The order of chemical elements in the periodic table is primarily based on the element's atomic number. The atomic number of an element represents the number of protons in the nucleus of its atoms, which is a unique identifier for each element.
The periodic table arranges elements in order of increasing atomic number from left to right and top to bottom. This arrangement leads to the periodic repetition of similar properties, hence the name "periodic table."
The table is divided into periods (horizontal rows) and groups or families (vertical columns). Each period corresponds to the energy level or shell of electrons in an atom, while each group consists of elements with similar electron configurations and shared properties.
The periodic table's organization also highlights patterns in the periodicity of elemental properties such as atomic radius, electronegativity, ionization energy, and chemical reactivity. Elements within the same group often exhibit similar chemical behavior due to their similar electron configurations, while elements across a period tend to show a gradual change in properties.
The periodic table has evolved over time as new elements have been discovered and our understanding of atomic structure and properties has advanced. The modern periodic table, known as the long form, is based on the work of the Russian chemist Dmitri Mendeleev and the English chemist Henry Moseley, among others.