Atomic Theories

The Greeks! Even though the Greeks had very little in the way of high technology, they still felt that they could use the power of their brains to figure out what matter was made up of at the smallest levels.  As a result, lots of them talked about it a lot.

Democritus He came up with a model of the atom that said: Atoms are solid and indestructible. Different atoms have different shapes and sizes – this is why different materials have different properties. Because Aristotle disagreed with him and everybody thought that Aristotle was a big hotshot, practically nobody paid attention to Democritus.  The moral of the story:  Don’t mess with Aristotle.  Or something like that.

Law of Conservation of Mass The amount of stuff you form in a reaction is equal to the amount of stuff you started with.

Law of Definite Composition  Every chemical compound has one and only one chemical formula.  For example, no matter what process you use to make water, the formula will always be H2O.

Law of Multiple Proportions  If two elements can combine to form more than one chemical compound, the ratio of the mass of one element that combines with a fixed mass of the other element will be a whole number ratio for the compounds. 

Law of Multiple Proportions Since this doesn’t make any sense, let’s use the example of two compounds where hydrogen reacts with oxygen:  H2O and H2O2 (hydrogen peroxide).  In the first compound, the amount of oxygen that’s needed to combine with 2 grams of hydrogen is 16 grams.  In the second compound, the amount of oxygen that’s needed to combine with 2 grams of hydrogen is 32 grams.  Since the ratio of 32/16 works out to a 2:1 ratio, it follows this law.

Dalton’s Atomic Theory In the 1800’s, some English guy named John Dalton came up with his own idea of what atoms were like.  His theory included the following ideas: Everything is made of atoms (which is true!) All atoms of an element are identical in every way (which is false, because of the existence of isotopes). Atoms of different elements are different (which is true).

Dalton’s Atomic Theory Atoms can’t be broken (which is true for chemical reactions, but not for nuclear ones). Atoms combine in whole number ratios to form compounds (i.e. you can’t have half an atom in a compound – this isn’t really surprising, given his idea that you can’t break an atom) – this is true. In chemical reactions, atoms are rearranged (this is true).

Thomsen’s Cathode Ray Experiment Found was that when he connected these big long hollow tubes to batteries, a beam of light would go from one end to another. If there was nothing in the tube to start with, where’d the light come from? He figured, it must come from the electrodes – since the electrodes were made of atoms, the atoms must somehow be coming apart.

Cathode Ray Tube

Thomsen got a magnet and held it near the beam Thomsen got a magnet and held it near the beam.  When he did this, he found that the beam would bend toward the positive side of the magnet and away from the negative side.  From this, he figured that the beam must contain very small particles from the atom and that they must have negative charge. 

So he developed the PLUM PUDDING MODEL of the atom: Basically, the negative particles are mixed throughout the atom like plums in pudding… or chocolate chips in cookie dough! *Note: His model was almost instantly disproved

Plum Pudding Model

Rutherford and the gold foil He found that when he shot alpha particles (Helium nuclei) at gold foil some particles passed through and some bounced either straight back or at angles. This led to his development of an atom model in which the electrons are located in the mostly empty space and the protons are found in the middle.

Rutherford’s model of the atom It was a bad model

Discovery of the Neutron In 1932, James Chadwick discovered the neutron (which has no charge at all) by doing some really complicated experiments

Niels Bohr Neils Bohr was one of the guys that didn’t buy Rutherford’s model, due to the discovery that when you add energy to an atom, it gives off light that has only a few very particular colors.

Niels Bohr He came up with the idea that electrons traveled only in certain circular paths around the nucleus, much as the planets circle the sun.

Bohr’s Model

Bohr’s Model  When energy is added to the electrons, the electrons jump from their normal orbit (called the “ground state” orbital) to a higher energy orbital farther from the nucleus (called the “excited state” orbital).