Every element is uniquely defined by the number of protons in the nucleus of its atoms, known as the atomic number. A free element is an element that exists in its uncombined state, meaning its atoms are not chemically bonded to atoms of a different element. This existence represents the pure form of the substance as found on the periodic table.
Defining the Chemical Identity of Free Elements
The defining characteristic of a free element is its complete chemical purity. This means the substance is composed entirely of atoms that share the exact same atomic number. For example, a sample of free oxygen contains only oxygen atoms, all of which have eight protons. The atoms within a free element maintain electrical neutrality, meaning the number of electrons equals the number of protons, resulting in an oxidation state of zero. This neutral state is the baseline chemical condition for classification as a free element.
The Molecular Forms of Free Elements in Nature
The term “free element” does not necessarily mean the substance exists as individual, single atoms. In nature, free elements adopt various stable molecular structures, with the bonds formed exclusively between atoms of the same type. Some elements, like the noble gases helium (\(\text{He}\)) and neon (\(\text{Ne}\)), are monatomic, existing as completely separate, unbonded atoms due to their inherent chemical stability. Other elements achieve stability by bonding with an identical partner, forming diatomic molecules such as oxygen (\(\text{O}_2\)), nitrogen (\(\text{N}_2\)), and hydrogen (\(\text{H}_2\)).
These diatomic molecules are still considered free elements because the chemical bond is homologous, meaning it is between two atoms of the same element. A significant portion of the Earth’s atmosphere, approximately 99%, is composed of the free elements \(\text{N}_2\) and \(\text{O}_2\). Other elements form larger, more complex structures; for instance, sulfur often exists as octasulfur (\(\text{S}_8\)) molecules in ring form. Carbon, in its free state, forms vast network solids, such as the crystalline lattice of diamond or the layered sheets of graphite. Noble metals like gold (\(\text{Au}\)) and platinum (\(\text{Pt}\)) are also found as free elements, existing as metallic solids where the atoms are bonded only to other atoms of the same metal.
How Free Elements Differ from Compounds and Ions
Free elements stand in direct contrast to chemical compounds, which involve the chemical bonding of two or more different types of elements. For example, water (\(\text{H}_2\text{O}\)) is a compound because it is formed by a chemical bond between hydrogen and oxygen atoms. Breaking a compound down via a chemical reaction yields its constituent free elements, but the compound itself possesses a unique set of physical and chemical properties.
Free elements also differ from ions, which are atoms or molecules that have acquired a net positive or negative electrical charge. An ion forms when an atom gains or loses one or more electrons, creating an imbalance between protons and electrons. Sodium (\(\text{Na}\)), as a free element, is neutral, but the sodium ion (\(\text{Na}^{+}\)) has lost an electron and carries a positive charge. This charge is the key chemical distinction, as free elements always exist in a neutral state, while ions are defined by their non-zero electrical charge.