P.A. Cox - Inorganic chemistry (793955), страница 2
Текст из файла (страница 2)
The motion ofelectrons changes when chemical bonds are formed, nuclei beingunaltered.Nuclei contain positive protons and uncharged neutrons. The number ofprotons is the atomic number (Z) of an element. The attractive stronginteraction between protons and neutrons is opposed by electrostaticrepulsion between protons. Repulsion dominates as Z increases and thereis only a limited number of stable elements.Isotopes are atoms with the same atomic number but different numbers ofneutrons. Many elements consist naturally of mixtures of isotopes, withvery similar chemical properties.Unstable nuclei decompose by emitting high-energy particles.
Allelements with Z>83 are radioactive. The Earth contains some long-livedradioactive elements and smaller amount of short-lived ones.Actinium and the actinides (I2)Origin and abundance of theelements (J1)Electrons and nucleiThe familiar planetary model of the atom was proposed by Rutherford in 1912 following experiments by Geiger andMarsden showing that nearly all the mass of an atom was concentrated in a positively charged nucleus.
Negativelycharged electrons are attracted to the nucleus by the electrostatic force and were considered by Rutherford to‘orbit’ it in a similar way to the planets round the Sun. It was soon realized that a proper description of atoms requiredthe quantum theory; although the planetary model remains a useful analogy from the macroscopic world, many of thephysical ideas that work for familiar objects must be abandoned or modified at the microscopic atomic level.The lightest atomic nucleus (that of hydrogen) is 1830 times more massive than an electron.
The size of a nucleus isaround 10−15 m (1 fm), a factor of 105 smaller than the apparent size of an atom, as measured by the distances betweenatoms in molecules and solids. Atomic sizes are determined by the radii of the electronic orbits, the electron itselfhaving apparently no size at all. Chemical bonding between atoms alters the motion of electrons, the nuclei remainingunchanged.
Nuclei retain the ‘chemical identity’ of an element, and the occurrence of chemical elements depends onthe existence of stable nuclei.A1–THE NUCLEAR ATOM3Nuclear structureNuclei contain positively charged protons and uncharged neutrons; these two particles with about the same mass areknown as nucleons. The number of protons is the atomic number of an element (Z), and is matched in a neutralatom by the same number of electrons.
The total number of nucleons is the mass number and is sometimes specifiedby a superscript on the symbol of the element. Thus 1H has a nucleus with one proton and no neutrons, 16O has eightprotons and eight neutrons, 208Pb has 82 protons and 126 neutrons.Protons and neutrons are held together by an attractive force of extremely short range, called the stronginteraction.
Opposing this is the longer-range electrostatic repulsion between protons. The balance of the two forcescontrols some important features of nuclear stability.• Whereas lighter nuclei are generally stable with approximately equal numbers of protons and neutrons, heavier oneshave a progressively higher proportion of neutrons (e.g. compare 16O with 208Pb).• As Z increases the electrostatic repulsion comes to dominate, and there is a limit to the number of stable nuclei, allelements beyond Bi (Z=83) being radioactive (see below).As with electrons in atoms, it is necessary to use the quantum theory to account for the details of nuclear structure andstability.
It is favorable to ‘pair’ nucleons so that nuclei with even numbers of either protons or neutrons (or both) aregenerally more stable than ones with odd numbers. The shell model of nuclei, analogous to the orbital picture of atoms(see Topics A2 and A3) also predicts certain magic numbers of protons or neutrons, which give extra stability. Theseare16Oand 208Pb are examples of nuclei with magic numbers of both protons and neutrons.Trends in the stability of nuclei are important not only in determining the number of elements and their isotopes (seebelow) but also in controlling the proportions in which they are made by nuclear reactions in stars. These determine theabundance of elements in the Universe as a whole (see Topic J1).IsotopesAtoms with the same atomic number and different numbers of neutrons are known as isotopes. The chemicalproperties of an element are determined largely by the charge on the nucleus, and different isotopes of an element havevery similar chemical properties.
They are not quite identical, however, and slight differences in chemistry and inphysical properties allow isotopes to be separated if desired.Some elements have only one stable isotope (e.g. 19F, 27Al, 31P), others may have several (e.g. 1H and 2H, the latteralso being called deuterium, 12C and 13C); the record is held by tin (Sn), which has no fewer than 10. Natural samplesof many elements therefore consist of mixtures of isotopes in nearly fixed proportions reflecting the ways in which thesewere made by nuclear synthesis.
The molar mass (also known as relative atomic mass, RAM) of elements isdetermined by these proportions. For many chemical purposes the existence of such isotopic mixtures can be ignored,although it is occasionally significant.• Slight differences in chemical and physical properties can lead to small variations in the isotopic composition ofnatural samples. They can be exploited to give geological information (dating and origin of rocks, etc.) and lead tosmall variations in the molar mass of elements.4SECTION A–ATOMIC STRUCTURE• Some spectroscopic techniques (especially nuclear magnetic resonance, NMR, see Topic B7) exploit specificproperties of particular nuclei.
Two important NMR nuclei are 1H and 13C. The former makes up over 99.9% ofnatural hydrogen, but 13C is present as only 1.1% of natural carbon. These different abundances are important bothfor the sensitivity of the technique and the appearance of the spectra.• Isotopes can be separated and used for specific purposes. Thus the slight differences in chemical behavior betweennormal hydrogen (1H) and deuterium (2H) can be used to investigate the detailed mechanisms of chemical reactionsinvolving hydrogen atoms.In addition to stable isotopes, all elements have unstable radioactive ones (see below).
Some of these occur naturally,others can be made artificially in particle accelerators or nuclear reactors. Many radioactive isotopes are used inchemical and biochemical research and for medical diagnostics.RadioactivityRadioactive decay is a process whereby unstable nuclei change into more stable ones by emitting particles of differentkinds. Alpha, beta and gamma (α, β and γ) radiation was originally classified according to its different penetratingpower. The processes involved are illustrated in Fig.
1.• An α particle is a 4He nucleus, and is emitted by some heavy nuclei, giving a nucleus with Z two units less and massnumber four units less. For example, 238U (Z=92) undergoes a decay to give (radioactive) 234Th (Z=90).• A β particle is an electron. Its emission by a nucleus increases Z by one unit, but does not change the mass number.Thus 14C (Z=6) decays to (stable) 14N (Z=7).• γ radiation consists of high-energy electromagnetic radiation. It often accompanies α and β decay.Fig.
1. The 238U decay series showing the succession of α and β decay processes that give rise to many other radioactive isotopes and end with stable206Pb.A1–THE NUCLEAR ATOM5Some other decay processes are known. Very heavy elements can decay by spontaneous fission, when the nucleussplits into two fragments of similar mass. A transformation opposite to that in normal β decay takes place either byelectron capture by the nucleus, or by emission of a positron (β+) the positively charged antiparticle of an electron.Thus the natural radioactive isotope 40K (Z=19) can undergo normal β decay to 40Ca (Z=20), or electron capture togive 40Ar (Z=18).Radioactive decay is a statistical process, there being nothing in any nucleus that allows us to predict when it willdecay.
The probability of decay in a given time interval is the only thing that can be determined, and this appears to beentirely constant in time and (except in the case of electron capture) unaffected by temperature, pressure or thechemical state of an atom. The probability is normally expressed as a half-life, the time taken for half of a sample todecay. Half-lives can vary from a fraction of a second to billions of years.
Some naturally occurring radioactive elementson Earth have very long half-lives and are effectively left over from the synthesis of the elements before the formation ofthe Earth. The most important of these, with their half-lives in years, are 40K (1.3×109), 232Th (1.4×1010) and 238U (4.5×109).The occurrence of these long-lived radioactive elements has important consequences. Radioactive decay gives a heatsource within the Earth, which ultimately fuels many geological processes including volcanic activity and long-termgeneration and movement of the crust.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
