Proton

A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 e (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an electron (the proton-to-electron mass ratio). Protons and neutrons, each with a mass of approximately one dalton, are jointly referred to as nucleons (particles present in atomic nuclei).

One or more protons are present in the nucleus of every atom. They provide the attractive electrostatic central force which binds the atomic electrons. The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number (represented by the symbol Z). Since each element is identified by the number of protons in its nucleus, each element has its own atomic number, which determines the number of atomic electrons and consequently the identity and chemical characteristics of the element.

The word proton is Greek for "first", and the name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by atomic collisions.

At sufficiently low temperatures and kinetic energies, free protons will bind electrons in any matter they traverse.

Free protons are routinely used for accelerators for proton therapy or various particle physics experiments, with the most powerful example being the Large Hadron Collider.

Description

Protons are spin- fermions and are composed of three valence quarks,

Following the discovery of the atomic nucleus by Ernest Rutherford in 1913, Antonius van den Broek proposed that the place of each element in the periodic table (its atomic number) is equal to its nuclear charge. Van den Broek speculated that the nucleus contained alpha particles with four positive charges and two electrons, the first version of the nuclear-electron hypothesis. (The modern model of two positive protons and two neutrons would take many years to discover). By 1920 he concluded that these hydrogen nuclei were a constituent part of the nitrogen nucleus. This result has been described as the discovery of protons.

Rutherford initially assumed that the alpha particle merely knocked a proton out of nitrogen, turning it into carbon. Patrick Blackett's cloud chamber images in 1925 demonstrated that the alpha particle was absorbed. If the alpha particle were not absorbed, then 3 charged particles, a negatively charged carbon, a proton, and an alpha particle, would be expected. The 3 charged particles would create three tracks in the cloud chamber, but only 2 tracks in the cloud chamber were observed. Blackett proposed that the alpha particle is absorbed by the nitrogen atom. Heavy oxygen (17O), not carbon, was the product. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate through the interstellar medium. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay. Protons also result (along with electrons and antineutrinos) from the radioactive decay of free neutrons, which are unstable.

Stability

The spontaneous decay of free protons has never been observed, and protons are therefore considered stable particles according to the Standard Model. However, some grand unified theories (GUTs) of particle physics predict that proton decay should take place with lifetimes between 1031 and 1036 years. The experimental lower bound for the mean lifetime is .

The mean lifetime measures decay to any product. Lifetimes for decay to specific products is also measured. For example, experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of for decay to an antimuon and a neutral pion, and for decay to a positron and a neutral pion.

Quarks and the mass of a proton

In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of protons and neutrons is explained by special relativity. The mass of a proton is about 80–100 times greater than the sum of the rest masses of its three valence quarks, while the gluons have zero rest mass. The extra energy of the quarks and gluons in a proton, as compared to the rest energy of the quarks alone in the QCD vacuum, accounts for almost 99% of the proton's mass. The rest mass of a proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles confined to a system is still measured as part of the rest mass of the system.

Two terms are used in referring to the mass of the quarks that make up protons: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark.

The constituent quark model wavefunction for the proton is

<math display="block">\mathrm{|p_\uparrow\rangle = \tfrac{1}{\sqrt {18 \left(2| u_\uparrow d_\downarrow u_\uparrow \rangle + 2| u_\uparrow u_\uparrow d_\downarrow \rangle + 2| d_\downarrow u_\uparrow u_\uparrow \rangle - | u_\uparrow u_\downarrow d_\uparrow\rangle -| u_\uparrow d_\uparrow u_\downarrow\rangle - | u_\downarrow d_\uparrow u_\uparrow\rangle

- | d_\uparrow u_\downarrow u_\uparrow\rangle - |d_\uparrow u_\uparrow u_\downarrow\rangle-| u_\downarrow u_\uparrow d_\uparrow\rangle\right)}.</math>

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. The most recent calculations

A third kind of high precision measurement agrees most closely with the value given by the muonic hydrogen spectroscopy but unexplained differences remain. The exact nature of what these measurement mean has also been questioned.

Pressure inside the proton

Since the proton is composed of quarks confined by gluons, an equivalent pressure that acts on the quarks can be defined. The size of that pressure and other details about it are controversial.

In 2018 this pressure was reported to be on the order 1035 Pa, which is greater than the pressure inside a neutron star. It was said to be maximum at the centre, positive (repulsive) to a radial distance of about 0.6 fm, negative (attractive) at greater distances, and very weak beyond about 2 fm. These numbers were derived by a combination of a theoretical model and experimental

Compton scattering of high-energy electrons. However, these results have been challenged as also being consistent with zero pressure and as effectively providing the pressure profile shape by selection of the model.

Charge radius in solvated proton, hydronium

The radius of the hydrated proton appears in the Born equation for calculating the hydration enthalpy of hydronium.

Interaction of free protons with ordinary matter

Although protons have affinity for oppositely charged electrons, this is a relatively low-energy interaction and so free protons must lose sufficient velocity (and kinetic energy) in order to become closely associated and bound to electrons. High energy protons, in traversing ordinary matter, lose energy by collisions with atomic nuclei, and by ionization of atoms (removing electrons) until they are slowed sufficiently to be captured by the electron cloud in a normal atom.

However, in such an association with an electron, the character of the bound proton is not changed, and it remains a proton. The attraction of low-energy free protons to any electrons present in normal matter (such as the electrons in normal atoms) causes free protons to stop and to form a new chemical bond with an atom. Such a bond happens at any sufficiently "cold" temperature (that is, comparable to temperatures at the surface of the Sun) and with any type of atom. Thus, in interaction with any type of normal (non-plasma) matter, low-velocity free protons do not remain free but are attracted to electrons in any atom or molecule with which they come into contact, causing the proton and molecule to combine. Such molecules are then said to be "protonated", and chemically they are simply compounds of hydrogen, often positively charged. Often, as a result, they become so-called Brønsted acids. For example, a proton captured by a water molecule in water becomes hydronium, the aqueous cation .

Proton in chemistry

Atomic number

In chemistry, the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl− anion has 17 protons and 18 electrons for a total charge of .

All atoms of a given element are not necessarily identical, however. The number of neutrons may vary to form different isotopes, and energy levels may differ, resulting in different nuclear isomers. For example, there are two stable isotopes of chlorine: with 35 − 17 = 18 neutrons and with 37 − 17 = 20 neutrons.

Hydrogen ion

thumb|220px|Protium, the most common isotope of hydrogen, consists of one proton and one electron (it has no neutrons). The term hydrogen ion () implies that that H-atom has lost its one electron, causing only a proton to remain. Thus, in chemistry, the terms proton and hydrogen ion (for the protium isotope) are used synonymously.

In chemistry, the term proton refers to the hydrogen ion, . Since the atomic number of hydrogen is 1, a hydrogen ion has no electrons and corresponds to a bare nucleus, consisting of a proton (and 0 neutrons for the most abundant isotope protium ). The proton is a "bare charge" with only about 1/64,000 of the radius of a hydrogen atom, and so is extremely reactive chemically. 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. In aqueous solution, it forms the hydronium ion, H3O+, which in turn is further solvated by water molecules in clusters such as [H5O2]+ and [H9O4]+.

References

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External links

Particle Data Group at LBL
Large Hadron Collider
MIT proton visualization project:
Inside the Proton, the 'Most Complicated Thing You Could Possibly Imagine', Quanta Magazine, Oct 19 2022
Visualizing the Proton, Arts at MIT, 2022