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Lesson 7: VSEPR

# Molecular polarity

Like bonds, molecules can also be polar. In a polar molecule, electron density is unevenly distributed throughout the molecule, resulting in regions of partial negative charge and regions of partial positive charge. Molecular polarity depends on both individual bond polarities and molecular geometry, the latter of which we can predict using VSEPR theory. Created by Jay.

## Want to join the conversation?

• Why is it called the dipole moment? why not 'Dipolarism' or 'Dipolarity'?
• A moment is a measure of a turning force about an axis.
When you place a molecule with an electric dipole in an electric field, a force acts to turn the molecule so that the positive and negative ends line up with the field.
The magnitude of the turning force is given by the formula
µ = q × d
where q is the amount of charge and d is the distance between the two charges.
µ is the turning moment.
That's why it's called the dipole moment.
• Can someone show me the math to do one of the (mu) equations listed in the video?
• For HCl, the charge on the H is about 2.8 x 10^-20 C (and the charge on Cl is -2.8 x 10^-20 C) and the bond length is 1.27 Å (where 1 Å = 10^-10 m).
`µ = q • d = 2.8 x 10^-20 C • 1.27 x 10^-10 m = 3.6 x 10^-30 C•m`
The conversion factor from C•m to D is: 1 D = 3.3 x 10^-30 C•m

Thus, we get :`µ = 3.6 x 10-30 C•m * (1 D / 3.3 x 10^-30 C•m) = 1.1 D`
These numbers, more details, and other similar calculations can be found at:
http://wps.prenhall.com/wps/media/objects/3081/3155729/blb0903.html
• : "it is really much easier to make this using a Molymon set"
What did he say?
• molymod set to create 3d structures of molecules. you've probably seen them in the chemistry lab when you construct organic compounds with the coloured balls (atoms) and stick thingies (bond)
• How to identify which element is more electronegative than the other
• Electronegativity is a general periodic trend that all elements follow. In general, electronegativity increases as you move to the right and up the periodic table. Thus, the MOST electronegative elements can be found in the upper, right hand corner of the periodic table, and the LEAST electronegative in the lower, left hand corner.

A handy trick that I learned in my orgo class was to make a "phone call" (F-O-N-Cl) for electronegativity, with F (flouriine) being the most electronegative element and all other elements decreasing from there.
• Is there a formula for determining m(u)? Or did i miss it somewhere?
• at around , jay mentions this.
μ=Q*d
where Q is the magnitude of the charges and d is the distance between them
• Does anybody understand what a temporary dipole moment is? Not just a dipole moment?
• An excited molecule goes from a relaxed state or (S0) state to an excited state (S1) the absorption of light is very quick pico/femto seconds and the excited molecule before relaxing back is very likely to undergo a change in dipole moment before the charges go back to the initial electronic rearrangement. Another example of a temporary dipole moment is when a molecules which are apparently non-polar (but even polar ones) are affected by electric fields so that they can become slightly polar....there are other examples of temporary dipole moments but they are quite complex.....
• How can I arrange different molecules in increasing dipole moments? For example between H2O, CBr4, H2S, HF, NH3, CO2?
• First, decide whether the molecules are dipoles or not. (Two of the ones you listed aren't, by the way.)
Next, look at the electronegativities of the different atoms in the dipolar molecules. Arrange the molecules in order of increasing difference of electronegativity between the elements in the molecule. For example, the difference between the electronegativities of nitrogen and hydrogen is 0.84. The difference between fluorine and hydrogen is 1.78. Therefore, HF has a greater dipole moment than NH3 does.
You may find this link interesting:
http://www.vias.org/genchem/physprop_12595_03.html
• Around , why are the dipole moments of the two unbonded electron regions pointed away from the O atom? I feel like they would be pointed towards the O atom because the electron density is in the O atom, which would mean that the net dipole moment for the water molecule is zero.
• This is because the oxygen atom is more electronegative as compared to the carbon. Hence, the oxygen atoms draw electrons away from the carbon, leading to the dipole moment in that direction.
• What does dipole moment physically mean? Is it some sort of force?And why is its magnitude proportional to r? Pls help me get some intuition
• A moment is the turning effect of a force around a fixed point, as when you are turning a crank.
It is the product of the force times the length of the crank.
A dipole moment is the turning force a fixed charge has on a dipole in a molecule. The moment depends on the charge at the end of the dipole and its distance from the charge at the other end of the dipole (i.e., the separation of charge).
µ = q×d