The periodic table is a tabular arrangement of the chemical elements, and it is organized in order of increasing atomic number. There is a recurring pattern called the “periodic law” in their properties, in which elements in the same column (group) have similar properties. Generally, within one row (period), the elements are metals to the left, and non-metals to the right, with the elements having similar chemical behaviors placed in the same column.
Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. The chemical properties of the atom are determined by the number of protons, in fact, by the number and arrangement of electrons. The configuration of these electrons follows the principles of quantum mechanics. The number of electrons in each element’s electron shells, particularly the outermost valence shell, is the primary factor determining its chemical bonding behavior. In the periodic table, the elements are listed in order of increasing atomic number Z.
The total number of protons in the nucleus of an atom is called the atomic number (or the proton number) of the atom and is given the symbol Z. The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus. Therefore, the total electrical charge of the nucleus is +Ze, where e (elementary charge) equals 1,602 x 10-19coulombs. Each electron is influenced by the electric fields produced by the positive nuclear charge and the other (Z – 1) negative electrons in the atom.
The Pauli exclusion principle requires the electrons in an atom to occupy different energy levels instead of them all condensing in the ground state. The ordering of the electrons in the ground state of multielectron atoms starts with the lowest energy state (ground state). It moves progressively up the energy scale until each atom’s electrons have been assigned a unique set of quantum numbers. This fact has key implications for building up the periodic table of elements.