Atomic Number

Atomic number is the number of protons (positively charged particles) in the nucleus of an atom. It is denoted by the letter Z. This number determines the identity of a chemical element: any atom with six protons is carbon, one with eight protons is oxygen.

In a neutral atom (one carrying no charge), the number of electrons orbiting it equals the number of protons. Therefore Z also determines the chemical behavior of an element, because chemical bonding depends on the electrons in the outermost shell.

Difference between atomic number and mass number

Mass number (A) is the sum of the protons and neutrons in the nucleus. Atoms of the same element with the same atomic number but different numbers of neutrons are called isotopes. For example, carbon-12 and carbon-14 both have Z=6, but their mass numbers differ.

History

The periodic table of elements was originally organized by atomic mass. In the early twentieth century, experiments involving the scattering of alpha particles and the analysis of X-rays from elements revealed that nuclear charge was the true property distinguishing elements. The periodic table was subsequently reorganized by atomic number.

Changes to atomic number

Chemical reactions do not change atomic number; only the distribution of electrons changes. However, nuclear reactions, such as radioactive decay, can change the number of protons, transforming one element into another.