Radiophysics. Халюшева Г.Р. - 36 стр.

         9 Unit 9

         9.1 Text 9
                                                        A few words about atoms

         Operation of all electronic devices involves methods of producing and controlling a flow of electrons. Therefore,
we shall first learn what electrons are, how they are made available, and some laws that govern their action under different
conditions.
         Elements. All matter is composed of one or more than a hundred elements found in nature, such as hydrogen,
oxygen, nitrogen, carbon, lead, iron, silver, gold. An element is a substance that cannot be separated chemically into
anything but itself. For example, hydrogen cannot be separated into anything but hydrogen no matter how small it is
divided, nor can gold be changed chemically into anything but gold.
      An atom is the smallest division that can be made of any element. Smallest and
simplest is the hydrogen atom, while at the other end of the scale of elements is the
uranium atom. It is the heaviest and most complex known, neglecting neptunium,
plutonium, americium and curium, created artificially.
          Up to about 80 years ago we believed the atom to be the final division of matter. Then evidence began to
accumulate that possibly it was composed of particles. Today through almost countless experiments by scientists we know
this is so. Some 12 particles have been found in the atom. Of these, the three that we are most interested in are the
following:
          1. The electron, smallest subdivision of electricity, has a negative charge.
          2. The proton, an elementary particle, has a positive electrical charge equal to the negative charge on the electron,
but it weighs 1,836 times as much.
          3. A neutron weighs about the same as a proton but is without an electrical charge; therefore it is electrically
neutral.
          Hydrogen Atom. This atom has a proton that serves as a nucleus
          about which a single electron rotates. Here we have a solar system consisting of one sun, a single proton with a
positive electrical charge about which revolves a single planet – an electron with a negative electrical charge.
          Atoms and electrons are so small that they cannot be seen under the most powerful microscopes. The hydrogen
atom, smallest and lightest, has a diameter of about four billionths of an inch. But the electron's diameter is only about one–
hundred–thousandth part of the atom's diameter. Compared with their dimensions, space is very great between electrons and
between them and the nucleus about which they revolve. According to Rutherford's conception, if we imagine the electron
enlarged to 30 feet diameter, it would be 300 miles from its proton, which would be the size of a pea.
          At ordinary temperatures the electron of the hydrogen atom has a velocity of about 75 miles per second in the orbit
around its proton. The proton with its plus charge attracts the electron with its equal, negative charge so that they tend to fall
together just as the earth tends to fall into the sun from gravitational pull. Because of their velocity, however, both earth and
electron tend to fly out into space but are held in their orbits by the attraction between them and their sun or proton.
          Practically the total mass of the atom is in its nucleus. In the hydrogen atom the mass of the proton, or nucleus, is
1,836 times that of the electron. Because of its comparatively great mass the proton is very dense and, therefore, small
compared with the electron.
          Isotopes. We may better understand helium, the next atom in the electron scale, if we consider the isotope of the
hydrogen atom. Among all atoms, some are isotopes. They are identical with all the other atoms in a given element, but
their atomic weight is different, which is due to the addition of one or more neutrons to the normal number in the nucleus.
For example, the hydrogen atom has a single proton for a nucleus whereas its isotope has a proton and neutron bound
closely together. Because the neutron has practically the same mass as the proton, the atomic weight of the hydrogen isotope
is double that of the hydrogen atom. One in about every 5,000 atoms of hydrogen is an isotope and is known as "heavy
hydrogen." Water containing this isotope is called "heavy water."
          The helium atom, next in the electron scale, has a compact nucleus with two electrons travelling around it in an
orbit.
          The nucleus includes two protons with a positive charge equal to that of the two electrons. The neutrons have no
charge but increase the atomic weight of the atom by an amount equal to their mass. A helium atom may be considered as
two hydrogen isotopes compressed into a single atom; consequently, it has double the atomic weight.
          The conception of the structure of the atom here presented is according to the Bohr theory that assumes the
electrons to be negatively charged particles moving in definite orbits. This theory has been discarded, however, for one that
pictures electrons as wavelike disturbances in space about a nucleus. But this does not in any way affect our discussion of
the atom's electron system. The atom as we show it serves our purpose for a practical and simple conception of a complex