electronegativity
Summary
Electronegativity is a relative measure of how strongly an atom pulls shared bonding electrons toward itself. It generally increases across a period and up a group, with fluorine highest, and a larger difference between bonded atoms makes a more polar bond.
Detail
Meaning: Electronegativity is a relative, unitless measure (Pauling scale) of how strongly an atom attracts the shared electrons in a covalent bond. Fluorine is highest at about 4.0, followed by O (3.5), N (3.0), C (2.5) and H (2.1). It generally increases across a period, as nuclear charge rises and radius shrinks, and decreases down a group, as bonding electrons sit farther from the nucleus. In a bond between unlike atoms the electrons spend more time near the more electronegative atom, giving it δ− and the other atom δ+, a polar bond.
On the MCAT: Expect to rank bonds by polarity, draw the dipole direction, or pick the most electrophilic carbon, such as a carbonyl carbon made δ+ by oxygen. It also explains acidity: nearby electronegative substituents withdraw electron density through bonds (the inductive effect), stabilize the conjugate base, and lower pKa. Hydrogen bonding needs H bonded to N, O or F.
Common confusion: Polar bonds do not guarantee a polar molecule: in symmetric CO₂ and CCl₄ the bond dipoles cancel. Electronegativity is also distinct from electron affinity and ionization energy, which are energies of isolated atoms.
Sources
- Organic Chemistry by Paula Yurkanis Bruice
- Organic Chemistry by John McMurry
- Chemistry: The Central Science
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.