The periodic table shows the chemical elements, ordered by their atomic number, electron configuration, and its chemical properties. Table rows are called periods and columns are called groups.
Russian chemist Dmitri Mendeleev was the first to publish a recognizable periodic table in 1869. He developed it to illustrate periodic trends of the then-known elements but he also predicted some properties of unidentified elements that were expected to fill gaps within the table.
The first 98 elements exist in nature, although some are found only in trace amounts and others were synthesized in laboratories before being found in nature.
Elements 99 to 118 have been synthesized in laboratories or nuclear reactors. Higher atomic Elements are currently being pursued: these elements would begin an eighth row, and theoretical work has been done to suggest possible candidates for this extension.
The elements
Each chemical element has a unique atomic number (Z) and represents the number of protons in its nucleus. Most elements have different numbers of neutrons among different atoms, these variants are referred as isotopes.
The elements are listed in order of increasing atomic number (the number of protons in the nucleus of an atom). A new row (period) is started when a new electron shell has its first electron.
Columns (groups) are determined by the electron configuration of the atom, elements with the same number of electrons in a particular subshell.
Elements in the same group generally have similar chemical properties because they interact with other elements giving, taking or sharing electrons from the same last shell.
