Atoms in molecules are held together by sharing electrons. A shared pair of electrons is called a chemical bond. Not all atoms share electrons equally, however. Some of them are more selfish than others. Electronegativity is a measure of an atom's ability to attract a pair of shared electrons to itself. The difference in electronegativity between two elements determines to what extent the electrons they share are shared unequally. The way in which electrons are shared helps determine many of the molecule's important properties like solubility.
Electronegativity is most simply defined as the pulling power of an element on electrons it shares in a bond. When two atoms share electrons, something like a tug of war exists between them. The electrons will spend more time around the more electronegative element, and the greater the difference in electronegativity, the more unequal the sharing will be. If the two elements have similar electronegativities, by contrast, they will share electrons in a more or less equal way.
Chemists have tried to come up with various ways to measure electronegativity across the years. The two most popular scales are those devised by chemists Robert Mulliken and Linus Pauling. In the Mulliken scale, the electronegativity is the average of the amount of energy it takes to remove an electron from an atom and the amount of energy released when the atom gains an electron -- the ionization energy and the electron affinity. The Pauling scale is more complicated and is based on the amount of energy it takes to break a bond between two elements. The two scales give different numbers for each element, but the results vary across the periodic table in basically the same way, so they are more or less interchangeable. Both scales are unitless, so each element is assigned a number to indicate how electronegative it is relative to other elements. Higher numbers mean increased electronegativity in both scales.
As you go either up the periodic table or toward the right, electronegativity tends to increase. Chlorine, for example, is more electronegative than bromine, which is more electronegative than iodine. Oxygen is more electronegative than nitrogen, which is more electronegative than carbon. Fluorine is the most electronegative element, with oxygen following in second place. Carbon and hydrogen have similar electronegativities, so they tend to share electrons more or less equally.
You can use differences in electronegativities to classify bonds into different categories. If one element is far more electronegative than the other, it will keep the electrons, rather than sharing them, so it will have a negative charge while its partner has a positive charge. The attraction between the opposite charges will hold them together. This type of bond is called an ionic bond and is fairly common when elements like sodium in the far-left column of the periodic table react with elements like chlorine on the far right. Sodium, for example, has an electronegativity of 0.9 on the Pauling scale, while chlorine has an electronegativity of 3.2. As a general rule of thumb, bonds where the electronegativity difference is greater than two are ionic.
In cases where one element is somewhat more electronegative than the other, the electrons will be shared, but shared unequally and will spend more time around the more electronegative atom. The oxygen-hydrogen bond in water and the oxygen-carbon bond in carbon dioxide are common examples. Oxygen has an electronegativity of 3.4 on the Pauling scale as compared to 2.6 for carbon and 2.2 for hydrogen, so it is significantly more electronegative. This type of bond is called a polar covalent bond. Finally, if two elements have similar electronegativities they will share electrons in a partnership called a nonpolar covalent bond. Carbon and hydrogen bonding is the most common example. As a general rule of thumb, polar covalent bonds have electronegativity differences in the range from 0.3 to 2. The greater the difference in electronegativity, the more polar the bond. This is only a rule of thumb, however, and there are exceptions.
Some of a molecule's important properties, like solubility, are related to how polar it is -- in other words, how unequally the electrons are shared in the molecule. A general rule of thumb for solubility is that like dissolves like, meaning that highly polar solvents are good at dissolving highly polar molecules while nonpolar solvents are good at dissolving nonpolar molecules.
