Chapter 10 · Chemical Bonding
10.8Electronegativity and Polarity: Why Oil and Water Don't Mix
12 min · two checks
Predict
Why do water and oil separate?
The idea
- Use electronegativity differences to judge bond polarity.
- Decide whether a whole molecule is polar.
Electronegativity is an atom’s pull on bonding electrons. It increases toward fluorine, the most electronegative element, and decreases down a group. If two atoms differ, the shared pair sits closer to the more electronegative one. That bond is polar and has a dipole: partial negative on the greedy atom, partial positive on the other. A difference near zero is essentially nonpolar covalent. A large difference, roughly above 1.8 to 2 on common scales for a metal/nonmetal pair, is treated as ionic. The boundaries are guidelines, not cliffs.
A molecule is polar if its bond dipoles do not cancel. CO₂ has two polar bonds aimed opposite each other in a line, so the molecule is nonpolar. Water’s bent shape means the O–H dipoles add up, and water is polar. Oil molecules are mostly C–H and C–C, weakly polar or nonpolar, and they huddle together rather than mix into water’s hydrogen-bonded network. “Like dissolves like” is this paragraph in slogan form. Soap works because one molecule has a polar head and a nonpolar tail, bridging the two worlds.
Keep these
- Electronegativity rises up and to the right; fluorine is highest.
- Polar bonds plus shape decide molecular polarity. Symmetry can cancel dipoles.
- Polar solvents mix with polar solutes. Nonpolar with nonpolar.
Worked path
Decide whether CF₄ is polar. The C–F bond is polar, and the shape is tetrahedral.
Observe
Four identical polar bonds, tetrahedral symmetry.