# Why can atoms only gain or lose electrons and not protons?

I know that an object can become net negative or net positive by losing or gaining electrons, and having more or fewer protons than electrons but why can't protons be transferred too?

The energy required to remove an electron from an atom is called its ionization energy. Typical ionization energies are five or ten electron-volts. A visible-light photon carries an energy somewhere under $$\rm3\,eV$$ and cannot ionize most free atoms. There is enough ultraviolet light in sunlight that atoms on Earth can be preferentially ionized during the daytime, which drives lots of interesting chemistry. However typical temperatures on Earth $$(T=\rm300\,K$$, $$k_BT = \frac{1}{40}\rm eV)$$ are low enough that atoms typically don’t ionize spontaneously. The relative stability of atoms against ionization allows stable molecules to exist.

The energy required to remove a proton from a nucleus is called the proton separation energy. Typical proton separation energies are five or ten million electron-volts. In an environment where proton separation was happening, there would be so much energy kicking around that all of the nuclei would be completely ionized, with no bound electrons at all. If you, a biological person made of molecules like DNA and protein, were to visit such an environment, you wouldn’t be made of molecules any more after your visit, and you would therefore have forgotten your question.

It’s not that protons can’t be transferred. It’s just that if we lived in a place where proton transfer was common, we would have a very different perspective on chemistry.

You may actually be aware of some consequences of one nucleon-transfer reaction. Energetic radiation from outer space can cause spallation when interacting with the Earth, either with the atmosphere or with the heavier nuclei under Earth’s surface. Some of the spallation products are free neutrons, which thermalize and behave like a (very tenuous) component of Earth’s atmosphere. The most common species in the atmosphere is nitrogen-14, which interacts with thermal neutrons by

$$\rm ^{14}N + n \to p + {}^{14}C$$

The carbon-14 will beta decay back to nitrogen, with a half-life of about 5000 years. So if you find an object made of carbon, you can measure the ratio of carbon-14 versus carbon-12 and learn whether that carbon was recently distilled from the atmosphere. Carbon mostly exits the atmosphere to be concentrated in living plants (and in eaters of recently-living plants), while non-organic carbon does not accumulate new carbon-14: a carbonaceous object with carbon-14 in it was probably alive in the geologically recent past, and figuring out how recent is straightforward.

• Love this: "It’s just that if we lived in a place where proton transfer was common, we would have a very different perspective on chemistry." It's too often forgotten that our conceptual framework is shaped by Earth being in a certain temperature and pressure regime. If intelligent life can exist on icy planets or on neutron stars, they will have a very different idea of the line between basic and exotic processes and, moreover, of the line between stable and unstable due to the timescales on which they operate. Jan 24 at 12:25
• Aside from your first paragraph, most of this doesn't really contribute to answering the question. You talk a lot about the consequences of a system where protons are transferred, but the question isn't "what if protons were easily transferred?" It is "why aren't protons easily transferred?" Jan 24 at 16:29
• Concerning your second paragraph: Randall Munroe once described this sort of environment as one in which "you would stop being biology and start being physics". Jan 24 at 16:31
• Respectfully, @BlackThorn, the answer is that proton transfer takes more energy than electron transfer. Sometimes when I answer this question I add that the extra energy comes from a nuclear interaction which is stronger than electricity, imaginatively called the “strong nuclear interaction.” But at the level of this question that’s just a name, which doesn’t add much. There are parts of the universe where it’s hot enough for easy proton-transfer reactions, just like there are parts of the universe where it’s too cold for chemistry; we just don’t live there.
– rob
Jan 24 at 17:31
• In an environment with a lot of proton transfers would chemistry even be a meaningful concept? With all the electrons stripped off and atoms routinely changing what type they are what chemistry could exist? And wouldn't such an environment fairly rapidly decay to just hydrogen, anyway? Jan 25 at 4:45

If an atom gained a proton, it would become a different atom. For example, if a hydrogen atom gained a proton, it would become a helium atom (for a sec forget that helium which you find in nature has also 2 neutrons).

Having this in mind, it is perfectly possible to have a process that changes the number of protons, but as a result, we get a different atom (a particle with a different name).

It is also important to note that it is far easier to exchange electrons than protons. This is because we extract an electron by overcoming the Coulomb force, while the proton is bounded by a nuclear force (thus, the processes in which this occurs are called nuclear processes).

• You might want to add that protons are about 2000 times more massive, and thus harder to move, than electrons. Jan 23 at 2:19
• @M.Enns But the mass difference isn’t the reason proton separation costs more energy than electron ionization. This answer (v1) correctly states that the difference is the strong interaction versus the electromagnetic interaction.
– rob
Jan 23 at 15:20

Some artificially made isotopes can emit protons.

• Why the answer got downvote? I can't see anything wrong here. Did the answer get downvote cause of the poster didn’t explain anything? Jan 23 at 4:55
– rob
Jan 23 at 5:13
• @rob : I am afraid I don't understand your comment. For example, according to Physics Today vol. 55, 9, 17 (2002), "Evidence Found for a New Type of Radioactivity: Two-Proton Emission", iron-45 nuclei, manifesting two-proton emission, were creating by striking a beryllium or nickel target with a beam of nickel-58 ions. How is this "adding protons or removing neutrons"? Jan 23 at 5:38
• To make iron-45 from nickel-58, you remove two protons and eleven neutrons. Then the iron-45 emits more protons.
– rob
Jan 23 at 6:12
• @rob : so it is not just "removing neutrons". And iron-45 emits protons a few milliseconds after that - an eternity compared to the collision time. I don't see any "circular reasoning". Jan 23 at 6:44

Yes, atoms (and molecules) can gain or lose protons. It is called "Chemistry of acids".

You mix an acid and water. A proton is exchanged and you now have a negative ion (the acid minus one proton) and a positive ion (a water molecule with one extra proton).

• In this scenario, and atom bound to a positive hydrogen ion has one proton that it “can” lose, and some larger number of protons in its nucleus that it “cannot” lose. All of the protons are identical, but their circumstances are not.
– rob
Jan 23 at 14:33
• What's more, in a mult-proton acid all "acidic" protons are differently hard to lose. Jan 23 at 18:17
• An atom forming a bond with a proton is not "gaining" a proton exactly. Jan 24 at 23:45
• Even though the question asked about atoms losing protons, and this answer by default is talking about molecules losing protons, I think this answer is a valuable addition to the other answers. Proton transfer is the simplest and most ubiquitous chemical reaction, and almost as fundamental as electron transfer in how matter reacts. Proton transfer is also a way to change the electrical charges of the species involved, and that seemed to be something the OP was interested in. Jan 25 at 6:55

Ever wondered why the strong force is called "strong"? It is way stronger then the EM force (and all others at this scale), and it is in your example usually way easier to overcome the EM force then the strong.

OK, the quarks constituting the protons do possess EM charge, and confinement is very complicated and the EM force does contribute to the whole proton as a bound quantum object, but what we can really say is that the proton (its constituents) is bound by the strong force. Just like the protons and neutrons are bound by the residual strong (nuclear) force. In this case, the EM force (the charge of the protons) does contribute to the whole picture of the nucleus being a bound QM object, but the nuclear force is what overwhelmingly dominates.

Simply said, it is much easier to tear off (remove) an electron in your example from an atom (and overcome the EM force), them to remove a proton (and overcome the strong force).

That being said, it is not impossible as you can see form other answers (by @rob and @redgiant), to like in your example to remove or add a proton to an atom, it is just that we happen to live in a universe where QM is the underlying theory and energetically favorable QM processes occur more likely.

• I find the wording of the answer confusing. Your last paragraph seems to suggest that quantum mechanics is dominant over electromagnetism, which I don’t think is what you intended to say.
– rob
Jan 25 at 4:19
• @rob thank you I edited. Jan 25 at 4:55

They can and do all the time. The energy either required or released is considerable so this results in far fewer instances in which the requirements are met. The sun uses quantum tunneling to do this as the gravity and heat are insufficient alone.

The electrons are bound to the nucleus by the electromagnetic force, while a proton is bound to the nucleus by the strong nuclear force (at least, in a multi-nucleon atom; a nucleus could consist of just a single proton, in which there isn't really a difference between the proton losing the electron and the electron losing the proton). The strong nuclear force is, as the name implies, stronger than the electromagnetic force. Moreover, the range is much smaller; protons are right in the nucleus, while electrons are a distance away. So it takes much more energy to remove a proton from an atom. If a molecule contains $$^1$$H, then it can "lose a proton" by ejecting the hydrogen atom but keeping the electron, which takes much less energy than a nucleus losing a proton.

Another regular way in which an atom can "lose a proton" is when an inner electron is captured by the proton-rich nucleus of a naturally-occurring radioactive isotope, such as potassium-40 converting to argon-40.

p + e → n + ν

(The earlier answer from @akhmeteli referred only to artificial isotopes.)

This also provides a partial answer to Why don't electrons crash into the nuclei they "orbit"? where electron capture is also mentioned.

Electrones are in motion, orbiting around the nucleus where the proutons are situated. Therefore, it is comparatively easy to add or remove electrones to the atom thereby ionising it.