Electronegativity values increase from left to right across a period and decrease from top to bottom in a group, influencing the type of bond that will form between two atoms.

Electronegativity is a measure of how strongly an atom pulls on shared electrons. Across a period, atoms get closer together and have more protons, so they pull harder, making electronegativity rise from left to right. Down a group the atoms get larger and the outer electrons are farther from the nucleus, so the pull weakens and electronegativity falls. Because of these trends, when a highly electronegative atom meets a less electronegative one, the shared electrons shift toward the stronger puller, forming a polar covalent bond, while very similar electronegativities give a nonpolar covalent bond. For example, hydrogen (low electronegativity) bonding with fluorine (high electronegativity) creates a highly polar covalent bond, whereas hydrogen bonding with hydrogen itself yields a nonpolar covalent bond.