A table of the elements listed in order of increasing atomic number (See Appendix table 5). Arguably the greatest discovery in chemistry occurred when Dmitri Ivanovich Mendeleyev (1834–1907) tried to organize the elements he knew into a coherent pattern that made chemical sense. He listed them in order of increasing mass; similar elements recurred at regular, periodic, intervals. His stroke of genius was to leave gaps, where chemical sense decreed, for undiscovered elements. For example, arsenic did not resemble aluminium, yet it was the next element he knew after zinc. So Mendeleyev concluded that two unknown elements, eka- aluminium and eka-silicon (eka is Sanskrit for ‘first’), lay undiscovered. He was uncannily accurate in his predictions for the properties of these unknown elements, so his fame was secured when the elements gallium and germanium were discovered.
Moseley\'s law showed that the correct quantity to use for ordering the elements is the atomic number and not the relative atomic mass. Nevertheless, as the two orders are similar, Mendeleyev\'s table worked very well, despite the odd irregularity such as the fact that argon has a larger mass than potassium.
The horizontal rows of the periodic table are called periods, and the vertical columns are called groups. The groups are collected into one of four blocks, according to the last orbital to be filled. The s block is on the left of the table and the p block on the right, with the d block in between. The f block is often shown at the bottom of the table, essentially to save space across the page.
There are general trends in the behaviour of the elements down a group and across a period. Groups contain elements which are similar to each other. The formulae of compounds of the elements within a group are usually very similar. For example, the chlorides of the elements in Group 2 have the formulae BeCl{TAG(tag=>sub)}2{TAG}, MgCl{TAG(tag=>sub)}2{TAG}, CaCl{TAG(tag=>sub)}2{TAG}, SrCl{TAG(tag=>sub)}2{TAG}, BaCl{TAG(tag=>sub)}2{TAG}; their oxides have the formulae BeO, MgO, CaO, SrO, BaO. The changes down a group are often subtle; in general, metallic character increases down a group, with Group 14 showing the largest change in behaviour. At the top of Group 14 is the nonmetal carbon, in the middle is the metalloid germanium, and at the bottom is the metal lead. Trends across a period tend to be much more dramatic. The formulae of the chlorides and oxides across period 3 from sodium to silicon change in a systematic way: NaCl, MgCl{TAG(tag=>sub)}2{TAG}, AlCl{TAG(tag=>sub)}3{TAG}, SiCl{TAG(tag=>sub)}4{TAG} and Na{TAG(tag=>sub)}2{TAG}O, MgO, Al{TAG(tag=>sub)}2{TAG}O{TAG(tag=>sub)}3{TAG}, SiO{TAG(tag=>sub)}2{TAG}. The bonding in both series of compounds changes from ionic to covalent; NaCl is a solid which dissolves in water whereas SiCl{TAG(tag=>sub)}4{TAG} is a liquid which hydrolyses violently in water. In general, metallic character decreases across a period. In period 3, the element on the left of the period, sodium, is a reactive metal, whereas the two elements on the right are chlorine, a reactive nonmetal, and argon, a very unreactive nonmetal. Sodium oxide, Na{TAG(tag=>sub)}2{TAG}O, is an alkaline solid whereas the oxide of chlorine with the similar formula, Cl{TAG(tag=>sub)}2{TAG}O, is an acidic gas. The structure of Na{TAG(tag=>sub)}2{TAG}O is well understood using ionic bonding whereas Cl{TAG(tag=>sub)}2{TAG}O is composed of individual covalently bonded molecules. |
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