Electronegativity

Electronegativity is the tendency of an atom to attract the shared electrons in a covalent bond. Fluorine, oxygen and nitrogen are among the most electronegative atoms. Malay: keelektronegatifan · Chinese: 电负性.

When two atoms share a pair of electrons, the pair is not always shared equally. A more electronegative atom pulls the shared electrons closer to itself, so its end of the bond becomes slightly negative while the other end becomes slightly positive. Electronegativity increases across a period, as the nuclear charge rises, and decreases down a group, as the atoms get larger. The most electronegative atoms therefore sit at the top right of the periodic table, with fluorine the highest of all.

Example. In a water molecule the oxygen atom is much more electronegative than hydrogen, so it draws the shared electrons towards itself. This gives oxygen a small negative charge and each hydrogen a small positive charge, making water a polar molecule and allowing it to form hydrogen bonds. In a molecule of two identical atoms, such as Cl₂, the two atoms have equal electronegativity, so the electrons are shared evenly and the bond is non-polar.

Confusion to avoid. Electronegativity is about attracting shared electrons in a bond, not about gaining or losing electrons to form ions. Do not confuse it with reactivity either; a very electronegative atom is not automatically the most reactive in every reaction. Note too that electronegativity only has meaning when electrons are actually being shared, so it is a covalent-bonding idea rather than an ionic one.

Electronegativity connects covalent bonding to the ideas of the polar molecule and the hydrogen bond in this chapter, and it is the reason certain covalent substances have unexpectedly high boiling points, a point that comes up in SPM Chemistry.