Atomic Theory

The History of the Models of the Atom


Democritus was a thinker but not a scientist.  His idea was that everything was made up of "atomos", or indivisible ultimate particles.  Of course he thought there were atoms of fish, rocks, or water.  The word "atom" is his.
The Democritus "model" was a mini-speck of stuff.  He has no idea how to describe it really, this is some artist's interpretation of his words.
John Dalton was the English farmer who got bored with his chickens and corn, so he invented chemistry in his barn.  He used the idea of atoms, and gave each one a unique "mass", which somehow took care of all their different properties.
The Dalton model was similar to the first one (but it's in color!).  He did come up with the Atomic Theory (learn that!) and so he is the Father of Modern Chemistry.  It looks somewhat like the "two ball" in pool, it's known as the BILLIARD BALL model.
J.J. Thomson has the coolest name, and he DISCOVERED the electron!  Where to put it was a problem.  Clearly the atom could not be a billiard ball like Dalton suggested, so he fashioned it from his wife's dessert (her plum pudding).
The plum pudding model put the electrons "into" some sort of positive mass of the atom.  He KNEW the atoms were neutral but didn't really understand why.  Still, the man discovered the electron!
Earnest J. Rutherford is your teacher's chemistry HERO.  He did the Gold Foil experiment to help him interpret (correctly) that the atom has negative electrons flying around, making up a relatively LARGE volume.  But the nucleus must be positively charged to keep the atom neutral.
The Rutherford model is almost correct.  The positive nucleus is surrounded by "far away" electrons which are negative.  Atoms are neutral, except that the atoms seemed to be mostly not there.  He is credited with the discovery of the proton.
Neils Bohr, a student of Rutherford saves his teacher's model by postulating that the electrons are not flying "around" any which way, rather they are in ORBITS, like the planets around the Sun.
The Bohr model has electrons in ORBITS or ENERGY LEVELS.  Electrons can gain energy and jump up to higher energy levels (further from the nucleus).  When these unstable electrons emit that energy they can return to the ground state.  The energy given off is seen as SPECTRA.
This man is not in our curriculum (too bad).  He is James Chadwick, and in 1932 he discovered the neutron.  (wow).
The Modern, or wave-mechanical model of the atom keeps the electrons in energy levels (Bohr) but makes them NOT like planets, but more zones or regions of most likely at any moment.  The atomic model is now a tiny nucleus with protons and neutrons, surrounded by the "cloud" of where the electrons might be at any moment.
Another look at the wave-mechanical model, showing the regions/zones that the electrons travel in.  Electrons can act as waves of energy or bits of mechanical matter at the same time.  if you can't understand that (I can't either), welcome to advanced chemistry!