January 3 – Rutherford's Physics Trumps Alchemy

Posted on January 3, 2015

This sculpture of Rutherford "splitting the
atom" and discovering the nucleus (although
in reality, he wore clothes as he did his
famous experiments) is found in Eastern
China, in the Qingpu Oriental Land Garden.
For centuries people had tried to create gold—precious, beautiful, heavy, shiny gold—out of “base metals.” All the greedy tinkering with materials in this attempt—experimental messing-around that went by the name alchemy—actual grew into the experimental methods of an actual science, chemistry. (Do you see the relation of the words alchemy and chemistry?)

But, it turns out, no amount of chemical tinkering can turn one element, such as lead, into another. No gold for the alchemists, no matter how hard they tried!

On this date in 1919, Ernest Rutherford did not create gold out of another element, but he did use gold foil in his experiment, and he did succeed in—for the first time ever—using science to change one element into another: 

He sent alpha particles through thin gold foil and into pure nitrogen.

And he created oxygen.

(Alpha particles are helium nuclei: two protons and two neutrons. Actually, when Rutherford sent the helium nuclei into pure nitrogen, he not only created oxygen but also leftover protons.)

This was an incredible step forward for physics, but in a way it is all of a piece with Rutherford's other accomplishments. Because of Rutherford's experiments with radioactive materials, the gold foil experiment, and later a fully controlled splitting of a nucleus, Rutherford was able to develop a model for atoms that was much more accurate than earlier models.

Um...what exactly are atoms?

Matter is largely made up of atoms. In the way olden days, people used atom to mean the smallest possible quantity of matter, which could not be further divided.

Indivisible? Then...why do we need a model? If it's indivisible, how could there be structure?

Of course, the important words in the last sentence are “in the way olden days.” Modern scientists of the early 1900s knew that there must be structure in atoms, and although they could not be divided by chemical means, Rutherford showed that natural radioactivity was in fact atoms disintegrating into smaller parts. Theoretically, humans could deliberately “split atoms.”

So what model of atomic structure did scientists come up with?

In 1904 J. J. Thomson suggested the plum pudding model of an atom.

Electrons had been discovered in 1894, and Thomson suggested that the negatively-charged electrons were scattered about in a sort of positively-charged “soup” or pudding, like raisins in plum pudding or blueberries in muffin batter.

And Rutherford disproved this model?

When Rutherford ran his gold foil experiment, he hypothesized that the alpha particles would continue through plum-pudding atoms uninfluenced by their consistent “mixture” type of structure. However, he discovered that a small portion of alpha particles were deflected, which would only make sense if the positive charge in an atom was found only in a small central area rather than throughout the entire atom.

Do we still use Rutherford's model?

Yes and no. Thanks to Rutherford and Niels Bohr, we know that protons (+ charge) and neutrons (no charge) are in a central nucleus, but our quantum physics insists that this familiar looking atomic model, which has electrons orbiting the nucleus rather like planets orbit the sun...

...is too simplistic. Instead, electrons form a “probabilistic” cloud around the dense nucleus.

But quantum physics is hard to draw or even think about. Most “structure of the atom” model-making activities go back to the simpler Rutherford-Bohr model. 

Also on this date:

Fruitcake Toss Day

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