Chemical symbols are shorten abbreviations of the names of the 109 known elements. The use of symbols for the chemical elements existed long before a systematic method was developed. the alchemist associated the symbols of the planets not only with the days of the week, but also with the seven metals known at the time: gold, silver, iron, mercury, tin, copper , and lead. By the beginning of the nineteenth century, there were about 26 known elements, but by the beginning of the twentieth century, therer were more than 81.