IB Chemistry - Atomic Theory

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Syllabus ref: 2.1

"Big fleas have little fleas upon their backs to bite them, and little fleas have lesser fleas, and so ad infinitum". Jonathan Swift.

Although atoms are said to be the smallest building blocks of matter, they themselves consist of smaller 'sub-atomic' particles. So what are sub-atomic particles made from? That, happily, remains the domain of the physicist and, as such, has no place in this book!

Evidence for sub-atomic particles

Atoms are the fundamental building blocks of matter. However, even atoms are constructed of smaller, sub-atomic particles. It was originally thought that the atoms were small indivisible particles (atomos = indivisible), however a paradigm shift occured with the discovery that the atoms themselves have a sub-structure.

Evidence for this emerged though the experiments conducted in the nineteenth century on cathode rays, culminating in the experiments of Rutherford (the nuclear atom) and Chadwick (neutrons) in the early 20th century.

It is now known that even these sub atomic particles themselves have an even deeper sub-structure - the quarks!


The sub-atomic family

The fundamental sub-atomic particles are protons, neutrons and electrons. The protons and neutrons are held together by strong nuclear binding forces in the nucleus of the atom. The electrons may be considered to be tiny particles that exist in regions of space known as orbitals around the atom.

This model of the atom is precisely that, a model. It is impossible to see atoms and, in order to be able to describe their properties, we use models representing this microscopic world that is invisible to us.

Similarly, the sub-atomic world is a strange place with unusual forces acting over infintesimally small distances. The rules of behaviour that govern the macroscopic world often break down in this strange environment, and it is important to understand that our representations and models are necessarily limited here.

The picture of the atom above is easy to discuss and is comfortably familiar. However, if you consider the actual dimensions of an atom compared to the nucleus you can see just how inaccurate even this simple picture is.

Hydrogen atomic radius: 3.7 x 10-11 m

Hydrogen nucleus radius: 8 x 10-16 m

You should appreciate that the nuclear radius is much smaller than the atomic radius by a factor of about 100,000. This means that the atom is mostly empty space with a very solid and tiny nucleus. This was originally demonstrated by the scattering experiments of Ernst Rutherford.