electronegativity
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Imagine two people sharing a blanket, but one keeps tugging it toward their side. In a chemical bond, atoms 'share' electrons, but some atoms tug the shared electrons harder than others. Electronegativity is a measure of how strongly an atom in a bond pulls the shared electrons toward itself.
Precisely, electronegativity is a relative number describing an atom's tendency to attract a bonding pair of electrons. It is usually given on the Pauli scale, running from about 0.7 for the weakest pullers (like cesium) up to 4.0 for fluorine, the strongest. Across a period it rises and down a group it falls, so nonmetals near the top right of the periodic table are the most electronegative.
Electronegativity matters because the difference between two bonded atoms decides the bond's character: a small difference gives a roughly even-sharing covalent bond, a moderate difference a lopsided polar bond, and a large difference a bond so one-sided it becomes ionic. The caveat is that it is a derived, relative concept, not a directly measured property of a lone atom — an atom's value can even shift a little depending on what it is bonded to.
Fluorine (4.0) is far more electronegative than hydrogen (2.2). In an H–F bond the shared electrons spend much more time near the fluorine, giving fluorine a partial negative charge and hydrogen a partial positive one — a strongly polar bond, even though no electron has been fully transferred.
A large electronegativity gap makes a bond strongly polar.
Do not confuse electronegativity with electron affinity. Electron affinity is the energy released when a free, isolated atom grabs an extra electron; electronegativity describes the pull within an existing bond. They tend to trend together but are distinct ideas.