Countdown to the SI redefinition: How long would it take to eat a mole of Easter eggs?

In case you missed it, the redefinition of the International System of Units (SI) is going into effect next month on World Metrology Day, 20 May 2019. Each month we are bringing you a blog post featuring one of the units of the SI. This month we are focusing on a unit that has particular significance to chemists and therefore, our scientists at the UK National Measurement Laboratory at LGC: the mole, the unit used to measure the (usually very large) number of things, usually atoms or molecules.

602 214 076 000 000 000 000 000, or approximately 602 trillion, has 24 digits. As unimaginably large as this number may seem to us, we encounter trillions of things constantly in everyday life. In a regular sized balloon, for example, there are about 50 trillion air molecules (including about 10 trillion oxygen molecules). A standard painkiller contains around 1.7 trillion molecules of the active ingredient. And one litre of mineral water contains about 1 trillion sodium atoms.

As you can see, when we look at extremely small things such as atoms and molecules, we start having to deal with very large numbers. Physics and chemistry often deal with atoms and molecules but scientists don’t always want to have to juggle such large numbers. To make things more manageable, large particle numbers are combined into imaginary bundles for counting. This ‘bundle’ – the mole, the unit of an amount of substance – is our unit of the month for April.

Using this way of counting, instead of saying that a balloon contains “about 10 trillion oxygen molecules”, we can say there is “about 0.02 moles of oxygen”. The conversion factor between the number of particles and the amount of substance is the so-called Avogadro constant, N_A. This states that one mole contains 602 214 076 000 000 000 000 000 particles. Although we aren’t usually that interested in the exact number of oxygen atoms in a balloon, in other areas of life, such as medicines or food, it is critical that we know exactly how much of something there is and the mole becomes really important.

At the beginning of the 20th century, before the mole was introduced, terms such as “gram atom” and “gram molecule” were used to describe quantities of chemical elements or compounds. These quantities referred to the relative atomic or molecular weights and the reference value for these relative weights was the atomic weight of oxygen, generally agreed to be 16. However, there was some disagreement between the physics and chemistry communities as to which exact definition of oxygen was meant [1]. In 1960 the International Union of Pure and Applied Physics (IUPAP) and the International Union of Pure and Applied Chemistry (IUPAC) agreed to assign the value of 12 to the so-called atomic weight of the isotope [2] of carbon with the mass number 12 (¹²C), correctly referred to as the relative atomic mass A_r(¹²C). From then on, this was used as the reference value.

At the suggestion of these international bodies responsible for agreement on standards and atomic weights (IUPAP, IUPAC and ISO), a definition of the mole as a unit for the quantity of the substance was developed at the end of the 1960s. The definition was adopted into the International System of Units (SI) by the General Conference on Weights and Measures (CGPM) in 1971. It reads:

1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol”.
2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

From 20 May 2019, the definition of the mole will be as follows: