The quantum atom

If we return to our atom; quantum physics is going to render Bohr's model redundant and replace it by a model more vague... more difficult to represent.

In effect, we've seen that the quantum electron is a double agent, victim of wave-particle duality.  The wave associated with the electron corresponds in fact to a probability to find the said quantum electron at a given location. 
The particle is no longer a classical material point but a bundle of probabalistic waves, a superimposition of potential movements. Electronic orbits must give way to the notion of orbitals, (a sort of fuzzy and probabalistic sphere), in which the electron will be in some ways diluted all around the nucleus.

It's not until physicists interact with the atom to observe the electron, that it appears as a particle: It's as though the undulatory electronic cloud suddenly reduces into a solid material particle. If it is necessary to risk an image to illustrate this curious phenomena, one could imagine the electron as a sub-marine which emerges, long enough for a measurement, from its probabalistic ocean. Later, it submerges and it will be impossible to an observer from the surface to localise it with any precision: one could do no more that define the volume of the ocean where the submarine could probably be found.
 

electronic orbital	observed electron
wave like nature	particle like nature

This new model of electronic clouds obeys a key principle of quantum physics: The Heisenbergs indeterminacy principle.
 
 

The shape of orbitals: If we were reduced to the scale of the atom, what would we see? The appearance of an atom is determined by its visible orbital, that is to say the one which is outermost.  The orbital of hydrogen is the simplest: it's spherical because its electon is alone.  Other atoms, like oxygen and carbon have far more complex orbitals because their external electronic layer contains several electrons diluted into as many distinct orbitals.  Quantum physics rules out two electronic orbitals being superimposed in the same place, these outermost orbitals therefore repel each other and offer up a more complex appearance.

Heisenbergs principle of indeterminacy
 

Besides the fact that the german physicist Werner Heisenberg is the father of this law (sometimes also called the uncertainty principle), what does this principle tell us? that it is impossible to determine with precision and simultaneously, the position and the velocity of a particle like the electron. The notion of an exact trajectory doesn't make any sense for particles. This quantum paradox (another one!) is associated with the difficulty to observe an electron... How do you observe it?

We can't observe something without illuminating it with light. Now at the scale of the infinitesimaly small, that poses a completely new problem. The least photon which strikes or interacts with an electron will modify its initial trajectory or make it change orbitals. At this scale, the photon becomes a projectile which could determine the position of the electron, but which would at the same time modify its speed and its trajectory; these cannot therefore be known at the same time. The slightest measurement interferes with the object being measured, and changes it!
 
 

the quantum owlet

Dare we have a new image to illustrate this principle:
Deep in the woods at night, a nature lover hears the hooting of an owl. If he would like, at the same time, see the feathered creature, he would have to turn a torch on him: But then it's a good bet that the surprised owl will stop singing. This gives rise to the insoluble dilema: We can't both hear and see the owl at the same time... Alas!