« Stoichometry with gases |
Since fluorine is the most electronegative atom, why isn't HF a strong acid, too? The HCl bond should be less polarized than the HF bond.
Dear 5110 student,
You are right, F is more electronegative than Cl, and if the only factor were electronegativity, HF would be stronger than HCl. But it isn't, HF is a weak acid. There is a competing factor.
The size of the halogens (or any group) changes more dramatically than their electronegativities as you go up and down the group. The F- ion is tiny compared to the Cl- ion, so negative charge is concentrated on this tiny ball, hating itself, so to speak. Big, fluffy iodide with a negative charge is the most stable, because it can distribute the -1 charge over a larger surface. So the charge looks more like zero everywhere.
Your electronegativity rule is a good one and does work as you go across a row, rather than down a column (H2O vs. HF for example) because size changes less than electronegativity across a row in the periodic table. For my own comprehensive review on acid base chemistry, see Things Everyone Should Know About Acid Base Chemistry
Let me know if that clears things up!
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