<text><span class="style10">lements and the Periodic Table (6 of 8)</span><span class="style7"></span><span class="style10">Valency</span><span class="style7">Russian chemist Dmitri Ivanovich Mendeleyev, the discoverer of the Periodic Table, was aware from the work of other scientists that, when the atoms of elements combined, each one had a characteristic ability to combine with a particular number of those atoms, such as hydrogen, which could combine with only one other atom. Chlorine could combine with only one hydrogen atom; oxygen could combine with two; nitrogen could combine with three; and carbon could combine with four hydrogen atoms. This ability was called valency. By about 1870 at least sixty-three elements were known and Mendeleyev was now able to arrange these in the order of their atomic weights. When he did so and considered the valencies of the atoms, he at once discovered an interesting fact. Lithium had a valency of one; the next element, beryllium, had a valency of two; the next, boron, had a valency of three; and the next, carbon, had a valency of four. After that was nitrogen with a valency of three; oxygen, with a valency of two; and fluorine, with a valency of one. This could have been a coincidence, but Mendeleyev found that the pattern of rise and fall of valencies ΓÇô a periodic characteristic ΓÇô held throughout the whole table. Mendeleyev also knew that the elements fell naturally into groups with similar chemical properties ΓÇô groups like fluorine, chlorine, bromine and iodine (now called the halogens) or the metals lithium, sodium and potassium. When he arranged his list ΓÇô still in order of atomic weight ΓÇô in vertical columns of identical valencies, he found to his astonishment that all the elements in each column fell into such groups. Chlorine was under fluorine, bromine was under chlorine and iodine was under bromine. The table was thus aptly called the Periodic Table.jem/ry</span></text>
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<id>23</id>
<text>ΓÇó QUANTUM THEORY AND RELATIVITYΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó CHEMICAL BONDSΓÇó CHEMICAL REACTIONSΓÇó METALS</text>
