[student8607]

I'm to determine if CH₃NH₂ has a net dipole moment.

First I drew this lewis structure


Now I know with the Carbon all the H's have arrows pointing towards the Carbon.
Nitrogen is more EN so there is an arrow going to C.
N is also more EN than H, so there are two arrows facing it.
So I have this


I know it obviously has a dipole moment, but how am I to "add up" these lines to determine the net polarity?

[student8607]

Can I just say the 3 arrows towards N form a triangle and cancel.
The up and down on the C cancel.
So the net is: -->

[azmanam]

If you've taken physics, treat them as vectors, and find the sum of the vectors.

If not, do  it qualitatively.  The up arrow and the down arrow cancel each other out.  There are two horizontal right arrows.  There are two diagonal left arrows.  The slope of the two diagonal arrows are equal and opposite, thus the vertical component of those arrows cancel.  The horizontal component of those arrows is additive, so there are also two 'arrows' pointing to the left.

So the only net arrows to worry about are the two right arrows and the two horizontal components of the left arrows.  The sum of the horizontal components of the left arrows are smaller than the right arrows, so the left arrows cancel some of the right arrows, but not all.

Small dipole moment with the + on the left and the arrow pointing right.

[azmanam]

you can also think about it graphically if it makes more sense...

[student8607]

That's what I thought. Fantastic.

[student8607]

Still talking about dipole moments.

I understand that if things are symmetrical, like trigonal planar and linear that they will not have a net polarity.

So does that mean something like PCl₅ is also symmetrical and wont have a dipole moment?
If not, then I am guessing it will have a slight +---> D?

[macman104]

Correct, PCl5 has no net dipole.

The 3 trigonal planar Cl cancel eachother out, and the two vertical Cl cancel eachother.

[student8607]

The 3 trigonal planar Cl cancel eachother out, and the two vertical Cl cancel eachother.

Thanks. That makes a lot of sense. Never looked at it like that!