[10] Protons were therefore a candidate to be a fundamental or elementary particle, and hence a building block of nitrogen and all other heavier atomic nuclei.

Wilhelm Wien in 1898 identified the hydrogen ion as the particle with the highest charge-to-mass ratio in ionized gases.

In 1917, Rutherford performed experiments (reported in 1919 and 1925) which proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of protons.

[24] These experiments began after Rutherford observed that when alpha particles would strike air, Rutherford could detect scintillation on a zinc sulfide screen produced at a distance well beyond the distance of alpha-particle range of travel but instead corresponding to the range of travel of hydrogen atoms (protons).

After observing Blackett's cloud chamber images in 1925, Rutherford realized that the alpha particle was absorbed.

Depending on one's perspective, either 1919 (when it was seen experimentally as derived from another source than hydrogen) or 1920 (when it was recognized and proposed as an elementary particle) may be regarded as the moment when the proton was 'discovered'.

He named this new fundamental building block of the nucleus the proton, after the neuter singular of the Greek word for "first", πρῶτον.

Rutherford spoke at the British Association for the Advancement of Science at its Cardiff meeting beginning 24 August 1920.

[28] Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle".

Free protons are found naturally in a number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity.

[32] Free protons of high energy and velocity make up 90% of cosmic rays, which propagate through the interstellar medium.

However, some grand unified theories (GUTs) of particle physics predict that proton decay should take place with lifetimes between 1031 and 1036 years.

Experimental searches have established lower bounds on the mean lifetime of a proton for various assumed decay products.

This experiment was designed to detect decay to any product, and established a lower limit to a proton lifetime of 2.1×1029 years.

[41][42][43][44] In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of protons and neutrons is explained by special relativity.

The internal dynamics of protons are complicated, because they are determined by the quarks' exchanging gluons, and interacting with various vacuum condensates.

Lattice QCD provides a way of calculating the mass of a proton directly from the theory to any accuracy, in principle.

These recent calculations are performed by massive supercomputers, and, as noted by Boffi and Pasquini: "a detailed description of the nucleon structure is still missing because ... long-distance behavior requires a nonperturbative and/or numerical treatment ..."[51] More conceptual approaches to the structure of protons are: the topological soliton approach originally due to Tony Skyrme and the more accurate AdS/QCD approach that extends it to include a string theory of gluons,[52] various QCD-inspired models like the bag model and the constituent quark model, which were popular in the 1980s, and the SVZ sum rules, which allow for rough approximate mass calculations.

As a muon is 200 times heavier than an electron, resulting in a smaller atomic orbital, it is much more sensitive to the proton's charge radius and thus allows a more precise measurement.

As a consequence it has no independent existence in the condensed state and is invariably found bound by a pair of electrons to another atom.

The free proton, thus, has an extremely short lifetime in chemical systems such as liquids and it reacts immediately with the electron cloud of any available molecule.

Also in chemistry, the term proton NMR refers to the observation of hydrogen-1 nuclei in (mostly organic) molecules by nuclear magnetic resonance.

The Apollo Lunar Surface Experiments Packages (ALSEP) determined that more than 95% of the particles in the solar wind are electrons and protons, in approximately equal numbers.

For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable.

For the remainder of each lunar orbit, the Moon is in a transitional region known as the magnetosheath, where the Earth's magnetic field affects the solar wind, but does not completely exclude it.

Research has been performed on the dose-rate effects of protons, as typically found in space travel, on human health.

[67] There are many more studies that pertain to space travel, including galactic cosmic rays and their possible health effects, and solar proton event exposure.

The American Biostack and Soviet Biorack space travel experiments have demonstrated the severity of molecular damage induced by heavy ions on microorganisms including Artemia cysts.

[68] CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests.

[69] The magnetic moment of antiprotons has been measured with an error of 8×10−3 nuclear Bohr magnetons, and is found to be equal and opposite to that of a proton.