As scientists explored nature at a fundamental level, they discovered certain basic quantities and physical constants that governed nature. The fact that they existed, meant that there was an inherent order in the scheme of things, at the physical level. One of the many such fundamental quantities is Avogadro's number, which is related to the concept of molar mass in chemistry. One of the central concepts in science, up on which all the theoretical ideas are based, is stated as 'Everything is made up of atoms.' Atoms in turn, combine to form molecules, increasing the order of complexity, to create the beautiful structures and compounds that make our world. The atomic radius is the order of 10^-10 m and therefore every bit of any substance contains more than trillion billion molecules. Due to the microscopic size, one would think that it would be impossible to count the number of these atoms and molecules. However, there are ways in which these numbers can be quantified and that's where the Avogadro's constant comes in. What is Avogadro's Number? This physical constant is named after the Italian scientist Amedeo Avogadro, whose work led to the search of a number that could actually quantify the number of atoms and molecules in a gas. He was the first to suggest that the volume occupied by any gas has to be directly proportional to the number of atoms contained in it. He also postulated that two ideal gases at the same temperature, with identical volume and pressure values, will have the same number of molecules. It was Jean Perrin, a French physicist who first defined the Avogadro's constant as the number of molecules in one gram mole of Oxygen. Later it was defined to be the number of atoms contained in 12 gm of Carbon-12. A mole of any substance is its molecular weight expressed in grams. Jean Perrin was awarded the 1926 Noble Prize in physics, for his work devoted to the calculation of this number. There are several physical techniques of measuring this constant. According to the current experimental data, the value of this constant is known to be 6.0221415 × 10²³ per mole. It is a scientific quantity, very similar to a daily use unit like a 'dozen'. Just like a dozen objects are always 12 in number, a mole of any substance, always contains 6.0221415 × 10²³ atoms, molecules or ions. Applications The constant can be used in several ways to determine other physical quantities. It can be used to determine the mass of a single molecule or atom of a substance or chemical element. When you know the total mass of a mole of a substance and the number of molecules contained in it, you can easily get the mass of a single molecule, by dividing the molar mass, by constant. It can be further used to determine the number of molecules in a substance with any variable amount of mass. All one has to do is calculate the number of moles contained within the specified mass of the substance and multiply it with the constant to get the total number of molecules. Thus the constant is a numerical quantity that quantifies the number of atoms, molecules and ions in a mole of any substance. It is a numerical connection between the microscopic and macroscopic world, which is helpful in quantifying the actual number of atoms or molecules that take part in a reaction.