values. Finally, de Broglie gave a direct physical way of interpreting this condition. He assumed that an electron orbit would be stable (allowed) only if it contained an integral number of electron wavelengths. Although the analysis provided by de Broglie was a promising first step, gigantic strides were made subsequently with the development of Schrodinger's wave equation and its application to atomic systems.
Quantum mechanics predicts that the wave function for the hydrogen atom in the ground state is spherically symmetric; hence the electron can be found in a spherical region surrounding the nucleus. This is in contrast to the Bohr theory, which confines the position of the electron to points in a plane. The quantum mechanical result is often interpreted by viewing the electron as a cloud surrounding the nucleus.
The exclusion principle
How many electrons in an atom can have a particular set of quantum numbers? This important question was answered by Pauli in 1925 in a powerful statement known as the Pauli exclusion principle: No two electrons in an atom can ever have the same set of values for the set of quantum numbers.
The Pauli exclusion principle explains the electronic structure of complex atoms as a succession of filled levels with different quantum numbers increasing in energy, where the outermost electrons are primarily responsible for the chemical properties of the element. If this principle weren't valid, every electron would end up in the lowest energy state of the atom and the chemical behavior of the elements would be grossly different.
Atomic transitions
An atom will emit radiation only at certain frequencies that correspond to the energy separation between the various allowed states. At ordinary temperatures, most of the atoms in a sample are in the ground state. If a vessel containing many atoms of a gas is illuminated with a light beam containing all possible photon frequencies (that is, a continuous spectrum), only those photons of energies, and so on, can be absorbed. As a result of this absorption, some atoms are raised to various allowed higher energy levels, called excited states.
An incident photon can cause atomic transitions either upward (stimulated absorption) or downward (stimulated emission). The two processes are equally probable. When light is incident on a system of atoms, there is usually a net absorption of energy, because when the system is in thermal equilibrium, there are many more atoms in the ground state
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than in excited states. However, if the situation can be inverted so that there are more atoms in an excited state than in the ground state, a net emission of photons can result. Such a condition is called population inversion. This is the fundamental principle involved in the operation of a laser. One interesting application of the laser is holography: the production of three-dimensional images of objects.
Energy bands in solids
In solids, the discrete levels of isolated atoms broaden into allowed energy bands separated by forbidden gaps. The separation and electron population of the highest bands determines whether a given solid is a conductor, an insulator, or a semiconductor. An electron can have any energy within an allowed energy band, but cannot have an energy in the band gap, or the region between allowed bands. The highest filled band is called the valence band and the next higher empty band is called the conduction band. In quantum terms, electron energies increase if there are higher unoccupied energy levels for electrons to jump to.
A conductor has a highest-energy occupied band which is partially filled, and in an insulator, has a highest-energy occupied band which is completely filled with a large energy gap between the valence and conduction bands. Semiconductor is a material with a small band gap of about 1eV whose conductivity results from appreciable thermal excitation of electrons across the gap into the conduction band at room temperature. The process of adding impurities, called doping, is important in making devices having well-defined regions of different resistivity.
The invention of the transistor by John Bardeen (1908-1991), Walter Brattain (1902-1987), and William Shockley (1910-1989) in 1948 totally revolutionized the world of electronics. For this work, these three men shared a Nobel prize in 1956. By 1960, the transistor had replaced the vacuum tube in many electronic applications. One simple form of the transistor, called the junction transistor, consists of a semiconducting material in which a very narrow n region is sandwiched between two p regions. This configuration is called a pnp transistor. Another configuration is the npn transistor. The outer regions are called the emitter and collector, and the narrow central region is called the base.
Invented independently by Jack Kilby at Texas Instruments in late 1958 and by Robert Noyce at Fairchild Camera and Instrument in early 1959, the integrated circuit has been justly called "the most remarkable technology ever to hit mankind." In simplest terms, an integrated circuit is
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a collection of interconnected transistors, diodes, resistors, and capacitors fabricated on a single piece of silicon known as a chip.
1.What experiment showed that Thomson’s model is not right?
2.Why the emission spectrum is reliable technique for identifying elements?
3.What does quantum mechanics predict about electron and nucleus?
4.What’s the Pauli exclusion principle consist from?
5.What equally probable processes an incident photon can cause?
6.Why the integrated circuit has been justly called "the most remarkable technology ever to hit mankind"?
Exercises
1. Rearrange the letters in the anagrams to form equivalents for the Russian words:
Ядро |
cleusnu |
Подоболочка |
shellsub |
Расщепление |
tingsplit |
Легирование полупроводника |
pingdo |
Эмиттер |
teremit |
2. Match the words in A with the words in B to form word combinations:
A |
B |
Ground |
gap |
Fine |
transistor |
Population |
inversion |
Band |
circuit |
Integrated |
structure |
Junction |
state |
3. Fill in the gaps with the missing words from the list:
solar spectrum, nucleus, quantum numbers, energetic state, integrated circuits
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1.In order to explain why electrons in this outer region of the atom were not pulled into the _____, Rutherford viewed them as moving in orbits about the positively charged nucleus in the same way that planets orbit the Sun.
2.The various absorption lines observed in the _____ have been used to identify elements in the solar atmosphere, including one that was previously unknown.
3.Radiation is emitted by the hydrogen atom when the electron "jumps" from a more energetic initial state to a less _____.
4.All these _____ were postulated to account for the observed spectra of elements.
5._____s were invented partly to solve the interconnection problem spawned by the transistor.
UNIT 29
NUCLEAR PHYSICS
Read the following texts. Study the thesaurus on fig. 29, answer the questions after the texts
In 1896, the year that marks the birth of nuclear physics, Henri Becquerel (1852-1908) discovered radioactivity in uranium compounds. A great deal of activity followed this discovery as researchers attempted to understand and characterize the radiation that we now know to be emitted by radioactive nuclei. Pioneering work by Rutherford showed that the radiation was of three types, which he called alpha, beta, and gamma rays. These types are classified according to the nature of their electric charge and their ability to penetrate matter. Later experiments showed that alpha rays are helium nuclei, beta rays are electrons, and gamma rays are highenergy photons.
In 1911 Rutherford and his students Geiger and Marsden performed a number of important scattering experiments involving alpha particles. These experiments established the idea that the nucleus of an atom can be regarded as essentially a point mass and point charge and that most of the atomic mass is contained in the nucleus. Further, such studies demonstrated a wholly new type of force: the nuclear force, which is predominant at distances of less than about 10~14 m and drops quickly to zero at greater distances.
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Other milestones in the development of nuclear physics include:
-the first observations of nuclear reactions by Rutherford and coworkers in 1919, in which naturally occurring α particles bombarded nitrogen nuclei to produce oxygen,
-the first use of artificially accelerated protons to produce nuclear reactions, by Cockcroft and Walton in 1932,
-the discovery of the neutron by Chadwick in 1932,
-the discovery of artificial radioactivity by Joliot and Irene Curie in 1933,
-the discovery of nuclear fission by Hahn, Strassman, Meitner, and Frisch in 1938,
-the development of the first controlled fission reactor by Fermi and his collaborators in 1942.
Some properties of nuclei
All nuclei are composed of two types of particles: protons and neutrons. The only exception is the ordinary hydrogen nucleus, which is a single proton. In describing some of the properties of nuclei, such as their charge, mass, and radius, we make use of the following quantities:
-the atomic number Z, which equals the number of protons in the nucleus,
-the neutron number N, which equals the number of neutrons in the nucleus,
-the mass number A, which equals the number of nucleons in the nucleus (nucleon is a generic term used to refer to either a proton or a neutron).
The nuclei of all atoms of a particular element must contain the same number of protons, but they may contain different numbers of neutrons. Nuclei that are related in this way are called isotopes. The isotopes of an element have the same Z value, but different N and A values.
The size and structure of nuclei were first investigated in the scattering experiments of Rutherford. Using the principle of conservation of energy, Rutherford found an expression for how close an alpha particle moving directly toward the nucleus can come to the nucleus before being turned around by Coulomb repulsion. Rutherford concluded that the
positive charge in an atom is concentrated in a small sphere, which he called the nucleus, with radius no greater than about 10-14m. Because such small lengths are common in nuclear physics, a convenient unit of length is the femtometer (fm), sometimes called the fermi.
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