electronegativity
/ ee-lek-troh-neg-uh-TIV-uh-tee /
Picture two people sharing a blanket on a cold night. If one of them is a blanket-hog, the blanket ends up bunched on their side. Electronegativity is an atom's blanket-hogging power: a measure of how strongly an atom, when bonded to another, pulls the shared bonding electrons toward itself. Some atoms tug hard (oxygen, fluorine), some tug gently (carbon, hydrogen), and the difference between two bonded atoms decides which way the electrons lean.
Chemists put numbers on this with the Pauling scale, where fluorine is the greediest at about 4.0, oxygen about 3.4, nitrogen 3.0, carbon 2.5, and hydrogen 2.1; metals sit low. The trend across the periodic table is simple: electronegativity rises going up and to the right, because small atoms with a strong nuclear pull hold electrons most tightly. When two atoms of equal electronegativity bond, they share evenly and the bond is nonpolar; when they differ, the more electronegative atom takes a larger share of the electron cloud and the bond becomes polar.
Electronegativity matters because it quietly drives most of organic chemistry. The electronegativity difference between two bonded atoms tells you where a bond is polar, which end carries a partial negative charge, and therefore where a molecule is likely to be attacked by an incoming nucleophile or electrophile. The polarity of a carbonyl C=O, the acidity of an O-H, the inductive pull of a chlorine — all trace back to electronegativity. Note it is a relative, comparative property of an atom in a bond, not a fixed force you can measure on a lone atom in isolation.
In a C-O bond, oxygen (EN 3.4) outpulls carbon (EN 2.5), so oxygen carries a partial negative charge and carbon a partial positive one. In a C-C or C-H bond the difference is tiny, so those bonds are essentially nonpolar.
An electronegativity difference turns an even-shared bond into a polar one, with a built-in plus and minus end.
Electronegativity is a property of an atom within a bond and is comparative; it is not the same as electron affinity or ionization energy, though all three are related.