The Atom in History

 

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Most people think that the atom is a very boring thing to study.  However, that can’t possibly be true – after all, lots of people have studied it for thousands of years.  It must be exciting!  Let’s take a look at what we’ll discuss here:

 

  • The Greeks talk and talk and talk about atoms.
  • Some new ideas crop up from somewhere or other.
  • Dalton actually says something smart about atoms.
  • Thomsen plays with electricity.
  • Rutherford plays with gold.
  • Rappin’ Neils Bohr and the planetary model

 

Let’s get started!

 

 

The Greeks and their imaginary atom:

 

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 was one of these guys.  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.

 

 

Some ideas that nobody had ever thought of before:

 

For a really long time, nobody really thought that Aristotle was wrong.  Eventually, however, with the advance of science, people started to rethink their devotion to the dead Greek guy.  Here are some of the discoveries that changed this:

  • 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.  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.  Seems like a lot of words for a simple idea, huh?

 

 

Dalton and his not entirely imaginary atom:

 

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).
  • 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).
  •  

Overall, people seemed pretty happy with Dalton’s laws.  That is, until his idea of what atoms are like was disproved by…

 

 

Thomsen and his cathode ray tube:

 

Since every chemistry textbook in the world shows a picture of the cathode ray tube experiment, I’m not going to reproduce it – I suggest you turn to it, though, since it might help with my explanation.

 

Anyway, one day Thomsen was goofing around the lab with these cathode ray tubes he found somewhere.  What he found was that when he connected these big long hollow tubes to batteries, a beam of light would go from one end to another.  Since he had a lot of time on his hands, he decided to figure out what the deal was with the light.  After all, 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.

 

Among other things, 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. 

 

This led directly to his “plum pudding” model of the atom, named after a dessert that nobody can eat without throwing up.  Think of a chocolate chip cookie, instead.  His idea was that the dough in the chocolate chip cookie made up most of the atom and that it had positive charge.  The chips represented the little tiny bits of negative charge that made up the light he was messing around with – unlike the dough, they could leave the atom if you gave them a shove (with a battery, for example).

 

For this discovery, Thomsen is forever known.  This, despite the fact that his model was almost instantly disproved.

 

 

Rutherford and his gold foil:

 

Rutherford was a scientist who liked to play with radioactive stuff.  His favorite radioactive thing to play with was alpha particles, which are helium nuclei (they have a mass of 4 amu and a charge of +2).  One day, he decided to shoot a bunch of alpha particles at a really thin piece of gold foil.

 

When he did this (again, you can find MUCH better pictures of this in your textbook than I can make), he found that most of the particles went right through the foil, while some of them either passed through or bounced off at irregular angles.  Why is this?

 

His idea was to come up with a model of the atom in which most of the atom is empty space with electrons floating around in it.  The protons, however, are all concentrated in the middle of the atom (called the nucleus) – according to this model, the positively charged alpha particles would go straight through the atom most of the time and only be deflected on the rare occasions when they passed very close to the tiny nucleus.  For this discovery, we will always know Rutherford as the man with the gold foil.  And the bad model of the atom, because we now know that this model isn’t right, either.

 

 

Random interlude:  Chadwick and the discovery of the neutron

 

In 1932, James Chadwick discovered the neutron (which has no charge at all) by doing some really complicated experiments that I don’t understand even a little bit.  Don’t worry, though – your teacher probably doesn’t understand it either (unless they’re a nuclear scientist or something), so if you just remember that Chadwick discovered it, you’re probably fine.

 

 

Neils Bohr and the planetary model:

 

As tends to be the case with models of the atom, nobody really bought into Rutherford’s model for very long because it turned out to be wrong.  Neils Bohr was one of the guys that didn’t buy it, due to the discovery that when you add energy to an atom, it gives off light that has only a few very particular colors.  Since Rutherford’s model didn’t explain how this could be, Neils (as his buddies called him) came up with a different model (which, as is the case with many things, is much better drawn in your textbook).

 

His idea was that electrons traveled only in certain circular paths around the nucleus, much as the planets circle the sun.  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).  Since the world likes to exist at low energy more than at high energy, the electrons eventually return to their ground state orbitals.  When this happens, the energy that they absorbed is given off as light.  Since the color of light is very closely related to its energy, you only see very particular colors of light being given off by the very particular energy differences between the ground state and the excited state. 

 

His idea further went on to say that the energies of the orbitals were different for every atom.  As a result, the colors of light given off by every element is unique, which allows us to identify them via the magic of spectroscopy (specifically, this phenomenon is called atomic emission spectroscopy).

 

Like all models of the atom, this was overturned in about 15 minutes by a bunch of guys who invented something called quantum mechanics.  However, don’t feel sorry for the planetary model of the atom – it still has a healthy and thriving life in elementary school textbooks (which for some reason refuse to acknowledge the existence of quantum mechanics).

 

 

© 2007 Ian Guch – All Rights Reserved