An acid is a chemical that can give up a proton to another chemical. Two out of three textbook definitions agree on this (we can worry about the Lewis definition some other time). But why are there so many chemicals out there giving away free protons?
The story begins with a hydrogen atom (one proton, one electron).
A second electron would complete hydrogen’s valence shell, which is something all atoms want to do. So hydrogen will try to bond with another atom, in the hope that by sharing electrons through chemical bonds, it can get the extra electron it so desperately needs.
Sadly, this arrangement doesn’t always work out in hydrogen’s favor. Rather than getting to borrow some other atom’s electron, sometimes hydrogen gets cheated out of the one electron it already had. This can happen for several reasons, such as:
- Greedy Atoms: Some atoms, especially oxygen, chlorine, and fluorine, tend to hog electrons from other atoms. I described this electron-hogging tendency in a previous post on electro-negativity.
- Electrons Gone Wild: Some molecular structures allow electrons to run around inside the molecule. This is called electron delocalization, or sometimes electron resonance.
Whether due to electron delocalization, electro-negativity, or electro-something else, the single proton of a hydrogen atom can end up feeling neglected and lonely. And the more neglected and lonely that proton feels, the more likely it is that this will happen:
And the more likely it is that this will happen, the more acidic the chemical in question is said to be.
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Today’s post is part of a special series here on Planet Pailly called Molecular Mondays. Every other Monday, I struggle valiantly to understand and explain some concept in the field of chemistry. Please note: I suck at chemistry, but I’m trying to learn. If I made a mistake, please, please, please let me know so I can get better.