Proton

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, while a negative Cl- ion has 17 protons and 18 electrons for a total charge of -1.

All atoms of a given element are not necessarily identical however, as the number of neutrons may vary to form different isotopes. Again for chlorine as an example, there are two stable isotopes - 35Cl with 35 nucleons which are 17 protons and 35-17 = 18 neutrons, and 37Cl with 17 protons and 37-17 = 20 neutrons. Other isotopes of chlorine are radioactive.

[edit] Hydrogen as proton

Since the atomic number of hydrogen is 1, a positive hydrogen ion (H+) has no electrons and corresponds to a bare nucleus with 1 proton (and 0 neutrons for the most abundant isotope 1H). In chemistry therefore, the word "proton" is commonly used as a synonym for hydrogen ion (H+) or hydrogen nucleus in several contexts:

1. The transfer of H+ in an acid-base reaction is referred to "proton transfer". The acid is referred to as a proton donor and the base as a proton acceptor.
2. The hydronium ion (H3O+) in aqueous solution corresponds to a hydrated hydrogen ion. Often the water molecule is ignored and the ion written as simply H+(aq) or just H+, and referred to as a "proton". This is the usual meaning in biochemistry, as in the term proton pump which refers to a protein or enzyme which controls the movement of H+ ions across cell membranes.
3. Proton NMR refers to the observation of hydrogen nuclei in (mostly organic) molecules by nuclear magnetic resonance. This uses the property of the proton to have spin one-half.

[edit] History

Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen , and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle.

See also: William Prout and Prout's hypothesis

Prior to Rutherford, Eugen Goldstein had observed canal rays, which were composed of positively charged ions. After the discovery of the electron by J.J. Thomson, Goldstein suggested that since the atom is electrically neutral there must be a positively charged particle in the atom and tried to discover it. He used the "canal rays" observed to be moving against the electron flow in cathode ray tubes. After the electron had been removed from particles inside the cathode ray tube they became positively charged and moved towards the cathode. Most of the charged particles passed through the cathode, it being perforated, and produced a glow on the glass. At this point, Goldstein believed that he had discovered the proton.[4] When he calculated the ratio of charge to mass of this new particle (which in case of the electron was found to be the same for every gas that was used in the cathode ray tube) was found to be different when the gases used were changed. The reason was simple. What Goldstein assumed to be a proton was actually an ion. He gave up his work there, but promised that "he would return." However, he was widely ignored.

It is named after the neuter singular of the Greek word for "first", πρῶτον.