Element

Main article: Chemical element

The concept of chemical element is related to that of chemical substance. A chemical element is specifically a substance which is composed of a single type of atom. A chemical element is characterized by a particular number of protons in the nuclei of its atoms. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium.
Although all the nuclei of all atoms belonging to one element will have the same number of protons, they may not necessarily have the same number of neutrons; such atoms are termed isotopes. In fact several isotopes of an element may exist. Ninety–four different chemical elements or types of atoms based on the number of protons are observed on earth naturally, having at least one isotope that is stable or has a very long half-life. A further 18 elements have been recognised by IUPAC after they have been made in the laboratory.
The standard presentation of the chemical elements is in the periodic table, which orders elements by atomic number and groups them by electron configuration. Due to its arrangement, groups, or columns, and periods, or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as atomic radius, electronegativity, etc. Lists of the elements by name, by symbol, and by atomic number are also available.