Before the Common Era, the Greeks believed that matter could be split but only split a certain number of times. In the Early 1800s, physicist John Dalton believed atoms were tiny particles that made up elements, cannot be divided and that the atoms of the given elements have to be the same. The famous Joseph John Thomson later built upon this theory after cathode rays had been discovered. He viewed that these particles had negative charges and could be deflected by both magnetic and electric fields. Also, he noticed that their masses were incredibly small. Thomson therefore proposed atoms were made up of electrons moving around in a ‘sea’ of moving positive charge. His model is commonly referred to as the plum pudding model.
The next major step in the history of the atom was Ernest Rutherford’s gold-leaf experiment in 1909. The experiment was directing alpha particles towards a sheet of gold foil. All deflection was measured and Rutherford calculated and determined that a plum pudding atom would have lots of deflection. The results were astounding as most particles were not deflected and the small percentage of particles that were deflected was deflected through large angles. Very few particles were actually deflected towards the source of alpha emission. Here is an image of Rutherford’s gold leaf experiment.
In 1911, Rutherford proposed a creation of a new atomic model based upon the results of gold leaf experiment. He proposed that most of the atoms mass is within a nucleus which would be positive and that the negative electrons were orbiting the nucleus just like planets orbit the sun. Also, the overall positive and negative charges must be balanced. In 1918, Rutherford discovered the proton further proving his model and in 1932, James Chadwick discovered the neutron concluding that Rutherford’s discovery was indeed fact. The image below depicts
Rutherford’s ideas graphically in this version of the atom.
The Bohr model was another step towards the latest model of the atom as he attempts to create the connection between atoms and light. He believed that the colors of light in a gas correspond differently to energy levels within electrons. The key to his model was that electrons can only be at certain energy levels within the atom. The model depends on the connection between the difference of energy level and the frequency corresponding to this change. A significant statement of Bohr’s model was that for certain elements, only certain frequencies of light can be absorbed or emitted. This can be linked to what we now know as the absorption and emission spectrums. The diagram below should hopefully make more sense, as Bohr’s model can be rather confusing.
Now the more recent, Schrodinger and Heisenberg model mentions that electrons orbiting the nucleus no longer occur as it previously did with Rutherford and Bohr. The issue is that we cannot state the trajectory of an electron within an atom and all we can state are the probabilities of where the electrons might be. This is due to the uncertainty principle within quantum mechanics. The diagram below is officially the latest graphic image of the standard model of the atom and whether it is likely to change in the future well, we cannot rule this possibility out.
Now in the 21st century we have discovered all sorts of particles, hadrons, bosons, baryons, mesons, quarks, leptons and all the antiparticles so many believe that there is no further change arising. However, with the continued research of dark matter and exotic behavior with electrons theorized into being split to their electric and magnetic one dimensional fields, no one knows where this will take us. The deficiencies of the standard model occur due to the lack of knowledge of the origin of mass, neutrino oscillations, matter and antimatter asymmetry and the nature and mystery of dark matter. These are only a fragment of the possibilities of change within the atom and perhaps it may remain as the electron cloud model but it is the determination and inspiration of those that aspire to change the unchangeable, think the unthinkable and do the undoable that makes the world progress therefore by no means will the current model of the atom be the final one.