The Atom

In this three-part documentary series, Professor Jim Al-Khalili tells the story of one of the greatest scientific discoveries ever: that the material world is made up of atoms. Jim Al-Khalili obtained his PhD in theoretical nuclear physics from Surrey in 1989. He spent two years as an SERC Postdoctoral Fellow at University College London before returning to Surrey in 1991. He was appointed lecturer in 1992 and, in 1994, awarded an EPSRC Advanced Research Fellowship for five years following which he reverted to a full time lecturer in the Department at Surrey. He was elected Fellow of the Institute of Physics in 2000 and promoted to Senior Lecturer in 2001. In 2003, he was elected onto the Council of the BA (British Association for the Advancement of Science). Along with his teaching and research, the considerable time he spends on science engagement activities and popularising science has culminated in his promotion in October 2006 to a newly created chair in the Public Engagement in Science. He is author of several popular science books and appears regularly on radio and television. In 2007, he was awarded the Royal Society Michael Faraday Prize for Science Communication.

An atom is the smallest particle that comprises a chemical element. An atom consists of an electron cloud that surrounds a dense nucleus. This nucleus contains positively charged protons and electrically neutral neutrons, whereas the surrounding cloud is made up of negatively charged electrons. When the number of protons in the nucleus equals the number of electrons, the atom is electrically neutral; otherwise it is an ion and has a net positive or negative charge. An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element. The concept of the atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and the early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was not indivisible. The principles of quantum mechanics, including the wave–particle duality of matter, were used to successfully model the atom.

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses that can only be observed individually using special instruments such as the scanning tunneling microscope. More than 99.9% of an atom's mass is concentrated in the nucleus, with protons and neutrons having about equal mass. In atoms with too many or too few neutrons relative to the number of protons, the nucleus is unstable and subject to radioactive decay. The electrons surrounding the nucleus occupy a set of stable energy levels, or orbitals, and they can transition between these states by the absorption or emission of photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

Professor Al-Khalili takes us from the discovery of the atom to the development of quantum mechanics.

This episode tackles world-changing discoveries such as radioactivity, the Atom Bomb and the Big Bang, and tries to answer the biggest questions of all - why are we here and how were we made?

Al-Khalili discovers that there might be parallel universes in which different versions of us exist, and finds out that empty space isn’t empty at all, but seething with activity.

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