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Easy way to memorize VSEPR

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  1. Nov 13, 2014 #1
    So I'm just posting this here for anyone else currently trying to memorize the VSEPR table, or just for future reference for anyone else. There's not a whole lot online about memorizing this table so I thought I'd throw my method out there. There's really not a whole lot to it other than good ol' fashioned memorization but I personally found this "order" of memorizing it to be easiest.

    1. First off I find it simple to memorize the structures in groups according to steric number/ Bonding number.

    Steric number 2 = 1 structure
    Steric number 3 = 2 structures
    Steric number 4 = 3 structures
    Steric number 5 = 4 structures
    Steric number 6 = 3 structures.

    Last one kind of breaks the rule but I still find this helps.

    2. Now you have to memorize the structures in each steric number group.

    Steric number 2 = Linear (180 degrees)
    Steric number 3 = Bent and Trigonal Planar (Both 120 degrees)
    Steric number 4 = Tetrahedral, Trigonal Bipyramidal and Bent again. (109.5, 107.3 and 104.5 degrees)
    Steric number 5 = Trigonal Bipyramidal, See-Saw, T-Shaped and Linear (Mixture of 90, 120 and 180 degrees)
    Steric number 6 = Square Planar, Square Pyramid and Octahedral (All 90 degrees)

    3. Now this seems like a lot but, for me personally, I try to find a relation between all the groups.

    For groups 2 and 3, I just kind of remember that they're those shapes it's pretty straightforward I think.

    Group 4 are all tetrahedral shaped molecules (Think group "4" / "Tetrahedral") with slight variations on the bond angles based on the amount of lone pairs. Tetrahedral is the most uniform shape and it has a bond angle closest to 120, with a slight repulsion from the uppermost atom making a bond angle of 109.5. Trigonal Bipyramidal has a lone pair of electrons which pushes the "side atoms" a little further away from 120 forming an angle of 107.3. Lastly, this variation of bent has two lone pairs of electrons pushing the "side atoms" even further away making an angle of 104.5.

    Group 5, to me, are the "wierd shapes". This includes the trigonal bipyramidal, see-saw, T-shape and linear. I don't have much for this one. T shape is 90 degrees because in the letter T, only 90 degree angles are present. Linear is obviously 180 degrees. See-saw has 3 angles and i remember them by picturing an actual see-saw in my head with 1 angle along the top of the seat (180), 1 between the seats and the legs (90), and one between the two legs (120). and trigonal Bipyramidal has 2 angles, 90 and 120 degrees.

    Group 6 is the "square group" including: Square planar, square pyramid and octahedral. (I know the last one isnt a square but, close enough). All squares consist of only 90 degree angles so these all only have 90 degree angles.

    Using ALLLL of this info you can kind of intuitively put together the notation for each structure.

    This is how I remember them. It might not help everyone else, but basically the only thing you need to take from this if it makes no sense to you is:

    Memorize the steric numbers, from that you get the amount of structures so you know you're not forgetting any. If you find a way to relate the groups together, it helps memorizing the angles like such since the angles in each steric group are similar.

    Good luck, I hope this makes sense to other people as much as it makes sense to me and I apologize in advance if this isn't as clear as I mean it to be.

    Let me know what you think!
     
  2. jcsd
  3. Nov 14, 2014 #2

    DrDu

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    That makes me really sad!
    The VSEPR rules were set up to make it possible for a student to predict structures based on a minimal amount of intuitive rules, and now students end up just memorizing all possible outcomes.

    VSEPR certainly does not predict angles up to a tenth of degree where it is not dictated by symmetry arguments. E.g. H2S only has a bond angle of about 92 degrees and not 104.5.
     
  4. Nov 14, 2014 #3
    I'm really sad too because I have to have this memorized for my Chemistry midterm in 3 hours. I'm quoting information given by my chemistry professor and what's written in the textbook, so if you're really that depressed maybe you should talk to them about it.

    With this information, whether it's right or wrong in your eyes, I can draw a lewis diagram of a molecule, count the bonds and lone pairs and know what the bonding angles and type of structure is. This works for me and I can name every structure and bond angle off the top of my head with this.

    Also, regarding H2S having an angle of 92 degrees, this isn't exactly something that can be predicted with this table, but by using rules learned previously and by comparing, say, H2O (Bond angle of 104.5) you can notice that sulfur is a larger atom which means the bonds between S-H will be longer. Being farther away, they H atoms can swing closer together to be repelled as far away as possible. Thus, H2S has a smaller angle (92 degrees).

    This method I posted isn't supposed to be able to explain all those little details like that, but you have to remember, you were a student once too and I'm more than sure you relied on little things like this to get by. Whether it be acronyms, anagrams or rhyming schemes, these help simplify complicated ideas so that you can easily refresh your memory during an exam.
     
  5. Feb 18, 2017 #4
    Thanks a lot, this will really help me memorize for my test coming up. But I think you got the steric number 4 wrong I'm pretty sure it should be
    Steric number 4 = Tetrahedral, Trigonal Pyramidal, Bent. (109.5, 109.5, and 109.5 degrees)
    I've corrected the ones in bold.
     
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