By 1911 it was clear from the work of Ernest Rutherford that the atoms had a characteristic structure in which a cloud of electrons surrounded a very, very dense center (or "nucleus") consisting of protons and, (as the later discovered), neutrons.

This was Rutherford's picture of a nuclear atom, a picture that is still valid today, but some extra details have been added since his time.

According to Rutherford, the dense center of an atom takes up no more than one trillionth of the whole volume of the atom, but is where almost all the mass is concentrated.

Surrounding this dense center, and taking up almost all of the volume of the atom, were the super-light electrons which behaved as if they were particles (with momentum) one moment, and then waves (with wavelengths and frequencies) the next moment - or both at the same time!

The dense atomic center determined most of the physical properties of an atom - such as it's total mass. But it seemed equally likely that is was the number and arrangement of the electrons surrounding this center that determined the chemical properties of an atom or element, i.e. how it behaved in relation to other atoms.

The question was - how were the electrons arranged and how did this arrangement determine the chemical properties of that atom?

The easiest picture of the internal structure of a Rutherford atom would be to have all the electrons in fixed, stationary positions at different places in the volume of space surrounding the dense atomic center.

For each positively charged proton in the center there would be a negatively charged electron hovering at a certain place and position at a definite distance and location away from this center. Perhaps the chemical properties of atoms could then be explained by knowing just where these electrons were and what they were doing there?

It was an attractive idea, but not practical. Electrons and protons are oppositely charged particles (one positive and one negative). These opposite charges generate a strong force of attraction between them (like the opposite poles on a magnet), and if the electrons were not moving, they would be pulled steadily closer and closer to the positive protons and eventually the atom would collapse.

What kind of force would be strong enough to resist this force of attraction and keep the protons and electrons apart?

Centrifugal force seemed to be the answer.

Think of a rod of iron and a strong magnet. If these two objects are close enough they will attract one another strongly and come together with a crash!

But join them with a piece of string, and then spin the magnet round and round the iron rod. The outward force, called the centrifugal force, of the magnet as it moves in its circular path compensates for the equal and opposite force of attraction between the magnet and the iron rod, so the two objects stay together, but separate.

This is how and why the earth and the sun do not collapse together into one single molten mass. Despite the very strong gravitational force attracting the earth downwards towards fusion with the huge sun, the fact that the earth is circulating around and around the sun generates the outward centrifugal that keeps us apart (at least for as long as we are going to be alive!).

So Rutherford and others, including Hanataro Nagaoka, thought that electrons must be moving very rapidly in circular orbits thus using the centrifugal force of their motion to prevent them from being fatally attracted down and down into a massive meeting with the protons at the center.

Nagaoka even pictured the electrons behaving like little planets and just like our solar system, each electron would have its own orbit (so it did not crash into other electrons in other orbits!), and that the chemical properties of such atoms could then be related to the nature, spacing and characteristics of these electron orbits.

There was only one problem with this picture; electrons are not planets. If a negatively charged electron really moved in a circular obit in this way it would be constantly changing in direction (towards the center - actually), and thus "accelerating", in the classic meaning of this word.

An accelerating charged particle must be constantly changing in energy and emitting this energy in the form of electromagnetic radiation. This would leave the electron with a tiny bit less energy and this it would cause it move a bit slower than before. The centrifugal force keeping it apart from the atomic center would be a bit less, and thus the electron would have to move a tiny bit closer to the center of the atom. Hummmmm... we have a problem.

The more orbits the electron makes, the more energy it loses, the slower it moves and the closer it is pulled to the atomic center. Eventually, therefore, the electron would spiral down and down and finally crash into the center of the atom.

Rutherford's picture of an atom, while a good one and a big step forward in our understanding of atomic structure, had this one - fatal - flaw; electrons could not "orbit" the atomic center, slowly and constantly give up some of their energy, and then spiral down into the center of the atom. A new idea was needed.

That's when Neils Bohr, a Danish physicist started thinking, and by 1913 he had what he thought was the answer.