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141141N + n (neutron)C + H→It then reacts rapidly with oxygen to form radioactive carbon dioxide which becomes distributed throughout the atmosphere mixed with 12C carbon dioxide. The 14C radioisotope undergoes decay by emission of an electron to give the stable 14N isotope of nitrogen with a half-life of about 5730 years (one half of the 14C decays every 5730 years). 1414CN + e→−The balance between its formation and decay leads to a stationary state natural abundance of 14C in atmospheric carbon dioxide of about one part in one trillion (∼1 : 10 12). Atmospheric carbon dioxide is absorbed in plants by pho-tosynthesis (this process is called the fixationof CO 2) and the carbon is transferred to animals which consume plants as food. Consequently, as long as CO 2from the atmosphere is being incorporated, the 14C/ 12C ratio within a living system will remain constant. Once the fixation stops, however, and the radioactive decay of 14C continues, the 14C/ 12C ratio in the fixed carbon decreases with time. As we know the half-life of 14C, analysis of the radioactivity of organic materi-als of bioorganic origin enables us to estimate the time since the carbon dioxide was fixed. This technique is called radiocarbon dating , or simply carbon dating, and was developed in 1949 by Willard Libby (University of Chicago) who was awarded the Nobel Prize in Chemistry in 1960 for the work. Times of up to about 60 000 years can be estimated and the method is widely applied in archaeology. Libby and his team first demonstrated the accuracy of the method by showing that the age of wood from an ancient Egyptian royal barge estimated by radiocarbon dating agreed with the age of the barge known from historical records. Panel 1.1 Radiocarbon dating same energy. The four AOs of this second shell ( n= 2) can accommodate a total of up to eight electrons. The third shell ( n= 3) contains one 3s and three 3p orbitals (3p x, 3p y, and 3p z) plus a set of five degenerate 3d orbitals—a total of 9 AOs which (together) can hold up to 18 electrons. The relative energies of some of the atomic orbitals mentioned above for an unspeci-fied atom are shown in Figure 1.2. The s orbitals of increasing energy with principal quantum numbers 1–5 are shown in the column on the left; in the centre column, the p orbitals are seen to increase in energy starting from n=2; the five degenerate d orbitals only start with n=3 (and no higher ones are shown). None of the seven-fold degenerate f orbitals are shown as they are higher in energy and do not start until n=4; they are of minimal importance in organic chemistry. 1s2s3s4s2p3p3d4p5sEnergyFigure 1.2 Energy levels of atomic orbitals.
4 . . . 1 Atoms, Molecules, and Chemical Bonding—a Review As mentioned above, the atomic orbital occupied by an electron indicates the space available to it as well as its energy. An s orbital is spherical, while each p orbital is elongated and circularly symmetrical about one of the three mutually perpendicular Cartesian axes (so they are labelled p x, p y, and p z), as illustrated in