THE BOHR ATOM
One of the most perplexing problems for earlytwentieth-century physicists was why the atom emits a discrete pattern of spectral lines. By 1913, when the structure of atoms was reasonably well known, Niels Bohr (1885-1962), a Danish physicist, proposed a theory for the structure of the simplest atom, hydrogen, whose one electron orbits around a proton. He suggested that the electron can occupy only a selected number of prescribed concentric orbits about the nucleus, rather than having an unlimited and unspecified orbital distance. Also, the electron normally resides in the lowest energy orbit, which is the one closest to the nucleus. Orbits representing higher levels of energy are increasingly farther from the nucleus. The diameter of the first orbit corresponds to the normal size of the hydrogen atom, about 10-8 centimeter in diameter.
When the atom absorbs energy, it is said to be excited, and the electron appears in one of the outer orbits, which have successively higher energies than the lowest orbit does. The electron's change (up or down) from one allowed orbit to another is called an electron transition. An atom in a gas may acquire the internal energy necessary to excite an electron by random thermal collisions with other gas atoms, collisions with subatomic particles such as free electrons, or absorption of a photon traveling through the gas.
Of all the photons striking the atom only those possessing an amount of energy equal to the energy difference between a higher energy orbit and the one in which the electron is located will be absorbed and excite the atom. For example, in the hydrogen atom it takes 10.2 electron volts, or 1.63 x 10-11 erg, of energy to raise the electron from its lowest energy orbit to the next higher energy orbit. Photons with energies below 10.2 electron volts will not be absorbed, and consequently the electron will not be excited. Photons with energies in excess of 10.2 electron volts cannot raise the electron to the second orbit, but they may, if they have the right amount of energy, excite the atom by lifting the electron to even higher energy orbits.
How long will an excited atom remain that way? If a gas atom is excited, then in about a hundredmillionth of a second it will rid itself of any energy in excess of that of the lowest energy orbit by emitting the energy in the form of one or more photons. Somewhat like a ball bouncing down a staircase, the electron will arop in succession into one or maybe several lower energy orbits on its way to the lowest energy level, where it can reside indefinitely. With each downward transition a photon of electromagnetic radiation is emitted. This photon represents the energy difference between the two orbits between which the electron makes the transition. The greater the energy difference, the greater is the amount of energy released in the form of a photon and, consequently, the shorter is the wavelength of the photon. Bohr was led to such a model for the atom as the most straightforward means of accounting for the discrete amounts of energy contained in photons. (Laboratory experiments had already shown that light really possessed the properties of a discrete phenomenon.)
Besides emitting energy spontaneously, an excited atom, before it can emit a photon, may collide with another atom in the gas and transfer energy to it as kinetic energy of motion. In this case no photon will be emitted.
One of the most perplexing problems for earlytwentieth-century physicists was why the atom emits a discrete pattern of spectral lines. By 1913, when the structure of atoms was reasonably well known, Niels Bohr (1885-1962), a Danish physicist, proposed a theory for the structure of the simplest atom, hydrogen, whose one electron orbits around a proton. He suggested that the electron can occupy only a selected number of prescribed concentric orbits about the nucleus, rather than having an unlimited and unspecified orbital distance. Also, the electron normally resides in the lowest energy orbit, which is the one closest to the nucleus. Orbits representing higher levels of energy are increasingly farther from the nucleus. The diameter of the first orbit corresponds to the normal size of the hydrogen atom, about 10-8 centimeter in diameter.
When the atom absorbs energy, it is said to be excited, and the electron appears in one of the outer orbits, which have successively higher energies than the lowest orbit does. The electron's change (up or down) from one allowed orbit to another is called an electron transition. An atom in a gas may acquire the internal energy necessary to excite an electron by random thermal collisions with other gas atoms, collisions with subatomic particles such as free electrons, or absorption of a photon traveling through the gas.
Of all the photons striking the atom only those possessing an amount of energy equal to the energy difference between a higher energy orbit and the one in which the electron is located will be absorbed and excite the atom. For example, in the hydrogen atom it takes 10.2 electron volts, or 1.63 x 10-11 erg, of energy to raise the electron from its lowest energy orbit to the next higher energy orbit. Photons with energies below 10.2 electron volts will not be absorbed, and consequently the electron will not be excited. Photons with energies in excess of 10.2 electron volts cannot raise the electron to the second orbit, but they may, if they have the right amount of energy, excite the atom by lifting the electron to even higher energy orbits.
How long will an excited atom remain that way? If a gas atom is excited, then in about a hundredmillionth of a second it will rid itself of any energy in excess of that of the lowest energy orbit by emitting the energy in the form of one or more photons. Somewhat like a ball bouncing down a staircase, the electron will arop in succession into one or maybe several lower energy orbits on its way to the lowest energy level, where it can reside indefinitely. With each downward transition a photon of electromagnetic radiation is emitted. This photon represents the energy difference between the two orbits between which the electron makes the transition. The greater the energy difference, the greater is the amount of energy released in the form of a photon and, consequently, the shorter is the wavelength of the photon. Bohr was led to such a model for the atom as the most straightforward means of accounting for the discrete amounts of energy contained in photons. (Laboratory experiments had already shown that light really possessed the properties of a discrete phenomenon.)
Besides emitting energy spontaneously, an excited atom, before it can emit a photon, may collide with another atom in the gas and transfer energy to it as kinetic energy of motion. In this case no photon will be emitted.
