The substances consisting of atoms of one chemical element (homonuclear molecules) are called simple. Simple substances are a form of existence of chemical elements in a free look, i.e. elements, chemically not connected with any other elements. More than 400 types of simple substances are known.
Simple substances can be nonmetals and metals, it depends on type of a chemical bond. They are divided into atomic gases (He, Ar) and molecular (O2, O3, H2, Cl2). Simple substances possess allotropic modifications when the same chemical element forms several types of simple substances. This phenomenon can be owing to various structure of molecules and a way of their placement in crystals (allotropy of a form) or various structure of molecules (atoms) of one element (allotropy of structure). Abilities of elements to formation allotropicof modifications are caused by the structure of atom defining type of a chemical bond, the structure of crystals and molecules. Any allotropic modifications have property to pass each other. For one chemical element its allotropy differs on chemical activity and physical properties (for example, temperature of melting of diamond is higher, than fullerene, or ozone is more active than oxygen). Under normal conditions simple substances for eleven elements will be gases (Rn, H, N, F, Ne, O, Cl, Kr, He, Xe, Ar), for two – liquids (Hg, Br), for the others – solid bodies. At a temperature close to room, 5 metals will be in semi-fluid or liquid state because they have melting temperature close to room: Mercury (39 °C), Rubidium (39 °C), Caesium (28 °C), Frantsy (27 °C), Gallium (30 °C). You should not mix the concepts "simple substance", "atom" and "chemical element". Atom – concrete value since it exists really. Chemical element – an abstract, collective concept, of the nature chemical elements exist in the form of chemically connected or free atoms, i.e. complex and simple substances. Characteristics of simple substance (set of particles) and chemical element (the isolated atom of a certain look) are also various.