Protons

FIELDS OF STUDY: Nuclear Chemistry; Inorganic Chemistry; Organic Chemistry

ABSTRACT

The discovery and basic properties of protons are described. Protons are found in all atoms and provide half of the mass for about the first twenty elements. Each proton bears a positive charge that must be countered by the presence of neutrons in the nucleus.

The Identification of Protons

Various models of atomic structure were proposed throughout the nineteenth and twentieth centuries, the most useful being the "planetary" model and the "plum pudding" model. In the first, an atom was envisioned to be composed of a nucleus with electrical charges whirling about it like the planets around the sun. In the latter model, the atom was envisioned to be a round, positively charged blob with dots of negative electrical charge embedded throughout. In 1897, while conducting research on the nature of cathode rays in cathode ray tubes, J. J. Thomson (1856–1940) discovered that the cathode rays behaved more like streams of charged particles than like electromagnetic radiation. He called the particles "corpuscles" at first, though they soon became known as electrons. Based on this discovery, Thomson proposed the plum-pudding model of the atom in 1904.

Thomson’s discoveries verified that atoms had an internal structure consisting of fundamental particles, but the existence of subatomic particles other than electrons had yet to be proved. In 1909, Ernest Rutherford (1871–1937), Hans Geiger (1882–1945), and Ernest Marsden (1889–1970) performed what is known as the gold-foil experiment, which produced more conclusive results. By directing a beam of α (alpha) particles at a very thin target of gold foil, they found that while most of the particles passed directly through the foil as though it were not there, some of the particles were deflected at various angles, as though they had bounced off something very small and very dense inside the metal. These observations led Rutherford to propose the planetary model of atomic structure, with the majority of the atom’s mass concentrated in a small, dense nucleus. He continued experimenting with alpha particles, and in 1917 he discovered that the collision of an alpha particle with the nucleus of a nitrogen atom released a positively charged particle identical to a hydrogen nucleus—in other words, a proton, as the most common isotope of hydrogen contains no neutrons. This breakthrough is widely credited as the discovery of the proton.

Properties and Function of Protons

The elemental identity of every atom is determined by the number of protons in its nucleus, which is also the element’s atomic number. With the exception of hydrogen, the numbers of protons and neutrons in the nuclei of the first twenty elements are approximately equal (allowing for some variation in different isotopes), which means that protons account for about half of the mass of the atoms of these first twenty elements. Hydrogen is a special case because its most common form by far is protium, which has one proton and no neutrons in its nucleus. Protium is what is typically referred to when discussing hydrogen; the isotope with one proton and one neutron is called deuterium, and it is significantly less abundant. If a protium atom loses its sole electron, it forms the positively charged ion H⁺, which has no electrons or neutrons and thus is essentially a proton.

After element 20, which is calcium, the protons of each element are increasingly outnumbered by the neutrons. For example, zinc (atomic number 30) typically has 35 neutrons and 30 protons, while uranium (92) has 146 neutrons but only 92 protons. Each proton bears a single positive electrical charge, and the protons in a given atom must be countered by an equal number of electrons for that atom to be neutral. The neutrons in the nucleus interact more closely with the protons and stabilize them against the force of electrostatic repulsion that would otherwise drive them apart. Electropositive elements are those that tend to give up one or more valence electrons in order to form positively charged ions that are more stable than their neutral forms. The positive charge on such an ion results from the fact that the protons are no longer balanced by an equal number of electrons.

Protons in Chemical Reactions

Protons are most important in normal chemical reactions when those reactions involve the protium atom. Because the hydrogen atom has only a single electron, that electron’s involvement in bond formation tends to leave the positive charge of the hydrogen nucleus exposed. If hydrogen forms a compound with an element of high electron density, such as oxygen, nitrogen, or fluorine, this creates a polar molecule, in which the positive charge is concentrated at the hydrogen end of the molecule and the negative charge is concentrated at the other end. The polar molecules are then attracted to each other, with the electron-dense end of one molecule forming a hydrogen bond with the hydrogen atom of another molecule. (A hydrogen bond is not a true chemical bond, but rather a strong electrical attraction between molecules.) This fact is the reason that water, a highly polar molecule, has such high melting and boiling points relative to other compounds of similar molecular weight. Compounds that are able to release the H⁺ ion—that is, the proton—from their molecular structure when dissolved in water or another polar solvent are defined as acids. Acids are valuable active components of reaction processes as well as indispensable analytical tools.

PRINCIPAL TERMS

• atomic number: the number of protons in the nucleus of an atom, used to uniquely identify each element.
• electropositive: describes an atom that tends to lose electrons to form a positively charged ion.
• fundamental particle: one of the smaller, indivisible particles that make up a larger, composite particle; commonly used to refer to electrons, protons, and neutrons, although these are themselves composed of various actual fundamental particles, such as quarks, leptons, and certain types of bosons.
• nucleus: the central core of an atom, consisting of specific numbers of protons and neutrons and accounting for at least 99.98 percent of the atomic mass.
• protium: the essential form of hydrogen, containing one proton and no neutron; the most common form of matter in the known universe.

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