If your drawing the shape of a molecule, how do you know how many lines are wedged and how many are dotted? (Eg how many are pointing away and how many are pointing towards us)
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Shapes of molecules and Ions watch
- Thread Starter
- 13-04-2011 23:03
- 14-04-2011 10:10
Yeah, I really struggle with this concept too. When I asked my teacher, she said I should visualise it but as I'm not very conceptually minded I still don't really get it. For my AS exam in Jan I just learnt how you draw them. Generally molecules with:
2 pairs of electrons are just straight solid lines (linear - 180 degree angles)
3 pairs of electrons have one dotted, one wedged and one straight (trigonal planar - 120 degree angles)
4 pairs of electrons have two straight solid lines (one up and one to side of central atom), one wedged (straight down from central atom) and one dotted - tetrahedral (109.5 degree angles)
5 pairs of electrons have three straight solid lines (one straight up, one straight drown and one to side - they are all at 90 degrees to each other); one wedged and one dotted - trigonal bipyramidal (straight lines 90 degrees to each other, other three angles are 120 degrees)
6 pairs of electrons have two solid straight lines (straight up and straight below central atom); two wedged (both slightly below central atom) and two dotted lines (both slightly above central atom) - octahedral (bond angles of 90 degrees)
- 14-04-2011 10:31
Sorry, forgot to say: the bonds can rotate and are often drawn/shown differently, but there will still be the same number of dotted, wedged and straight lines.
In terms of lone pairs (eg in ions), there are really 3 rules you must remember which all occur because electrons are negatively charged:
1. Lone pair next to lone pair will have the biggest angle as they repel each other the most
2.Lone pair next bonding pair produce the second biggest angle
3.Bonding pair next to bonding pair produce the smallest angles (the angles I mentioned in my last post are all bonding pair/bonding pair angles)
The angles you will have to learn for molecules/ions with lone pair electrons are the new angles between the bonding pairs.
For example: ammonia has one 1 lone pair of electrons and adopts a tetrahedral shape (with lone pair at top instead). However, the lone pair has a greater force of repulsion on the bonding pairs than in a normal tetrahedral molecule (with no lone pairs).
This pushes the bonding pairs closer together so the new angle between bonding pairs is reduced from 109.5 (no lone pairs) to a new angle of 107 degrees.
- 14-04-2011 23:12
This is quite a difficult thing to grasp and if you are still struggling you may find this free software of help:
It allows you to rotate prebuilt molecules on your computer. They're mostly organic ones but hopefully once you understand some of them it should help.
Alternatively, if you prefer something physical you can look into a model kit. The cheapest i could find was this one at £19.99 - somewhat more expensive than when i bought mine 10 years ago:
Or the final option as one of my lecturers suggested is a blob of bluetack and a box of matches which you probably have lying around the house. These can fairly decently be used to visualise the molecules in 3D.
Hope that helps
- 15-04-2011 12:44
I like to think of it by using a piece of paper, which you can do in an exam.
You have an equatorial plane (i.e. the paper) and an axial plane (an imaginary line going at right angles through the middle of the paper). Let's say you have PCl5. This means all electrons are used for bonding and you have five bonds to draw, without lone pairs.
- Start off by drawing the P in the middle of the paper.
- Draw three bonds coming off it at 120 degrees to each other.
- Now hold the paper up to near eye level and imagine the other two bonds going up and down, above and below the paper.
- Finally, rotate the paper so that one of the bonds is pointing directly left or right. This is the one you draw as a normal line. The other two on the paper you draw as a back wedge and a front wedge, which should be obvious by looking at the piece of paper.
For an octahedral or square planar one, you should be able to see you'd need two back and two front wedges in the plane of the paper, since it's the only way to "see" all of them.
You should just get as much practice as possible. Eventually you'll realise there are only a limited number of shapes and they're all based either on the tetrahedron, the triangular pyramid or the octahedron.
(that is, until you get to uni, and they introduce the piano stool, the pogo stick, the boat, the chair, the sofa, the toaster and the rest of them).