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Homework Help: Chemistry Help

  1. Feb 29, 2004 #1
    Hi Everyone
    i have been having problems with my school calender lately, and had to change one course to a grade 12 chemistry, i just attended the class for the first time friday, and it has been 2 weeks since it started. My teacher is going all out with no mercy and giving me an assignment thats due in 2 days, i am honestly screwed if i dont get this done, i have missed the first unit of the course, and this assignment is assoictated with it, i dont have a text book yet, so i cant even try to learn it, i dont understand how she expects me to finish this, she said borrow a book from a classmate, and i dont know anyone...so this forum is my last resort, here it goes....

    1) Draw an orbital diagram (line and arrow) for : Ca^+2
    2) In chart form, generate a set of quantum #'s for the last 4 electrons in an atom having the electron configuartion ----> [Kr]5s^2,4d^1(subscript1), 4d^1(subscript2)
    3) Draw and name the shape of the following molecules: a) CO3^-2 b) SF2

    4) For the molecule PCl3 :

    a) draw its lewis structure ( i think i can do this part)
    b) Name the shape ( i think i got this also)
    c) illistrate the polarity of each bond (lost here)
    d) use the difference in electronegativites to determine the type of bond between P-Cl (lost also)
    e) is the molecule POLAR or NONPOLAR? EXPLAIN (no clue)

    5) What similarites and what diff would u expect in the molecular shape and intermolecular bonding of NH3 and BF3?

    I would be very grateful if anyone with the knowledge can help me with these questions thank you very much
  2. jcsd
  3. Feb 29, 2004 #2


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    Introductory texts can be very idiosyncratic --- translation: you'll be lots happier making the acquaintance of a class mate, and in getting a few clues from same regarding your teacher's idiosyncracies as well. Polar/non-polar? Bond type? This kind of stuff is highly text specific as far as where particular authors want to draw lines --- we can give you "correct" answers, but they might not be correlated with the author's or your teacher's "correct" answers.
  4. Mar 1, 2004 #3


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    Polarity means which side is more negative and which side is more positive; it's based on which atom the electrons are closer to. Electronegativity is how strong an atom is attracted to electrons and it tends to increase as you go up and right on the periodic table. For example, fluorine has high electronegativity but potassium has low electronegativity. The atom with higher electronegativity is more negative.

    There are 3 types of bonds.
    1. ionic
    2. polar covalent (sp?)
    3. nonpolar covalent (sp?)
    Ionic is when the difference in electronegativity between the two atoms is 1.7 or greater.
    Polar covalent is when the difference in electronegativity is less than 1.7 but greater than 0.1
    Nonpolar covalent is when the difference in electronegativity is less than 0.1

    Assuming that the bonds between P and Cl are polar, the question is basically asking whether the pulling of the Cl atoms cancel each other out. To answer this question, you would have to know how P makes its bonds. If the Cl atoms are as far away from each other as possible, the molecule will be nonpolar. If the Cl atoms are not as far away from each as possible, the molecule will be polar.
  5. Mar 1, 2004 #4
    for (2) the order is as such:

    (primary quantum number, secondary (angular momentum) quantum number, magnetic quantum number, spin quantum number)

    example: (2, 1, 0, -1/2)

    I don't really have a lot of time to explain each 'quantum number' right now...If I remember I'll post later

    ok I'm back so I'll finish my post

    Primary quantum number- Designates the principle energy level in which a given electron is located. For this, just look at the number beside the orbital type. ie. in 4p^2_x, the principle quantum number is 4.

    Secondary quantum number- This is related to the angular momentum (motion) of the electron and the shape of its orbital. This is based on the orbital type. 's', 'p', 'd' and 'f' orbitals are given the values 0, 1, 2 and 3, respectively. So a 'p' orbital would have a value of 1.

    Magnetic quantum number- This is related to the spatial orientation of the orbitals. In any given/possible energy level, there can only be a certain number of orbitals. There is a range for these numbers, based on the orbital type. For 's' orbitals, the number must be 0, since there can only be 1 's' orbital in any given energy level. 'p' however can have -1, 0 or 1, because there can be 3 'p' orbitals in any given/possible energy level. For 'd' orbitals it ranges from -2 to 2, and for 'f' orbitals it ranges from -3 to 3.

    Spin quantum number- Distinguishes between the "oppositely spinning" electrons in an orbital- The spin can either be -1/2 or 1/2. Since there can be only 2 electrons in any single orbital, this just shows us the difference between the two electrons.

    Just as an example, Hydrogen. Since it only has 1 valence electron, it is quite easy:

    The valence configuration is: 1s^1

    The 'address' for this is (1,0,0,1/2)...as side note, if you only have 1 electron in an orbital, it doesnt' matter what you set the spin to. It can be either 1/2 or -1/2.

    I'll also adress #5 briefly

    Ok, do you rememeber how to draw lewis diagrams from grade 11 chemistry (assuming you took it)?

    NH3 looks like this
    Code (Text):

    The lines are single bonds.
    Notice here the octet rule is satisfied for Nitrogen (And a exception of it for H, since Hydrogen needs only 2 to be stable)
    The free electron pair at the top repels the hydrogens, and they repel each other (We can use the VSEPR model...and if you need to know what that is, just look it up on the internet). This will look like a pyramid (It's shape is named trigonal pyramidal). The bond angles for this are approximately 109.5 degrees (in experimentation they are slightly less between the hydrogen atoms because the electron pair is more repulsive than the hydrogen atoms.

    BF3 is like this:

    Code (Text):

    F   F
     \ /
    There should also be 3 free electron pairs around each fluorine, but its too hard to do with text. The bond angles here are 120 degrees, and the shape is trigonal planar. It is NOT a 3 dimensional shape, as opposed to NH3, which is. Notice a difference?

    Hope I was helpful
    Last edited: Mar 1, 2004
  6. Mar 1, 2004 #5
    VERY HELPFUL thank you very much warr, i really appreciate it, i understand everything know, except 4ce, and 5, thx if anyone can help me with these then am done.
  7. Mar 2, 2004 #6
    ok now i understand everything except #5, if anyone can help me out thx
  8. Mar 2, 2004 #7
    I never was very good at chemistry, but I'll give #5 a shot anyway.

    NH3 can be shown to have 3 single bonds extending from the nitrogen to each hydrogen atom and a single free electron pair on the nitrogen. This means that it's going to have a tetrahedron shape with one arm missing. Can you show that on your own?

    Since the missing arm is actually a very negatively charged free electron pair, the molecule will be polar. It will have a large negative charge toward the free pair and a large positive charge toward the hydrogen atms. Additionally, because of this polarity, it has a strong intermolecular bond due to the positive hydrogens bonding with the negative free pairs (hydrogen bonding).

    BF3 can also be shown to have 3 single bonds extending from the boron atom to each fluorine atom and a single free electron pair on the boron, so it also has a tetrahedral shape. However, because each fluorine atom also has 3 free pairs, they are more negatively charged than the boron atom and its free pair. As such, this molecule is also polar, with the relatively negative direction being toward the fluorines. Even so, there is no significant positive charge (the fluorines are really negatives, whereas the free pair is only sort of negative--nothing is really positive like NH3's hydrogen), so one BF3 molecule will not bond easily with another.

  9. Mar 4, 2004 #8
    hey thx alot cookiemonster and everyone else, i got 93% on my assignment help appreciated thx again
  10. Mar 4, 2004 #9
    great job, glad to hear you did well
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