The periodic table is a listing of known elements, originally proposed by Russian chemist Mendeleev. Elements are substances that consist of only one type of atom. They are arranged left to right and top to bottom in order of increasing atomic number.

Rows are called periods. The period number of an element signifies the highest energy level an electron in that element occupies (in the unexcited state). The number of elements in a period increases as one moves down the periodic table.