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Chemistry Help!

Discussion in 'Pre-Medical Allopathic [ MD ]' started by TigerLilies, 07.04.05.

  1. TigerLilies

    TigerLilies Senior Member

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    Hi everyone, I had a few chemistry questions that I am unable to answer and I looked through my notes and text but I am still confused on. So I would be really grateful if someone could help me out.


    1. How many atoms in BF\3 reside in the same plane? How many in NH\3 reside in the same plane? Why?

    2. How many atoms are coplanar in CH\4 and XeF\4. And why?

    3. SF\4 is see-saw and XeF\4 is square planar. Is either of them symmetrical or isotropic?

    4. Using only the periodic table (but NOT Lewis or VSEPR structures) predict the geometries of the following molecule:

    A) AsH\3


    Thanks so much!!!! Even if you could help me with one, I'd really grateful!!

    Thanks.
  2. DrHopeless

    DrHopeless The Monkey is In

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    1. How many atoms in BF\3 reside in the same plane? How many in NH\3 reside in the same plane? Why?

    BF3 I believe is planar so all of them. NH3 is tetrahedral (well pyrimidal, lone pair) so 3 of them (imagine a pyramid).

    4. Using only the periodic table (but NOT Lewis or VSEPR structures) predict the geometries of the following molecule:

    A) AsH\3

    Well As is in the same periodic group as N so if NH3 is tetrahedral (pyrimidal, lone pair) so should AsH3

    I think those are correct don't blame me if they are wrong :laugh: Sorry about the other two, I haven't take basic chem in a while, forgot what all those words mean.
  3. Belfagor

    Belfagor Busy Breaking Bonaduce

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    1. All of the atoms in BF3 share the same plane. Boron is in group 3, and makes an exception to the octet rule. It is satisfied with 3 total bonds. It's molecular geometry is trigonal planar, since there are no unbonded electron pairs. NH3, however, is a tetrahedral structure with trigonal pyramidal molecular geometry because Nitrogen is in group 5, and prefers 5 total bonds. The unbonded electron pair creates a repulsion cloud that lies in its own plane, and the three Hydrogens are in their own plane as well.

    2. If by "coplanar" you mean the same plane, then the rules are similar to those in #1. CH4 is a tetrahedral strucuture with tetrahedral geometry. Carbon is found in group 4, and thus is satisfied with 4 total bonds all of which lie in the same plane. So, all the molecules are "coplanar", I suppose. Xenon is a noble gas, and is found in group 8. It is an exception to the octet rule, and prefers 8 bonds. Drawing the skeleton structure first will allow you to distribute all electrons. The remaining electrons (remember to always count the total # of electrons) are placed on the central atom, Xe. So, 4 Flourines are bonded to the Xe, and two unbonded electron pairs are remaining. After placing those on the Xe, you have a total of 6 electron pairs on the Xe which results in a Octahedral structure that has 2 unbonded pairs. 4 bonded and 2 unbonded pairs gives you a Square planar strucutre where the 2 unbonded pairs lie in an equatorial or sometimes axial plane to the bonded flourines. So 4 F's and 2 unbonded pairs share two different planes.

    I'm not going to continue to answer these questions since this looks like a homework problem, and I just gave you the structures of the compounds. You should be able to deduce the remaining answers from the first 2 questions since these are essentially the same question reworded.
  4. LJDHC05

    LJDHC05 Former Chicken Slayer

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    Both of the answers are correct...but you should really re-read your chem book if you're having trouble with those sorts of questions
  5. Belfagor

    Belfagor Busy Breaking Bonaduce

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    Agreed. There really is no way around memorizing the different shapes that result from Lewis structures. That being said, you have to be very sure you know how to draw Lewis structures or you're going to be getting all of these wrong.
  6. TigerLilies

    TigerLilies Senior Member

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    thanks, i was mainly confused on how can you determine whether or not a molecule is coplanar?

    thanks.
  7. TigerLilies

    TigerLilies Senior Member

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    also, these are just suggested homework problems that my prof gave me. they will not be graded. i'm using them to learn the material ahead. our grade is based only on three exams, final and lab.

    thanks once again.
  8. TigerLilies

    TigerLilies Senior Member

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    Thanks everyone. I've figured most of them out. However, I am confused on how you could determine the geometry of a particular molecule without using the VSEPR theory and just the periodic table. Isn't the VSPER theory that helps determine the shape of a particular molecule?

    This is the question that I was wondering about:

    Using only the periodic table (but NOT Lewis or VSEPR structures) predict the geometries of the following molecule:

    A) AsH\3

    ???

    Thanks so much for everything!!!!
  9. Belfagor

    Belfagor Busy Breaking Bonaduce

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    I recommend that on a test, if you have time, you draw a Lewis regardless.

    That being said, you can be pretty accurate at guessing geometry without actually drawing structures. Start with Arsenic. It's in group 5 along with Nitrogen. Basically, you can think to yourself "What does NH3 look like?". Because we already determined it's a tetrahedral trigonal pyramidal. Why? Because like Arsenic, Nitrogen is a Group 5 element that likes to have a total of 5 bonds attached to it. You know that hydrogen can only take one bond, and if you quickly add electrons 3H x 1 =3, 1AS x1= 5, you get a total of 8 electrons. Knowing that Arsenic can only distribute 3 bonds (and so 6 electrons) to the 3 hydrogens leaves 2 remaining electrons, which as I stated before, should be applied to the main atom. So, you're going to have an unbonded electron pair attached to Arsenic. Four total bonds, one unbonded electron pair gives a tetrahedral structure that is trigonal pyramidal.
  10. TigerLilies

    TigerLilies Senior Member

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    Thanks so much! Your explanation makes sense. I was thinking that by looking at the periodic table you could determine the hybridization of the molecule (sp3) since the atoms occupy an S and 3 p orbitals and that from the hybridization, you could determine that the molecule is trigonal pyramidal.

    What do you think of that? Your response makes much more sense and is clearly more logical, but I was just wondering if what I said made sense?

    Thank once again! :)
  11. Belfagor

    Belfagor Busy Breaking Bonaduce

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    This may work for some molecules, but not all (I'm not 100% on this). My recommendation is that when asking for hybridization, ALWAYS draw a lewis structure. Hybridization is the easiest thing ever once you break it down. Just count the electron domains, and you're set. But, if you deduce that a molecule is trigonal pyramidal, yes, it's going to be sp3 hybridized.
  12. TigerLilies

    TigerLilies Senior Member

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    Yeah and the question asked to predict the geometry using just a periodic table, not a Lewis dot structure. And I don't think that there's really a way of determining the hybridization without knowing the Lewis structure of the molecule, so I guess my explanation was bogus. :)
  13. TigerLilies

    TigerLilies Senior Member

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    Would I use the same reasoning to determine the geometry of a CO\2^2- ion using just a periodic table?

    Thanks.
  14. Belfagor

    Belfagor Busy Breaking Bonaduce

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    Yes, the exact same way. To be honest with you, these types of "without a periodic table" type of questions seem to be aimed at getting you to think conceptually. Once you understand the concept of groups and valence electrons, feel free to use the table. It's hard to tell without drawing the resonance structure, since C02 contains double bonds, but you can guess since it has an extra two electrons, it's going to have an unbonded lone pair somewhere that throws off its typical structure. When in doubt, draw it out.
  15. LJDHC05

    LJDHC05 Former Chicken Slayer

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    I guess the best way for me to explain this is through the term coordinate number. Take NH3 for example:
    N= 5 electrons
    3H= 3 electrons

    the N has 3 bonds and a lone pair making its coordinate number 4 (3+1=4). thinking about geometries, the easiest way you can put 4 ligands around an atom is in a tetrahedral arangement. by easiest i mean putting the most space between ligands.

    Looking at AsH3 we can use the same logic...
    As= 5 e-
    3H= 3 e-
    CN= 4--> tetrahedral

    BF3 however
    B=3 e-
    3H= 3e-
    CN=3--> trigonal planar places the maximum space between ligands

    CO2 ^-2
    C= 4e-
    20^-= 7e-
    CN=2 because 2 double bonds form--> linear molecule O=C=O


    Hope that works for you!
  16. TigerLilies

    TigerLilies Senior Member

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    Thanks a lot! But I wrote the molecule incorrectly, it's actually CO3^2-. Is that also a resonance structure? Because I'm confused, how can you determine whether or not a molecule is a resonance structure when you draw the lewis dot? Or do you just have to memorize the resonance structures like (NO3^-, SO2, etc.)

    Thanks once again, I really appreciate it! :)
  17. Belfagor

    Belfagor Busy Breaking Bonaduce

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    That's a good way of looking at it too, because it ties in coordinate numbers and cubic cell types. However, I think for most Gen Chem courses icoordinate numbers is the last topic that is covered.
  18. LJDHC05

    LJDHC05 Former Chicken Slayer

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    Advanced inorganic starts with it and builds on it and revolves around it and so on...shudder :scared: bad memories of too many hours studying chemistry as an undergrad...YAY FOR DIPLOMAs
  19. TigerLilies

    TigerLilies Senior Member

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    lol, i have no idea what coordinate numbers are, but i'll keep that in mind, thanks! it sounds like a term i could use at a cocktail party to sound smart, lol.
  20. LJDHC05

    LJDHC05 Former Chicken Slayer

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    Resonance is just moving electrons. Electrons dont act like dots on a page as most intro chem classes would have you believe, so they can move around and do things that you dont even want to think about describing mathematically (HOORAY QUANTUM MECHANICS!).

    But I digress, so say you have CO3^-2 you have 4 electrons on your central carbon atom and three oxygens to share them with. The easiest way to to this is draw a carbon with three Oxygens spaced evenly around it. give each oxygen an electron from C by drawing a bond. So now you have an extra electron floating around. It can go on any of the three oxygens in the form of the double bond. Resonance would be moving that extra electron from oxygen to oxygen thereby moving the double bond. Make sense?
  21. LJDHC05

    LJDHC05 Former Chicken Slayer

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    look it up in your chem text...it should be there. And it doesnt work at cocktail parties, unless you want to put someone to sleep or you're with a bunch of chem dorks (yes we have parties too!!! :) )
  22. TigerLilies

    TigerLilies Senior Member

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    Thanks so much! Yeah, I think I do. I've noticed that in many molecules, in which one of the bonds is a multiple bond (double, triple) and other bonds are single; the molecule is usually a resonance structure, like NO3^2- and SO2. is the case with all such molecules?

    thanks so much!!
  23. LJDHC05

    LJDHC05 Former Chicken Slayer

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    There are always different ways to draw a molecule, some are less correct than others. for the purposes of an intro level class, anything with an oxygen as a ligand can generally form a double bond, unless of course, the final product violates the octet rule on the central atom...best thing to do is practice practice practice! good luck :luck:
  24. TigerLilies

    TigerLilies Senior Member

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    Awww, I thought going on a tangent about coordinate numbers (when i don't even know what it is) and relating to the essence of molecular matter and such, would be a surefire way for me to be the super-cool 18 year old at a cocktail party! :(
  25. LJDHC05

    LJDHC05 Former Chicken Slayer

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    yeah, make sure you bring your lab goggles to the party if you go on about that kind of stuff. You wouldnt want to lose an eye if someone splashed their drink after falling asleep out of boredom at the mention of chemistry...yeah, I have renounced my major and talk about more exciting things like sports, the weather...come to think of it, I dont like cocktail parties, I prefer beer anyway
  26. TigerLilies

    TigerLilies Senior Member

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    Thanks so much! And thanks for everyone's responses, I really appreciate it as it's helped me a lot. I think it's time I go to bed, so I can wake up for my 8 o' clock class tomorrow! :(
  27. TigerLilies

    TigerLilies Senior Member

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    LOL, I've actually never been to a cocktail party in my life. I guess I should say a frat party. I'll be sure to have my trusty googles out--with them I'll be irresistable. Think of all the dates I could get!
  28. mercaptovizadeh

    mercaptovizadeh ἀλώπηξ

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    1.) In BF3, all four atoms: 1B and 3Fs, reside in the same plane. In NH3, the 3Hs reside in one plane and the N in another. Actually, NH3 undergoes an umbrella inversion, so the Hs are not fixed in a plane.

    2.) In CH4, none are coplanar. In XeF4, all are coplanar. You can tell this by counting the valence electrons. C gives you 4 and each H gives you 1, so CH4: 4 + 1x4=8. Assigning these electrons in connectivities, you get four bonds and no lone electron pairs on the C. In XeF4, Xe gives you 8 valence electrons and each F gives you 7, so XeF4: 8 + 7x4=36. Assigning the electrons, each F gets eight around it (including two in a bond to Xe), so 36-32=4 remaining electrons. These are 4 electrons are assigned in two electron pairs on Xe. Since Xe has 6 connectivities: 4 Fs and 2 electron pairs, it goes into tetrahedral, but the two electron pairs at the top and bottom of the octahedron don't count as part of the structure, so we only look at the 4 Fs, which thus lie in a plane, to give us all 5 atoms in XeF4 as coplanar.

    3.) XeF4 is definitely isotropic. SF4 is not (I think, but haven't taken the time to actually look).

    4.) Based JUST on the periodic table, you see that As is in the same group as NH3. Since NH3 is trigonal pyramidal, you could infer that AsH3 would be as well. When you look at the electrons, AsH3 would have: 5 + 1x3=8 electrons. Six go into the As-H bonds, so there is one lone pair that goes on As. This gives an overall tetrahedral structure, but since we ignore the electron pair in naming the structure, we say that it is trigonal pyramidal, as we inferred.

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