The word atom is derived from an idea first formulated by the ancient Greek philosopher Democritus.Democritus believed that everything was made from indivisible, imperishable and unchanging particles which he called atamos. The way in which the two atoms of hydrogen and one atom of oxygen combine is called covalent bonding. The term valence simply refers to the number of electrons in an atom's outer shell that are available for bonding. The next type of bond that can occur between atoms is called an ionic bond. An ion is an electrically charged atom that is formed when an atom gains or loses an electron. Each of the elements represented on the periodic table is composed of atoms of the same type. That is to say, each element is composed of either a single atom or a number of atoms of the same type.