In a hydrogen chloride molecule, the chlorine atom pulls the shared electrons closer to itself. The bond is polar, with a partial negative charge on chlorine and a partial positive charge on hydrogen. The tendency of an atom to attract electrons in a bond is called electronegativity. It is a relative property, not an absolute one. Fluorine is the most electronegative element, with a value of 3.98 on the Pauling scale. Cesium is one of the least, with a value of 0.79.

Electronegativity increases across a period and decreases down a group. That is because atomic radius decreases across a period, so the nucleus holds electrons more tightly. Down a group, the radius increases, and the outer electrons are farther from the nucleus, so they are held less tightly. The noble gases are usually excluded because they do not form bonds easily.

The difference in electronegativity between two atoms determines bond type. A difference greater than about 1.7 usually means an ionic bond. A difference between 0.4 and 1.7 means a polar covalent bond. A difference less than 0.4 means a nonpolar covalent bond. These thresholds are approximate, but they are useful for predicting bond character.

Electronegativity affects molecular properties. Polar molecules dissolve in polar solvents. Nonpolar molecules dissolve in nonpolar solvents. Hydrogen bonds form between hydrogen and highly electronegative atoms like oxygen, nitrogen, and fluorine. These interactions shape proteins, DNA, and water, making life possible.