An abridged history of the atomic model

Towards the end of the 19th century, scientists were considering the structure of the atom. The atom was deemed  to be a tiny piece of matter which helped answer questions regarding spectroscopy. Spectroscopy being the  interaction between matter and electromagnetic radiation which includes light. Atoms of different material  absorb and emit different wavelengths of electromagnetic radiation – this was how helium was detected in the  Sun before it was detected on Earth hence its name originating from Helos, the Sun.

J. J. Thomson believed the atom to be an indivisible piece of matter which, owing to the earlier discovery that  electrons have a negative charge, would be made up of a physically larger substrate that was positively charged  – it had been suggested that atoms were electrically neutral and the electrons would cancel out the charge of  the positive substrate. The electrons in Thomson’s model were thought to orbit within the substrate and so the  model became known as the ‘plum pudding’ model. This model had a couple of problems and the arguments against  it would not go away. To begin with, anything in circular motion is in a state of constant acceleration. To  clear up any confusion, acceleration is defined as a change in velocity (over time) where velocity is defined  as a speed with a direction. This means that a change in direction without a change in speed can be called  acceleration. So for electrons to orbit within a sphere, they would need to constantly change direction so as  not to fly out of the sphere and are therefore in a state of constant acceleration. All of this requires  energy.
Secondly, any acceleration of a charged particle like an electron will emit radiation of some type but, with  the exception of some, atoms do not emit radiation.
These problems did not stop the theory becoming accepted in many quarters as it mostly fitted the bill and  worked with the Periodic Table nicely however not everybody was a believer.

Rat Catcher, Going Postal
Helium atom ground state – An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case. The black bar is one angstrom (10−10 m or 100 pm).
User:Yzmo [CC BY-SA]
Ernest Rutherford doubted the plum pudding model as did some others. One idea from Philipp Lenard in 1903 was that an  atom was mostly empty space and this was pounced upon by Rutherford. He took Lenards experiment of firing  electrons at thin metallic strips a stage further and decided to fire alpha particles at gold foil. Remember  from previous postings that an alpha particle is a helium nucleus which has 2 protons and 2 neutrons, well,  Rutherford had discovered these too so why not! The alpha particles that Rutherford and his student Ernest  Marsden (who did most of the donkey work) used were streamed at very high velocity and the argument was thus:

If the plum pudding model is correct then with the alpha particle being 7000 times the mass of  an electron, a  collision with a stray electron would have no effect even considering the electrostatic repulsion of a  negatively charged electron and a positively charged alpha particle.
If the alpha particle collided with the positively charged substrate then at worst and given the high velocity  of the alpha particle, it would do no more than deflect it whilst it continues its journey through the gold  foil – ricocheting between the plums until it exits.

Sure enough, this is what was found, alpha particles weadling their way through the foil but then Rutherford  asked Marsden (who by then asked a colleague called Hans Geiger to assist) to stop looking at alpha particles  going through the foil but instead look for alpha particles that would be deflected back from the foil towards  the point of origin. When this was done, it was noticed that a number were indeed reflected back. Granted, not  a large number but nevertheless, a number that could not be ignored so what was going on?
This result helped Rutherford complete his theory. What was happening was that most alpha particles would  happily skim past the atoms in the gold foil because, as had been suggested, atoms were largely made up of  empty space. Should an alpha particle hit an electron belonging to a gold atom then, like a bowling ball  hitting a table-tennis ball, it would plow on through. If an alpha particle was seen to be reflected back then  this was the alpha particle colliding head on with the nucleus of a gold atom. Yes, there were obviously some  variation in the angle of deflection but it depends where it ‘hit’ the gold nucleus so it was to be expected.  It should be noted that the alpha particles and gold nucleus do not physically collide, both are positively  charged and so electrostatic repulsion replaces an actual collision.
This single experiment should have put paid to the plum pudding model but, like the facts not fitting the  theory of man made climate change today, many scientists decided that the evidence before them was wrong and  for good reason….except that they couldn’t think of one.

1 year later in 1911, Rutherford put together his model of the atom which had a central nucleus that was  entirely positive in charge and had electrons orbiting outside this nucleus. Rutherford made a number of  predictions based on his atomic model which were all confirmed in an experiment by Geiger and Marsden in 1913.

3D animation of an atom incorporating the Rutherford model

This is when scientists began to take more notice, especially a Danish physicist called Nils Bohr. Bohr  combined Rutherford’s atomic model with ideas on quantisation from Max Plank whilst studying the most basic  atom of all, the hydrogen atom. These ideas were the basis of quantum physics and were finally fed back in to  the study of spectroscopy (this is where we came in) which allowed predictions to be made and eventually found  to exist.

Bohr went on to refine Rutherford’s model of the atom and just as importantly, face head on one of the problems  associated with acceleration of the orbiting electrons which had dogged previous models.
Yes, the electrons should spiral in to the nucleus of an atom if the electron receives no more energy to  maintain its orbit however, based on Plack’s quantum hypothesis, the electron can only move in orbits where its  angular momentum is an integer multiple of ħ (where ħ = h/(2π) and h is Planck’s constant). In such an orbit,  the electron can orbit indefinitely and will emit no radiation.

Advances in the quantum model mean that we have now also moved on from the Bohr model of the atom though this  is still the model taught in schools. The orbits of electrons are not so much orbits as spheres of probability.  This means that, owing to Heisenberg’s uncertainty principle, we can’t know the orbit precisely however, we can  create a cloud of probability in which, the electron will definitely be located. This means that atoms look a  little different to the way that we have come to know them but that is the idea of a model – to simplify a  concept to enable it to be worked on without losing important detail.

Rat Catcher, Going Postal
3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
Geek3 [CC BY-SA]

The above are the clouds of probability for each electron shell. These shapes are combined for larger atoms  and can give some very impressive representations of atoms – atoms are not 2 dimensional!

© Rat Catcher 2017