[Author © Dr Phil Brown GRIC, PhD: Doc Brown's chemistry exam revision notes on history of atomic model theories suitable for students of UK GCSE/A advanced level and international IGCSE/O/Advanced A level chemistry courses, ~US grades 9-12 chemistry notes  [page updated RE-EDIT]

The development of the 'atomic model' is an excellent example of how new experiment evidence initiates the need to change an existing scientific model or even come up with a different theoretical model, in this case for atomic structure. You should appreciate that knowledge and understanding of atomic structure has evolved over time and as a scientific model of atomic structure, it must explain current experimental observations and pose questions for future investigations.

Any further change in an atomic structure hypothesis e.g. because of new evidence, must be re-tested out in the laboratory and the results checked by other research groups from around the world. Plus a transparent peer group review of any research paper to be published, which means scientists having their work checked by other scientists. I they don't agree, somebody has got something wrong or made false claims or just done the experiment badly! If further experimental checks don't back up a hypothesis, it must be modified or abandoned in favour of other ideas.

When a hypothesis is backed up by experimental evidence from different scientists and lots of cross-checking of results e.g. the Bohr theory of the atom, it becomes an accepted theoretical model of an atom. Even to this day the atomic structure model is still developing e.g. the hypothesis that neutrons and protons are made of quarks which are held together by gluons, but at this point I'm out of my depth! Nothing stands still in scientific theory, even with atomic structure, and the hypotheses of modern quantum physicists about the structure of the nucleus I find pretty 'whacky', actually, pretty incomprehensible!

The ancient Greeks thought that everything was made of four basic 'elements' - air, earth, fire and water. However, at the height of the great classical Greek civilisation, the Greeks Leucippus and Democritus and others ~400 BC, wondered what was the result of continually dividing a substance i.e. what was the end product or smallest bit i.e. what was left that was indivisible – the word atomic is from Greek adjective meaning 'not divisible'.

The Greeks idea was not forgotten and later revived by Boyle and Newton but with little chemical  progress.

 

However, in 1808 Dalton at the beginning of the 19th century, proposed his atomic theory that all matter was made up of tiny hard particles/spheres called atoms.

Dalton also proposed (correctly) the theory that different types of atoms (elements) combined together to give all the different substances of the physical world (all which of course is true, except for the 'hard solid indivisible spheres'!).

He also produced the first list of 'atomic weights' (we now call relative atomic masses) on a scale based on hydrogen – given the arbitrary value of 1 since it was lightest element known, and, as it happens, correctly so.

He was incorrect by stating that atoms were indivisible, because we now know that atoms consist of electrons, protons and neutrons and that atoms can be 'taken apart' by ionisation or nuclear changes  e.g. radioactivity.

Until the discovery of the electron, atoms were thought of as hard indivisible spheres, but brilliant 'JJ' Thompson changed all that. New experimental evidence led to a new scientific model of the atom with the discovery of the electron, recognised as the first known 'sub-atomic particle', change was on the way.

J J Thomson around 1897 proposed his 'plum pudding model' theory (picture on right) based on the growing evidence that atoms were themselves composed of even smaller more fundamental particles like the electron i.e. atoms were not hard indivisible spheres, so the solid sphere had to go.

He based his model on experiments that showed that atoms contained even smaller negatively charged particle called electrons which could be removed from atoms using a vacuum tube and applying a high potential difference (voltage) to a very low pressure gas. He showed that the mass of an electron was much less than the mass of an atom and that it had a negative electric charge. Therefore the 'hard indivisible sphere' model of an atom was wrong.

From his experiments Thomson envisaged a plumb pudding atom consisting of a positively charged 'pudding' (a sort of ball of positive charge) with just enough lighter negatively charged electrons embedded in it to produce a neutral atom. Note that both the positive charges and negative charges are evenly distributed through the sphere of the atom (shown later by Rutherford and Bohr etc. to be completely wrong).

The idea of positive particles balancing the negative particles was correct but the relative size and nature of the nucleus and distribution of electrons were not, BUT it was a more advanced model.

Ernest Rutherford, assisted by Hans Geiger and Ernest Marsden (the latter two were students of Rutherford at Cambridge University) conducted alpha particle scattering experiments (1902–1910, and described in detail below).

The famous alpha particle experiment in 1909 was designed to test the plum pudding theory of JJ Thomson.

By 1911, these experiments established

(i) minute nature of the nucleus even compared to the size of an atom.

(ii) the nucleus was positive and the positive charge varied from element to element.

(iii) the positive charge was concentrated in the nucleus and able to deflect other positive particles e.g. alpha particles.

Diagram of the famous Rutherford and Geiger–Marsden alpha particle scattering experiment

When positive alpha particle beams are fired on very thin layers of metals (e.g. very fine gold leaf) some rather surprising results were made by scientists of the early 20th century.

By using a 360^o charged particle detection system it was found that ...

3. most particles passed through un–deflected (as if there was nothing there!), this was expected. This was predicted from JJ Thompson's plumb pudding model, but all the alpha particles were expected to pass through or to be slightly detected (observation 2. below), though NOT big deflections.

2. a small proportion were deflected slightly (so there was something there!), this again was not unexpected.

1. about 1 in 20,000 were 'bounced' back through an angle of over 90^o, in other words were reflected backwards, a totally unexpected result and quite shocked the experimenters - not what they were expecting. This was because the JJ Thompson's plumb pudding model predicted the positive charge was spread out and not sufficiently concentrated to cause, for some alpha particles, a 180^o deflection! So, whatever was there, was substantial in mass and positive charge to cause the repulsion 'bounce' of the positive alpha particles, BUT what it was (the 'nucleus') it wasn't very big!

These results made it quite plain the JJ Thompson plumb pudding model was in some way wrong i.e. the positive charge was NOT spread throughout the volume of an atom, therefore a new model must be proposed to take into account the new results.

 

From a detailed mathematical analysis of the scattering experiment results, the only 'atomic model' which could account for the pattern was an atom consisting of ...

1. mainly empty space (which is why most alpha particles passed through undeflected), thus completely contradicting JJ Thompson's 'plum pudding' model. Other experiments showed that the electrons were orbiting in energy levels around the nucleus, but occupying virtually no significant volume in themselves as particles.

2a. a relatively minute positive centre (the nucleus) causing deflection (like charges repel, alpha particles are positively charged and so were being repelled by the 'later to be discovered' positive protons in the nucleus), we now know the nucleus is positive due to protons,

2b. a tiny dense centre of similar or greater charge or mass to an alpha particle (which we now call the nucleus), so most of the mass of an atom was in the central nucleus, we know the mass is made of protons and neutrons.

3. Most of the atom is mainly empty space with a cloud of negative electrons moving around the dense relatively massive positive nucleus.

Putting these three points together formed the basis of the modern picture of the 'nuclear atom', in other words the nuclear atomic model.

 

The Bohr model of the atom

BUT, there was still a puzzle to solve - why didn't the negative electrons collapse into the nucleus?

The great physicist Niels Bohr suggested the electrons orbited in energy levels (shells) with sufficient energy that prevented electrons from being attracted into the nucleus - but would experiments confirm this theory?

Later experiments did show that electrons are arranged in energy levels, sort orbits around the nucleus, ideas first proposed by scientists such as Bohr.

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances from the nucleus and each orbit was a specific electron energy level - a fixed electronic energies.

The theoretical calculations of Bohr agreed with experimental observations.

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge.

The name proton was given to these particles.

This was a necessary extension and modification to the Rutherford model of an atom, because this model could not account for why the electrons were not attracted to the nucleus.

Bohr's suggested that the negative electrons can only exist in certain specific energy levels (shells) at fixed distances from the nucleus and held in place by the positive nucleus. This theory added too, and complimented the Rutherford model of the atom, to gives a reasonably complete picture of an atom (at least for this academic level!)

e.g. on the right the 'Bohr' electronic diagram for sodium with the (Na) representing the nucleus.

and below, a more sophisticated diagram of a lithium atom.

Bohr envisaged the electrons orbiting the nucleus in specific energy levels (or fixed shells) at specific distances from the central nucleus with nothing in between. In other words the electrons have sufficient energy to keep away from the nucleus and be confined to these specific energy levels. The negative charge of the electrons was still balanced by the positive charge (protons) of the nucleus.

So now, as far as we can tell (at GCSE/A level anyway) an atom is quite well represented by the Bohr model of the atom (picture below) which moves the Rutherford nuclear model another step forward.

 

Diagrams of a 'Bohr' lithium atom

AND, most importantly, experimental results matched a theoretical mathematical model of simple 'electronic' atoms like hydrogen.

 

So by now, earlier theories of atomic structure, e.g. the 'plum pudding' model in which 'protons' and 'electrons' were scattered or arranged evenly across the atom, were superseded by the nuclear model of Rutherford and subsequently this was superseded by Bohr's electronic energy level model.

It was the only model that could explain the scattering of the high speed alpha particles by a small dense and positive atomic centre AND the behaviour of electrons.

Experiments had shown that the outer bits could be knocked off atoms and these had a very tiny mass and a negative charge, in other words the electron!

Further experiments showed that the nucleus (partly) consisted of positive particles with the same mass and charge as an ionised hydrogen atom, that is a proton (mass 1, charge +1).

INDEX of atomic structure exam revision notes