**The International System of Units (with the abbreviation of SI units) is one of the foundations of modern science. It consists of seven base units from which all others can be derived.**

**These are the meter for length; the kilogram for mass; the second for time; the ampere for electric current; Kelvin for thermodynamic temperature; candela for luminous intensity, and mole for the amount of substance. This is a coӧrdinated system of units that allows scientific results to be compared relatively easily, regardless of where they are made.**

**However, the SI system is far from perfect. One of the problems is that some of the units have values based on arbitrary objects, such as the kilogram. There is general agreement that this should be changed so that the units are based on the fundamental constants of nature and on specific numbers that are defined and therefore constant.**

**One of these numbers is Avogadro’s constant. This is currently defined as the number of atoms in 12 grams of carbon-12 and is known to be about 6.02214129 ×10^23. But the exact number depends on the definition of a kilogram, which for the moment is the mass of an arbitrary bar of platinum-iridium alloy hidden in a safe somewhere in Paris.**

**The general consensus is that it would be better to define Avogadro’s constant and let this determine the mass of the kilogram. But what number should serve?**

**click here.**