Categories: Guide to Space

Dipole Moment

[/caption]It has long been known that all molecules possess two equal and opposite charges which are separated by a certain distance. This separation of positive and negative charges is what is referred to as an electric dipole, meaning that it essentially has two poles. In the case of such polar molecules, the center of negative charge does not coincide with the center of positive charge. The extent of polarity in such covalent molecules can be described by the term Dipole Moment, which is essentially the measure of polarity in a polar covalent bond.

The simplest example of a dipole is a water molecule. A molecule of water is polar because of the unequal sharing of its electrons in a “bent” structure. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge while the hydrogen, in the center, has a partial positive charge. Because of this, the direction of the dipole moment points towards the oxygen.

In the language of physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system’s overall polarity – i.e. the separation of the molecules electric charge, which leads to a dipole. Mathematically, and in the simple case of two point charges, one with charge +q and one with charge ?q, the electric dipole moment p can be expressed as:p=qd, where d is the displacement vector pointing from the negative charge to the positive charge. Thus, the electric dipole moment vector p points from the negative charge to the positive charge.

Another way to look at it is to represent the Dipole Moment by the Greek letter m, m = ed, where e is the electrical charge and d is the distance of separation. It is expressed in the units of Debye and written as D (where 1 Debye = 1 x 10-18e.s.u cm). A dipole moment is a vector quantity and is therefore represented by a small arrow with a tail at the positive center and head pointing towards a negative center. In the case of a Water molecule, the Dipole moment is 1.85 D, whereas a molecule of hydrochloric acid is 1.03 D and can be represented as:

We have written many articles about dipole moment for Universe Today. Here’s an article about what water is made of, and here’s an article about molecules.

If you’d like more info on dipole moment, check out these articles from Hyperphysics and Science Daily.

We’ve also recorded an entire episode of Astronomy Cast all about Molecules in Space. Listen here, Episode 116: Molecules in Space.

Sources:
http://en.wikipedia.org/wiki/Electric_dipole_moment
http://en.wikipedia.org/wiki/Dipole
http://www.tutorvista.com/content/chemistry/chemistry-iii/chemical-bonding/degree-polarity.php
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dipole.html#c1
http://en.wikipedia.org/wiki/Water_molecule

Matt Williams

Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.

Recent Posts

Hubble and Webb are the Dream Team. Don't Break Them Up

Many people think of the James Webb Space Telescope as a sort of Hubble 2.…

2 hours ago

Scientists Have Figured out why Martian Soil is so Crusty

On November 26th, 2018, NASA's Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight)…

10 hours ago

Another Way to Extract Energy From Black Holes?

Black holes are incredible powerhouses, but they might generate even more energy thanks to an…

16 hours ago

Plastic Waste on our Beaches Now Visible from Space, Says New Study

According to the United Nations, the world produces about 430 million metric tons (267 U.S.…

1 day ago

Future Space Telescopes Could be Made From Thin Membranes, Unrolled in Space to Enormous Size

As we saw with JWST, it's difficult and expensive to launch large telescope apertures, relying…

2 days ago

Voyager 1 is Forced to Rely on its Low Power Radio

Voyager 1 was launched waaaaaay back in 1977. I would have been 4 years old…

2 days ago