The Avogadro Project

This sounds like some sort of clandestine secret government project I think but in fact, it is something far more interesting!

The Avogadro projects earliest rumblings began in the 1990’s and were linked in with defining the kilogram. Its results were published in January 2011 and updated in 2015. The ultimate aim of the project was to provide the most accurate determination ever of the Avogadro constant. It involved an international collaboration of many scientists.

As nearly all good IB students will tell you, the Avogadro constant is defined as the number of carbon-12 atoms in 12g and this comes to around 6.022 x 10²³ atoms but it should be remembered that a mole is number, and this number can be applied to anything. For example, a mole of tennis balls would be 6.022 x 10²³ tennis balls.

In order to calculate a precise value for the Avogadro constant, spheres of pure silicon were manufactured. The spheres were the roundest ever manufactured, so smooth that if the sphere was the size of the earth, the distance in height between the deepest ocean and largest mountain would be a mere 5m.

This silicon used was 99.9995% silicon-28 with any other atoms being other isotopes of silicon.

The purity and smoothness of the sphere allowed scientist to calculate the Avogadro constant to an uncertainty of The purity and smoothness of the sphere allowed scientist to calculate the Avogadro constant to an uncertainty of 10 parts in a billion(!)

In order to calculate the value, the mass and dimensions of the sphere were needed plus the spatial dimensions of silicon’s crystal lattice and the mass of an atom of silicon-28. This allowed the number of atoms in the sphere to be calculated and hence Avogadro’s constant.

And what was the answer? Well, it was surprisingly close to Avogadro’s original number. The number is 6.02214076 x 10²³!

I hope that this blog post has provided you with a great example of Nature of Science in action, in particular section 4, ‘The Human Face of Science’, point 4.1: ‘Science is highly collaborative and the scientific community is composed of people working in science, engineering and technology. It is common to work in teams from many disciplines so that different areas of expertise and specializations can contribute to a common goal that is beyond one scientific field.’ (Quoted in the IB chemistry guide, page 10).

How do you teach this particular aspect of the nature of science? Please feel free to share your ideas with us below.

Read more about the International Avogadro Project on these two websites:

https://www.bipm.org/en/bipm/mass/avogadro/

https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project