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.
Planet Pailly 
Reading this, I’m struck by the observation that larger scale patterns (in this case molecules) seem to happen as a result of asymmetries in smaller scale patterns (atoms). If hydrogen were naturally balanced, it’s not clear that we’d have much of a universe. It seems like the same thing would be happening at the quark level, leading to protons and neutrons.
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I hadn’t been thinking about it in that way. How does helium fit into the asymmetries of our universe?
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Good question. From what I’ve heard of helium, it, along with the other noble elements, mostly just do their own thing. Although a universe filled with helium would probably still collapse into stars, where the gravitational pressure on the core would fuse it into heavier elements.
But if every element were a noble gas, I wonder if the universe would have anything solid in it. It might all just be stars, gas giants, and black holes. (Although I might be showing here my cavernous ignorance of chemistry.)
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That’s basically the way I see it too. Noble gases don’t form chemical bonds, and a universe full of them would be pretty dull.
But helium is something like 20% of the ordinary matter in our universe. If it weren’t a noble gas, imagine how much more chemistry would be going on.
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