Some atoms have more affinity for electrons than do other atoms, and this affinity is based upon the size of the atom and the distance of its valence electrons from its positvely charged nucleus. The measure of an atom's electron affinity is called it's electronegativity. Some atoms are highly electronegative and steal electrons from any atom it bonds to; other atoms donate electrons to any atom that it bonds to.

As a general rule of thumb, as you go up and to the right of the periodic table, the electronegativity of an atom increases. Thus, as you go up and to the right on the periodic table, you reach the bully elements such as fluorine, oxygen, and nitrogen who like to hog electrons in a bond (fluorine, at the top right, is the most electronegative element). As you go down and to the left, you reach the 98-pound weakling elements such as the metals and silicon (and carbon, to some extent)

In the illustration below, carbon atoms are shown as "weaker" than fluorine atoms at holding onto the bonding electrons (important note: "atoms" not drawn to scale. Fluorine, despite being heavier, is actually smaller than carbon, although is much more of an electron pig).

Electron hogs such as fluorine selfishly steal electron density from less electronegative atoms like carbon, creating a bond dipole

 

 

Using electronegativities

If there is a difference in electronegativity between two bonding atoms (a carbon bonded to fluorine for example), you get a separation of charge, or a dipole moment. In the C-F bond example, carbon has some of its electron density stolen from it by the highly electronegative fluorine atom and becomes partially positive charged, while fluorine, having stolen the electron density, becomes partially negative charged. You can use the dipole vector (shown below) to indicate the direction of the dipole moment in a bond, with the head pointing towards the direction of negative charge.

It is very important to understand how electronegative atoms attached to carbons form dipoles, as it is essential in the understanding of the reactivity of organic molecules. It's a good idea to look at many different bonds and predict the dipole moment of the bond using the dipole vector.