| 释义 |
atom
The smallest part of an element that can exist. Atoms consist of a small dense nucleus of protons and neutrons surrounded by moving electrons. The number of electrons equals the number of protons so the overall charge is zero. The electrons may be thought of as moving in circular or elliptical orbits (see Bohr theory) or, more accurately, in regions of space around the nucleus (see orbital).
The electronic structure of an atom refers to the way in which the electrons are arranged about the nucleus, and in particular the energy levels that they occupy. Each electron can be characterized by a set of four quantum numbers, as follows: (1) The principal quantum number n gives the main energy level and has values 1, 2, 3, etc. (the higher the number, the further the electron from the nucleus). Traditionally, these levels, or the orbits corresponding to them, are referred to as shells and given letters K, L, M, etc. The K-shell is the one nearest the nucleus.
(2) The orbital quantum number l , which governs the angular momentum of the electron. The possible values of l are (n - 1), (n - 2), …, 1, 0. Thus, in the first shell (n = 1) the electrons can only have angular momentum zero (l = 0). In the second shell (n = 2), the values of l can be 1 or 0, giving rise to two subshells of slightly different energy. In the third shell (n = 3) there are three subshells, with l = 2, 1, or 0. The subshells are denoted by letters s (l = 0), p (l = 1), d (l = 2), f (l = 3). The orbital quantum number is sometimes called the azimuthal quantum number .
(3) The magnetic quantum number m , which governs the energies of electrons in an external magnetic field. This can take values of +l, +(l - 1), …, 1, 0, -1, …, -(l - 1), -l. In an s-subshell (i.e. l = 0) the value of m = 0. In a p-subshell (l = 1), m can have values +1, 0, and -1; i.e. there are three p-orbitals in the p-subshell, usually designated px, py, and pz. Under normal circumstances, these all have the same energy level.
(4) The spin quantum number ms , which gives the spin of the individual electrons and can have the values +1/2 or -1/2.
According to the Pauli exclusion principle, no two electrons in the atom can have the same set of quantum numbers. The numbers define the quantum state of the electron, and explain how the electronic structures of atoms occur. ATOMIC THEORY| c.430 BC | Greek natural philosopher Empedocles (d. c.430 BC) proposes that all matter consists of four elements: earth, air, fire, and water. | | c.400 BC | Greek natural philosopher Democritus of Abdera (c.460–370 BC) proposes that all matter consists of atoms. | | 306 BC | Greek philosopher Epicurus (c.342–270 BC) champions Democritus’ atomic theory. | | 1649 | French philosopher Pierre Gassendi (1592–1655) proposes an atomic theory (having read Epicurus). | | 1803 | John Dalton proposes Dalton's atomic theory. | | 1897 | J. J. Thomson discovers the electron. | | 1904 | J. J. Thomson proposes his ‘plum pudding’ model of the atom, with electrons embedded in a nucleus of positive charges. Japanese physicist Hantaro Nagaoka (1865–1950) proposes a ‘Saturn’ model of the atom with a central nucleus having a ring of many electrons. | | 1911 | Ernest Rutherford discovers the atomic nucleus. | | 1913 | Niels Bohr proposes model of the atom with a central nucleus surrounded by orbiting electrons. British physicist Henry Moseley (1887–1915) equates the positive charge on the nucleus with its atomic number. Frederick Soddy discovers isotopes. | | 1916 | German physicist Arnold Sommerfield (1868–1951) modifies Bohr's model of the atom specifying elliptical orbits for the electrons. | | 1919 | Ernest Rutherford discovers the proton. | | 1920 | Ernest Rutherford postulates the existence of the neutron. | | 1926 | Erwin SchrÖdinger proposes a wave-mechanical model of the atom (with electrons represented as wave trains). | | 1932 | James Chadwick discovers the neutron. Werner Heisenberg proposes a model of the atomic nucleus in which protons and neutrons exchange electrons to achieve stability. | | 1939 | Niels Bohr proposes a ‘liquid drop’ model of the atomic nucleus. | | 1948 | German-born US physicist Marie Goeppert-Meyer (1906–72) and German physicist Hans Jensen (1907–73) independently propose the ‘shell’ structure of the nucleus. | | 1950 | US physicist Leo Rainwater (1917–86) combines the ‘liquid-drop’ and ‘shell’ models of the nucleus into a single theory. | |