The Atom
Introduction
This article will deal with the basics of atomic theory and the history of atomic theory from the ancient greeks to the modern nuclear model. We will see how and why people use the model of atoms and why they have been modified so much. Atomic theory was very much driven by experimental observations so it will also be important for us to look at the experiments that led to these theories.
The Greeks
This is where our journey starts around 400BC with the greek philosphers Leucippus and Democritus. They came up with the concept of Atomos where an atom was something which could not be split any further, ie the smallest elemnt of an object that was visible. Atomists at the time argued that this was not an infinite process that there was a smallest unit when the object became invisible. The idea of atoms was eventually dropped in favour of the four elements model where the world was made of fire,earth,air and water. To see more on early atomic ideas in the greek era click here.
The Atomic Revolution
In the 1700's people became fascinated with the newly discovered phenomena of electricity and this was intensively researched, the ideas of current(the flow of electricity) and charge were constantly developing and in 1780 Charles Coulomb came up of a way to show the force between two electrical charges.
In the 1800's when a guy called John Dalton came along. He was very interested in the study of gasses and after many years of research eventually decided that the gasses he observed could be explained in terms of atoms.
Then in 1897 J.J Thompson set about examining a flow of electricity, this is what are called cathode rays where electricity is proppelled across a vacuum by a potential difference. Thomson showed that this stream could be deflected by both electric and magnetic fields and from this deduced that whatever was flowing must have a negative charge. This led to the idea of the cathode ray being a stream of particles which we now call electrons. By balancing the force of an electric field and that of a magnetic field he managed to work out the charge/mass ratio for an electron. The charge on an electron can be found through electrolysis(where electricity was passed through a solution of silver ions). It can be seen that the charge required to deposit one mole of material was a constant called the faraday constant, this is found to be ~96500C. Next we can work out the charge on each individual ion by dividing by Avogadro's constant($N_a = 6.02x10^{23}mol^{-1}$ , the number of atoms in one mole). This gives us a value of $1.6x10^{-19}C$ for the charge on an electron.
Thomson found that for an electron $\frac{e}{m} ~ 10^{11}Ckg^{-1}$ then using the results from electrolysis for the fundamental charge he derived a mass of $~10^{-30}kg$.
Thomson went further than this he developed his own model of the atom, in order to account for the electrically neutral complete atom he proposed a sphere of positive charge to counter the electrons. The electrons were free to move around in this charge as though it were a kind of liquid a very popular concept at the time. This model became known as the plum pudding model, physicists were able to derive rough sizes for atoms based on their density as solids and found that they are on a scale of about 0.1nm or 1 Angstrom($1x10^{-10}m$). You can see the idea of the plum pudding model in the diagram to the right.
From Puddings to Planets
The early 1900's was a very busy time for atomic theory next up was a Mr Ernest Rutherford, his famous experiment involved shooting alpha particles which are hydrogen atoms with the electrons removed so are positively charged. He collimated the alpha particles using slits then fired them at a thin sheet of gold only a few atoms thick. Rutherford expected that the atoms would pass straight through with very little deviation in accordance with the plum pudding model. What they actually detected was backscattering or deflection at angles > 90 degrees. This would be incompatable with the pudding model of Thomson. By counting the number of impacts on a phosphorous screen he found that ~ $\frac{1}{1000}$ of $\alpha$ particles were deflected at angles > 90 degrees. In 1911 Rutherford proposed a theory where instead of being distributed over the whole atom the positive charge was condensed into a very small nucleus at the centre of the atom surrounded by orbiting electrons(This became known as the Nuclear model). As you can see this model has a very empty atom with most of it been free space.
Stability Problems
There was one very obvious problem with Rutherfords model and that lay in the well established laws of classical electrodynamics. These laws stated that any accelerating charge would produce electromagnetic radiation. Since the electron accelerates radially as it orbits the nucleus, energy would be radiated away from the electron causing its orbit to decay. From observing atomic densities and calculating the time for these orbits to decay based on classical theories it was very clear this wasn't happening. This issue of stability is one of the main reasons why the idea was largely shunned.
A Quantum Step
The 1900's was the birth of quantum mechanics and interest was growing in the ideas of discrete physical models. Neils Bohr in 1913 took the step of incorporating recent developments in quantum ideals into a model of the Hydrogen atom. What he proposed was basically the Rutherford planetary system but he postulated that the oribits would remain stable at certain quantised radii due in turn to qunatized angular momentum. He also said that the only way the orbit could change is if it were to jump from one quantum state to another with no intermediate stages. The previously observed Rydberg emission lines of elements were explained almost perfectly by his theory and thus he formed one of the most influential theories of modern physics. There are some clear limitations of the theory though the main two being the fact that it does not predict decay rates/intensity of emission and also the fact that it doesnt work too well for much larger atoms than Hydrogen.
Links and Further Reading
I plan to write at least a few more articles on atomic theory which will be more in depth, especially a closer look at the quantum Bohr Model.