Notice that although our own environment has relatively high percentages of the heavier elements, the universe as a whole is still composed almost entirely of the hydrogen and helium that were probably formed in the Big Bang when the universe was first created. Molecules are formed from two or more atoms that have been combined chemically. Some elements can exist as individual atoms, but other are so chemically active that even in their pure form they combine into two-atom molecules, which are therefore referred to as diatomic . When the atoms of two different elements unite into a molecule, they form a compound, and their chemical symbols are combined to provide a molecular formula for that compound. Note that subscripts are used to designate the number of atoms of each element that are present in the molecule.
Some elements can combine in two or more ways by using different bonding structures.
If these multiple combinations occur in the same physical phase, they are called allotropes . These are not multi-element compounds but are made up entirely of atoms of the same element. The periodic table is an orderly arrangement of the chemical elements into horizontal rows called periods and vertical columns called groups . The elements are placed in the table in order of increasing atomic number and in such a way that those having similar chemical properties go into the same column. This is the basis for the famous periodic law of chemical properties.