The Gold Foil Experiment and The 250-Million-Ton Pea: The Composition of the Atom

This Atom Is Not To Scale

In a recent post about isotopic abundance, I used a prototypical image of a lithium atom to illustrate the basic structure of an atom.  However, the image was deliberately not drawn to scale to make the protons, neutrons, and electrons visible.  Let’s look at the basic composition of the atom to see why, and we owe this understanding to Ernest Rutherford.  First, let’s give some historical background about what motivated Rutherford to conduct this experiment; we first turn to the Plum Pudding Model by J.J. Thomson.

The Plum Pudding Model

Before 1911, the dominant theory of atomic composition was J.J. Thomson‘s “plum pudding” model.  Thomson hypothesized that an atom consisted of electrons as negatively charged particles (the “plums”) “floating” in a “pudding” of positive charge.

plum pudding model

Plum Pudding Model of the Atom

Source: Wikimedia Commons

The Gold Foil Experiment

The Set-Up

In 1909, Ernest Rutherford, Hans Geiger and Ernest Marsden began an experiment to test the plum pudding model.  They bombarded a thin gold foil with alpha particles, which are positive charged and sufficiently energetic to pass through the uniformly distributed “pudding” of positive charge in the atom.  A zinc sulfide screen was swivelled around the foil to capture the positions of the alpha particles after hitting the gold foil.  According to the plum pudding model, the mass and the positive charge of the “pudding” should be widely distributed throughout the volume of the atom.  Thus, they expected the large and densely positive alpha particles to easily pass through the pudding of positive charge in the gold foil with minor deflections, if at all.  (The electrons are too small to alter the path of the alpha particles – recall that a proton is almost 2,000 times heavier than an electron, and an alpha particle has 2 protons.)

The Surprising Result

Most of the alpha particles passed through the gold foil with no or minor deflections.  To their surprise, however, some of the alpha particles bounced backward.  Rutherford said that it was “as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”.

gold foil experiment

Expected and Actual Results of the Gold Foil Experiment

Source: Wikimedia Commons

Rutherford’s New Model of the Atom

Since the large and highly energetic alpha particles could only have been repelled by similarly positive and large particles, and since there were so few of these rebounds, Rutherford concluded that the alpha particles must have hit small but dense masses of positively charged particles.  This led Rutherford to propose a new model of the atom, in which most of the mass of the positive charge is concentrated in the middle in the nucleus, and electrons fill a large volume of space – much larger than the nucleus itself – around the it.

Proper Scaling of the Atom

An atom consists of mostly space.  It contains a highly dense mass – the nucleus – at the centre, and electrons filling a very large volume of empty space around it.  Thus, all images of atoms that you see in chemistry text books are falsely scaled so that the nucleus is much larger than it should be and the electrons are much smaller and closer to the nucleus than they should be – it’s the only way for them to be visible.  To get an intuitive understanding of the true dimensions of the subatomic particles, let’s scale the atom to the size of a football stadium.  In that case, the nucleus would have the size of a pea, but it would have a mass of 250 million tons!

References

I have learned about and seen the gold foil experiment in numerous books, and it is a very well known experiment, so you will have no problem finding resources to learn more about it if you wish.  Despite its place in mere high school and introductory chemistry courses, this is one of the most profound experiments in the history of physics and chemistry, and it’s a nice example of how experimentation is at the heart of the scientific method.

In case you’re curious, I read “Chemistry” by Olmsted and Williams (3rd edition) to brush up on the details.  If you want to replicate the calculations of the density of a nucleus, eHow has a nice example.

Your thoughtful comments are much appreciated!

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