-space and there is no gravity.  Now imagine that you have a big ball that can be attached to a bunch of little magnets with some string.  These little magnets are really powerful.  Ever try to push a magnet together the wrong way?  Same idea with the little magnets, they don't want to be anywhere near each other.  So 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 sides because of how hard the magnetic forces are pushing on each other.  What happens then, if we were to attach 3 magnets?  BAM, they push each other into a triangle so 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 an atoms 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.

Electron Geometry

It is important to realize that electron-geometry and molecular-geometry are NOT the same thing, but are related.  Keep that in mind as you read.  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 you measured the angle between the magnets, it would equal 180°.  For all of my examples, X will stand for the central atom, and Y will stand for the electrons.  I am also using the conventional wedges and dashed lines used in chemistry to indicate depth - if you don't understand them, go here.

This is known as a linear electron geometry.

When you 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 you 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 you get up to 5 magnets, something interesting happens - you 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 you pretend the bottom magnet isn't there, you can see how everything else forms a pyramid shape.  Then if you pretend the top magnet isn't there, you can see another pyramid.  Two triangular pyramids = trigonal bipyramidal.

The last electron geometry we're going to talk about is actually one of the simplest.  When you 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.  You can see where the octa (8) part comes from if you 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.

Those are the 5 basic shapes that make up all of electron geometry.  If you can remember those 5 shapes, you can figure out any shape in molecular geometry.  You will probably have seen in class 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.

That was part 1 about electron geometry.  Click here to see part 2 about molecular geometry.

Did I miss anything in my explanation?  Still not understand something?  Let me know

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