Radioactivity: the Discovery of the Nucleus

Today's picture heavy topic is Radioactivity: the Discovery of the Nucleus.

See, the thing is, until relatively recently, we didn't really know what we were made of. Some philosophical ideas had been thrown around for millennia that if you keep halving something that you would eventually get to a point where the cheese could no longer be halved. This tiny bit of something was referred to as "Indivisible", or ἄτομος (átomos). This was known as Atomism. The origin of this thought is widely credited to Democritus and Leucippus.

However people still believed that we were made up of the classical elements, and while such a theory is still prevalent in modern popular media, most college textbooks tend to prefer teaching the periodic table. This was due to the foundations laid by 17th century "natural philosopher" Robert Boyle with Corpuscularianism, similar to atomism, except "corpuscles" could in principle be divided. While he didn't create the idea, Robert Boyle argued in 1661 that matter was composed of various combinations of different "corpuscles" or atoms, rather than the classical elements of air, earth, fire, water and dragon.

Things didn't really progress for a while after, Corpuscuthingy being the prevalent thought of the time, until the science of chemistry was developed. In 1789, French nobleman and scientific researcher Antoine Lavoisier discovered the law of conservation of mass and defined an element as a basic substance that could not be further broken down by the methods of chemistry. Later, in 1805, discovered the John Dalton proposed that each element consists of atoms of a single, unique type, and that these atoms can join together to form chemical compounds, prompting him to become thought of as the originator of modern atomic theory. Further lines of reasoning were made by one Johann Josef Loschmid, who in a scientific landmark, worked out the size of molecules in air, and botanist Robert Brown, father of what has come to be known as Brownian motion.

In 1869, building upon earlier discoveries by such scientists as Lavoisier, Dmitri Mendeleev published the first functional periodic table. The table itself is a visual representation of the periodic law, which states that certain chemical properties of elements repeat periodically when arranged by atomic number.

Now bear with me, if you haven't left already, because here is where we finally near the content of the A2 syllabus (assuming you haven't found the last 4 paragraphs as fascinating as I have). In 1897, the electron was discovered by J. J. Thomson while pissing about with cathode rays. In doing so, he discovered that they were a component part of every atom, thus overturning the then prevalent belief that atoms are indivisible. Thomson postulated that atoms were therefore made up of the negatively charged electrons distributed, possibly in rings around a uniform sea of balancing positive charge. Thus was created one of my least liked scientific names, the Plum Pudding Model.

Douse it in brandy and set it alight, then get back to me.

Thus it was that in 1909, under the direction of Ernest Rutherford, Hans Geiger and Ernest Marsden bombarded gold foil sheets with α rays, then known to be positively charged helium atoms. Lets take a more detailed look at that. What Geiger and Marsden used was an evacuated metal box, containing an alpha source lined up with a gold foil, and a scintillator (A zinc sulphide screen that emitted light when hit by an alpha particle). What was observed was that most particles travelled straight ahead with little to no deflection. 1 in 2000 were deflected however, and very occasionally, 1 in 10 000 would be deflected at a greater angle that 90°. Some would even bounce right back to the source, an action that would be impossible for a diffuse cloud of positive charges, as Thomson had suggested. It was, in Rutherford's words "as incredible as if you fired a 15 inch naval shell at tissue paper and it came back." to give you an idea of just how surprising this was.

It would have to be at least triple ply

From this, Rutherford interpreted the results as suggesting that the positive charge of a heavy gold atom and most of its mass was concentrated in a nucleus at the centre of the atom. This was the creation of the Rutherford model. He went on to use Coulombs law of force and Newton's laws of motion to explain his results, and through the use of different metals in the same experiment he worked out that the magnitude of the charge of a nucleus was +Ze, where e was the charge of an electron, and Z was the atomic number of the element.



So there we have it. The history of the discovery of the nucleus. I could go on to the size and density, but that's decidedly mathsy, and while I love maths, it doesn't translate very well into the historical documentary format I have going here, and I fear I would end up breaking into degree level stuff, which I just do not need distracting me right now.

At any rate, I hope you enjoyed it. It has probably helped me, if not you, so at least there's that.

It is highly likely that this won't be a daily thing as it is entirely likely that I will be a) busy b) doing other things and more likely c) procrastinating.