Polarity

In polar bonds the electrons are shared unevenly between the two atoms. This causes a difference in charge resulting in two poles, one positive and one negative. Polar molecules are known as dipoles because they have two charged ends.

In nonpolar bonds the electrons are shared evenly between the two atoms making the bond neutral. If a bond contains only nonpolar bonds it will be a nonpolar molecule.

The shape of a molecule and the polarity of its bonds together determine whether a molecule is polar or nonpolar.

The opposite is true for large molecules, the polarity determines the Molecular Shape of a large molecule. A good example of this is protein which can contain thousands of atoms. These molecules have a variety of shapes, such as chains, rings, and balls.

Polar molecules form, when one atom has a stronger attraction to electrons than the other, or when there are electrons that are not bonded. One end is positive, the other negative, so that they will be attracted to each other and anything else with a charge.

The reason for hydrogen bonds is the polarity of the molecules. Polar molecules have their boiling points much higher than the molecular mass alone suggests. Water would boil at -100 C without the polarity of the molecule. The polarity is also the reason why liquid polar solvents are wet: the electric forces make the molecules stick to things.

One consequence is that a water jet (from a faucet) bends around a plastic comb, which is electrostatically charged by rubbing it to a woollen cloth. Mitsubishi Heavy Industries Corporation has drafted plans for an entire ferry, which moves using this phenomenon.

Polar substances usually dissolve to water, which is a highly polar substance. The polar solvents have so strong an attraction between their molecules that no nonpolar molecules can stay in their way. This is why nonpolar substances, like oil, don't dissolve in water. The strong attraction between the water molecules keeps them together and forces the oil molecules out, because they cannot break the strong hydrogen bonds to get into the water. The electric charge will cause even nonpolar substances to attract the polar molecules strongly, because the polar charge strengthens the van der Waals forces. You can see this, if you put a drop of oil on water: it will spread to a thin film. The forces keeping the oil drop together are smaller than the attraction to the polar water, so that the drop is attracted to the water. The area in contact with the water is maximised in a thin film.

Polar and nonpolar solvents don't repel each other. That is Bad Science, which even many chemists believe in, caused by false analogies and dumbing down in the name of good communication. More about this popular misconception can be found from Dr. Kevin Lehmann's Bad Chemistry page: http://www.princeton.edu/~lehmann/BadChemistry.html

Water is polar, because the oxygen molecule has eight electrons (an octet), but only two pairs of them are shared with the hydrogen atoms. The rest, the four electrons, form two pairs. (They pair up because one electron has its spin "up" and the other "down".) The two pairs are as far away as possible from the hydrogens and from each other. On a sphere, this means that they are in the vertices of an imaginary tetrahedron. Now you can imagine a tetrahedron with two vertices that have a negative charge. This means that there is a electric potential difference between the ends - the other side is positive, the other negative.

A side view of a water molecule:

:       (+)
:(+)     __  (+)
:       /  \
:     __|2H|__
:    /  \__/  \
:   /          \
:  |            |
:  |    (++)    |
:  |            |
:   \          /
:  (2e-)_____(2e-)
:
:  (-)   (-)    (-)

Polar molecules have one atom with a stronger electronegativity than the other. Hydrochloric acid is one, but it is a gas when pure at rtp. The chlorine atom has a higher electronegativity, so it "robs" some electron density from the hydrogen. Again, there is a potential difference between the ends. All compounds are a little polar, because there is always a difference in electronegativity. For some compounds, like oil, the difference is so small that they're almost nonpolar.

Thanks Professor Pi

Electrical Polarity

The source and destination of electron flow arbitrarily defined as Negative (-) as the source of electrons or EMF which flows through a circuit until it reaches the Positive (+) of the power supply.

The effects of electron flow direction are not obvious and for this reason (there is no obvious difference), many, perhaps the majority of circuits, are designed as if positive electricity flowed through the circuit downward toward the negative, which is often common ground or an actual connection to the earth which has a negative charge. Benjamin Franklin defined the polarity of electricity, and clouds may be positively charged, even though it "seems obvious" that lightning falls from the sky.

In AC circuits, the polarity reverses usually twice the rate of 50 or 60 times, one Hertz or cycle involving a positive half followed by a negative half. Since low frequencies are larger in wavelength and less affected by reactance (imaginary- heatless-resistance), aircraft may use 400 Hz frequency AC so the reactive circuits especially transformers are much less massive. AC is more complex than DC but used for power because it is less affected by real resistance which wastes so much heat that DC can not follow long wires without losing most of its power heating up the wires. AC overcomes resistance by using a transformer to reversibly exchange heat-producing current for voltage, which can push less current through more wire and thereby reduce total power loss.

Po*lar"i*ty (?), n. [Cf. F. polarit'e.]

1. Physics

That quality or condition of a body in virtue of which it exhibits opposite, or contrasted, properties or powers, in opposite, or contrasted, parts or directions; or a condition giving rise to a contrast of properties corresponding to a contrast of positions, as, for example, attraction and repulsion in the opposite parts of a magnet, the dissimilar phenomena corresponding to the different sides of a polarized ray of light, etc.

2. Geom.

A property of the conic sections by virtue of which a given point determines a corresponding right line and a given right line determines a corresponding point. See Polar, n.

© Webster 1913.