Physics: 1906

Thomson, Sir Joseph John (1856-1940), a British physicist, received the 1906 Nobel Prize in physics for his discovery of the electron. In 1937, his son and pupil, Sir George Paget Thomson, shared the Nobel Prize in physics with Clinton Davisson, an American physicist.

Thomson began in 1895 to investigate the mysterious rays that occurred when electricity was passed through a vacuum in a glass tube. Because the rays seemed to come from the cathode (negative electrical pole in the tube), they were called cathode rays. No one had succeeded in deflecting them by an electric force. Some scientists therefore assumed that cathode rays were like light waves. Thomson believed that they were really tiny particles of matter.

He built a special cathode-ray tube in which the rays passed through electric and magnetic fields that were perpendicular to each other. The rays became visible as a dot on the opposite end of the tube. By measuring the deflections of the dot as he changed the strength of the electric and magnetic fields, Thomson determined the ratio of the charge to the mass of the particle (symbolized as e/m). From the direction of their deflection, he decided that the particles were negatively charged. Because their e/m was always the same, he felt sure that they were a fundamental part of all atoms. These particles were later called electrons.

Thomson was also the first to separate isotopes of the chemical elements. This accomplishment spurred the invention of the mass spectrograph by his assistant, Francis W. Aston.

Thomson was born near Manchester and was educated at Manchester and Cambridge. Thomson's experimental work was invaluable to physics. His theoretical model of the atom, however, became obsolete after new models were proposed by Ernest Rutherford in 1911 and Niels Bohr in 1913.

About Electron

Electron is a negatively charged subatomic particle. A useful model of an atom portrays it as a tiny nucleus surrounded by electrons. The electrons are at various distances from the nucleus and are arranged in energy levels called shells. Electrons occupy almost the entire volume of an atom, but electrons themselves account for only a small fraction of an atom's mass. The chemical behavior of an atom is determined largely by the number of electrons in its outermost shell. When atoms combine and form molecules, electrons in the outermost shell are either transferred from one atom to another or shared between atoms.

Ordinarily, an atom has an equal number of electrons and protons, positively charged particles found in the nucleus. Each electron carries one unit of negative charge, and each proton carries one unit of positive charge. As a result, the atom is electrically neutral. If an atom gains electrons, it becomes negatively charged. If it loses electrons, it becomes positively charged. Electrically charged atoms are called ions.

Electrons are fundamental units of matter--that is, they are not made up of smaller units. The diameter of an electron is less than 1/1,000 the diameter of a proton. The mass of an electron in grams may be written with a decimal point followed by 27 zeros and a 9. Electrons are the lightest particles that have an electric charge.

The discovery of the electron is generally attributed to Sir Joseph John Thomson, a British physicist who identified it in 1897. In 1913, the American physicist Robert A. Millikan reported an accurate measurement of the electron's charge.

Proton, pronounced PROH ton, is a positively charged subatomic particle. A single proton constitutes the nucleus of an ordinary hydrogen atom. Protons, together with other subatomic particles called neutrons, make up the nuclei of all other atoms. All atoms of the same chemical element have the same number of protons. The number of protons in the atoms is called the atomic number of the element.

Ordinarily, an atom has an equal number of protons and electrons, negatively charged particles that surround the nucleus. Each proton carries one unit of positive charge, and each electron carries one unit of negative charge. As a result, the atom is electrically neutral.

Protons are made up of fundamental particles called quarks. A proton has a diameter of approximately one millionth of a nanometer. One nanometer equals one millionth of a millimeter (1/25,400,000 inch). The mass of a proton in grams may be written with a decimal point followed by 23 zeros and a 2.

The proton was first identified by the German physicist Wilhelm Wien in 1902. The British physicist Sir Joseph J. Thomson proved the identity of the proton in 1906.

Neutron is a subatomic particle. Neutrons, together with subatomic particles called protons, form the nuclei of all atoms except ordinary hydrogen, whose nucleus consists of a single proton (see PROTON). Neutrons and protons make up 99.9 percent of an atom's mass. A cloud of electrons around the nucleus accounts for the rest of the mass. In the nucleus, neutrons and protons are held together by a force known as the strong interaction or the strong nuclear force.

The number of neutrons in an atom of any chemical element is equal to the difference between the element's mass number (total number of protons and neutrons) and its atomic number (number of protons). The atoms of lighter elements contain about an equal number of neutrons and protons. Heavier elements contain more neutrons than protons.

Neutrons consist of fundamental particles called quarks. A neutron has no electric charge. Its diameter is approximately one millionth of a nanometer. One nanometer equals one millionth of a millimeter, or 1/25,400,000 inch. The mass of a neutron is slightly greater than that of a proton. A free neutron decays into a proton, an electron, and an antineutrino. Free neutrons have an average life span of about 15 minutes.

Sir James Chadwick, a British physicist, discovered the neutron in 1932. Today, scientists use neutrons to make various elements radioactive. They bombard atoms of the elements with neutrons in a nuclear reactor. After the nuclei of the atoms absorb neutrons, they decay by giving off radiation. When a nucleus of the uranium isotope U-235 is struck by a neutron, it becomes unstable and splits into two nearly equal parts. This process, called fission, releases a huge amount of energy and frees additional neutrons that cause more uranium nuclei to fission. A continuous series of such fissions, called a nuclear chain reaction, produces the energy in nuclear weapons and reactors.

Quark, pronounced kwawrk, is one of the three families of particles that serve as "building blocks" of matter. The other two families are the leptons and the fundamental, or gauge, bosons. Quarks are elementary particles--that is, they have no known smaller parts.

There are six types of quarks, each of which carries a fraction of an electric charge. Three of the quarks, called down (or d), strange (or s), and bottom (or b), have 1/3 unit of negative charge. The other three--the up (or u), charm (or c), and top (or t)--have 2/3 unit of positive charge.

A quark is always combined with one or two other quarks. Composite particles made up of quarks are known as hadrons. These include protons and neutrons, which form the nuclei of atoms.

There are two kinds of hadrons--(1) baryons and (2) mesons. A baryon is a three-quark combination. A proton is a baryon consisting of two u quarks and one d, while a neutron is a baryon made up of two d's and one u. A meson is made up of a quark and an antiquark. Antiquarks are the antimatter equivalents of quarks, opposite in electric charge and certain other properties.

Quarks have no measurable size. Physicists describe them as "pointlike." The t quark is the heaviest known elementary particle. Its mass is about 190 atomic mass units. This is almost as heavy as an entire atom of gold. The lightest quark, the u, has about 35,000 times less mass than the t.

The s, c, b, and t quarks are much heavier than the u and d. All the heavy quarks are unstable and they do not exist in ordinary matter. They usually break down into u's, d's, and other lighter particles in less than a billionth of a second. Physicists must create s, c, b, and t quarks with devices called particle accelerators. An accelerator causes subatomic particles to collide violently with one another to produce these quarks.

Two California Institute of Technology physicists, the American Murray Gell-Mann and Russian-born George Zweig, independently proposed the first theory of quarks in 1964. The original theory required only u, d, and s quarks to build all known hadrons. In the late 1960's and early 1970's, experiments showed that protons and neutrons contain parts much smaller than they are, and that these parts carry fractional charges. Discoveries in 1974, 1977, and 1995 proved the existence of the c, b, and t, in that order.