Periodic table

The periodic table arranges chemical elements by increasing atomic number, grouping elements whose properties recur in patterns across rows and columns.

Atomic number: The number of protons in an atom's nucleus, which defines its chemical element. Increasing atomic number sets the table's ordering principle.
Periodic law: The principle that elemental properties vary periodically when elements are ordered by atomic number. It states the recurring pattern the table is designed to reveal.
Dmitri Mendeleev: A Russian chemist who published a periodic classification of the elements in 1869. His table left gaps and predicted properties of elements not yet discovered.
Chemical bonding: The attraction that holds atoms or ions together in molecules and solids. Group and valence patterns help anticipate how elements form bonds.
Mendeleev's periodic table: Dmitri Mendeleev's 1869 tabular classification of elements, arranged mainly by atomic mass and recurring properties. Its gaps and mass-based ordering contrast with the completed modern table.
Electron configuration: The distribution of an atom's electrons among its atomic orbitals. Similar outer-electron arrangements produce recurring chemical behavior in columns.
Period (periodic table): A horizontal row of elements in the periodic table. Each row corresponds broadly to the filling of an electron shell.
Lothar Meyer: A German chemist who independently developed a periodic classification of the elements in the 1860s. His work showed that periodic patterns were emerging in more than one research program.
Electronegativity: A measure of an atom's tendency to attract shared electrons in a chemical bond. Its periodic pattern helps estimate bond polarity from element positions.
Spiral periodic table: A nonstandard periodic-table layout that places elements along a continuous spiral. It presents periodic recurrence without the familiar rectangular rows and columns.

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