Do the following molecules have permanent dipole movement?

Question

Yes or No

_______NF3
_______SiH4
_______N20


My guess:
I know that permanent dipoles occur when two atoms in molecules have differing eletronegativity, as when one atom attracts electrons more than another becoming more negative, the other atoms becomes more positive.

? yes------->NF3
?yes--------->SiH4
?yes--------->N20

Cambridge Soft's ChemFinder is disabled so I cannot get a close enough look at these. I just took some guesses. Please double check.

Thanks!

Answer 1

NF3 ----> polar
SiH4 ---> non-polar
N2O ----> polar

You're right about permanent dipoles occur when two different atoms form a covalent compound. But that's only one factor. The other is the structure of the molecule.

A simple compound made up of only 2 elements usually forms a tetrahedral shape (4 corners with one atom in the middle) between the lone pairs and bond pairs of electrons. In SiH4, all of the four pairs of electrons of silicon (which is in the middle of the tetrahedral) are shared with hydrogen atoms. The silicon atom is more electronegative (attracts electrons more) than the hydrogen atoms, but because all four of the electron pairs are shared with hydrogen, the effect is balanced out between the four covalent bonds and the molecule becomes non-polar.

In NF3, the fluoride atoms take up 3 of the 4 pairs, with the last pair as a lone pair. This gives the molecule a triangular pyramidal shape, with the nitrogen atom at the top corner. The fluorine atoms attract more electrons, so their side would be more electronegative than the side with the single lone pair.

N2O is more complex because it does not have a normal tetrahedral shape like the other two compounds. There is one wandering covalent bond in its structure and that travels between the N-N bond and the N-O bond (this is illustrated in the site I used). The travelling bond means that the polarity is swapped around, but the molecule is still polar.

Answer 2

Your first intuition is right, but then move further to consider the arrangement of those atoms.

In both NF3 and N2O they're not totally symmetric - in NF3 there's also a lone pair on the nitrogen and N2O is probably best represented as N(-)=N(+)=O, so yes, they both have permanent dipoles. Silane (SiH4) however, is a bit like methane (CH4), the hydrogens are arranged at the vertices of a tetrahedron so any dipoles resulting from the polarity of the Si-H bond simply cancel each other out. So this does not have a permanent dipole.

Answer 3

I think you mean "permanent dipole moment"....

Dipoles occur from differing electronegativities, but it is also necessary for the charge to be assymetrically distributed.

NF3 is trigonal planar with the positive nitrogen at the top and the negative fluorines at the bottom, while SiH4 (tetrahedral) has no dipole because the hydrogens are symetrically arrayed around the central Si.

Finally, I believe N20 is bent so it should have a permanent dipole.

Answer 4

NF3 is triangular planar there one N-F bond moment is cancelled by the magnitude of other two N-F bonds. Hence it has no permanent dipole moment.
SiH4 is tetrahedral. Each Si-H bond is polar but the total molecule has no permanent dipole moment as for the former reason.
For N2O, N=N=O, due to its linear nature it has a permanent dipole moment

Answer 5

NF3 and N2O do have permanent dipole moment, while SiH4 does not; given to it's spatial orientation and equivalence among bonds, the dipole moments cancel each other

Answer 6

you should be able to draw the lewis structure yourself and figure this out. what are you gonna do on the test? use chemfinder? I dont' think your teacher will like that one. as for chemfinder after 5 searches you have to register. its free and they don't send you junk so just do it