The most abundant elements in the universe are hydrogen and helium , which make up around 80% and 20% of all matter respectively. The rest of the heavy elements, although they represent a tiny fraction of the universe, exert a great influence on astronomical phenomena.
The periodic table contains more than 118 chemical elements , in a sort order according to their atomic weight. Of the total, some have been created artificially in particle accelerators or in atomic reactors; others in nature, forming part of toxic compounds or simple substances.
As a curious fact, the first classification of elements was the work of Antoine Lavoisier, who proposed that the elements be divided into matales, non-metals and transition metals. Although well thought out, this way of classifying them quickly was rejected , as experts considered that there were many differences, both in physical and chemical properties.
