The Avogadro number is also known as Avogadro`s constant denoted by N or N0. It is the number of constituent particles (usually molecules, atoms or ions) that are present in one mole. The international (SI) unit for the amount of substance is the amount, exactly 6.02214076×1023, and it is dimensionless. The term was named after the scientist Amedeo Avogadro (1776–1856). Different gases are made up of particles that have no similarity to one another. For example, chlorine molecules have large numbers of electrons, protons, and neutrons. They are bigger and occupy more volume than the hydrogen molecule, which has only two protons and two electrons. Early scientists recognized that there must be such size differences and assumed that collections of larger particles must have larger volumes than collections of an equal number of small molecules. According to Avogadro's hypothesis, equal volumes of gases at the same temperature and pressure contain equal numbers of particles. You can understand Avogadro's hypothesis more by thinking about the current explanation for gas pressure. Equal numbers to particles of different gases in equal volumes at the same temperature should exert the same pressure because the particles have the same average kinetic energy and are contained within equal volumes.