5.6 Polarity

Hydrogen sulfide has a boiling point of -59.55°C. Sulfur's heavier congener is selenium, and hydrogen selenide boils at -41.25°C. The next heavier congener is tellurium, and hydrogen telluride still only boils -2.2°C. The heavier the molecule, the warmer its compound boils. But hydrogen oxide, the lightest member of this series, boils all the way up at 100°C! What's going on?

Although covalently bound atoms share electrons, they don't do so equally. The electronegativity of each bonded atom determines how much it tries to hog all the electrons for itself. When a covalent bond happens between two atoms with different electronegativities, a dipole moment results, and the two atoms each develop a partial charge.

The bonds in water are between atoms of hydrogen, electronegativity 2.20, and oxygen, electronegativity 3.44. Consequently, the atoms each have a partial charge, negative for the oxygen and positive for the hydrogens. Since the two dipoles are on one side of the molecule - the molecule is bent - a water molecule is polar. So neighboring water molecules act like tiny magnets that attract each other by their opposite poles. That's how water is liquid when its heavier congeners are gases; hydrogen sulfide is nowheres near as polar as water, because sulfur's electronegativity of 2.58 is much closer to that of hydrogen. (Searching the internet reveals that about as many people consider H₂S polar as consider it nonpolar. It's certainly in a gray area.)

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