5.5.2 What's in an atom?

Inquiry: How can we know what an atom is like?

Maybe you remember from the beginning of the course in 4.1.1 Powers of Ten that you explored what the Universe looked like from the very smallest to the very largest scales.

In the last inquiry 5.5.1 Have you ever seen an atom? you explored the evidence that atoms exist, and we saw that Brownian motion was one suggestion that there were tiny particles, invisible (then) to even the most powerful microscopes.

In 1897, the British physicist J. J. Thomson made another remarkable discovery. He was experimenting with a newly discovered ‘ray’ of energy that seemed to be produced by some metals when they were heated and given a relatively negative electric charge. Since a negative electrode is called a cathode, these rays had been called ‘cathode rays.’ But Thomson's experiments using a Cathode Ray Tube (CRT) suggested that the rays were not actually ‘beams’ of energy - as light was considered to be - but consisted of tiny, electrically charged 'corpuscles'. 

This simulation app shows you how Thomson's CRT experiment worked (there is a full experiment activity you can try later!) Try:

Controlling the accelerating voltage V_{a} at a certain value

Changing the plate voltage V_{p}. Observe how the cathode ray (electron beam) moves.

With V_{p} fixed so that the electron beam is curving, change the current I through the magnetic coils. Can you find a current that makes the electron beam straight again?

The CRT experiment showed that the cathode ray was affected by electric and magnetic fields. We now know that these ‘corpuscles’ are particles, which we call electrons because they carry a negative electrical charge. Thomson was able to use his experimental evidence to determine a value for the ratio of the charge to the mass of these particles \frac{e}{m}

Thomson's discovery showed that there were particles even smaller than the estimated size of atoms. Thomson speculated that the electrons came from inside atoms. Since it was known that atoms were electrically neutral (did not carry a charge) unless ionised, he assumed that the rest of the atom was made from some positively charged matter, and the electrons were scattered around inside like fruit in an English ‘plum pudding.’ You can imagine a ‘plum pudding’ as a bit like a blueberry muffin!

So how did we understand the structure of matter now? As made from solid atoms, mostly made from positively charged ‘stuff’, but with some bits of negatively-charged stuff scattered through them. If we visualised what a thin sheet of, say, gold looked like, it would look like this:

Meanwhile, the quest to measure the size of an atom continued. At the University of Manchester in 1911, two experimental physicists Hans Geiger and Ernest Marsden were working on a project with their supervisor, Ernest Rutherford. They were trying to measure the size of atoms by irradiating a thin sheet of gold (gold foil) with another new discovery, ‘alpha radiation.’ 

In the experiment, a small piece of radioactive material in a lead container was used to produce a beam of ‘alpha rays.’ The rays were directed into an evacuated vessel at a very thin gold foil. The gold foil was surrounded by a screen that was coated with Zinc Sulphide, a chemical that would produce flashes of light when the alpha rays struck it.

Rutherford expected that the alpha rays would be deflected by the atoms in the gold foil, and they thought that they could determine the size of an atom by measuring the angles at which the flashes of light were produced by the alpha rays.

The stories about what happened next vary. Most historians think that Rutherford suggested one day that Marsden have a look to see if there were any flashes in front of the foil, where alpha particles were deflected back on their own path. If the atom was a solid ‘plum pudding’ this was thought very unlikely, because the charge of the positive ‘stuff’ was not thought to be concentrated enough to make an alpha particle go back on its tracks. Another story is that Marsden was checking the calibration of the microscope used to observe the flashes when he accidentally saw flashes occurring in front of the foil. While only about 1 in 8000 of the alpha particles were deflected in this way, it was very hard to explain with the ‘plum-pudding’ model. Rutherford commented “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Inquiry:  How do we represent the structure of atoms? 

Rutherford's interpretation of the results is often referred to as ‘splitting the atom,’ because he calculated that the alpha-particles could only be deflected back on their path if they encountered a very strong positive electrical charge that repelled them. Since the overall charge on the gold atoms was already known, this suggested that the positive charge was all concentrated in the middle of the atom. In short, Rutherford, Geiger and Marsden had discovered the nucleus.

When Rutherford ran the numbers, he realised that the nucleus was relatively tiny when compared to the atom as a whole. The atom was mostly empty space! To get a sense of this, imagine a peanut sitting on the centre-point of a soccer stadium! The peanut is the nucleus and the stadium is the atom!

A little later, in 1913 Danish physicist Niels Bohr proposed that the electrons were organised in orbits like those of the planets around the sun. Each orbit had its own specific or quantised energy - this was the beginning of quantum physics. 

Bohr's model is that used to visualise atoms and explain how they react chemically - although the story didn't end there…