Atomic Structure and the Alpha Scattering Experiment

John Dalton (1766 – 1844) John Dalton described atoms as tiny balls of material. He said the atoms of a particular element are all identical. These are original Dalton model atoms He used small wooden balls to model atoms and showed how they could combine to produce compounds

JJ Thompson (1856 – 1940) In 1897 at Cambridge University JJ Thomson discovered ‘Cathode rays’ On closer examination it was decided that these ‘rays’ were in fact tiny, negatively charged particles being emitted from atoms. He has discovered the electron! Suddenly it was realised that atoms were not the smallest particles that could exist. Thomson came up with a model for atoms based on his discovery. He said that because atoms themselves had no charge there must be a positive charge inside the atoms to cancel out the charge on the negative electrons. He said that atoms were solid lumps of positively charge material with the electrons scattered through it like the fruit in a plum pudding. ‘Plum Pudding’ model

Ernest Rutherford (1871 – 1937) In 1911 Ernest Rutherford, (a New Zealand scientist working in Britain) carried out an experiment that proved that atoms were not solid lumps of material as thought by Thomson but were in fact mostly empty space with a very small solid centre called the nucleus. He came up with a new model which had a solid, positively charged centre and the negative electrons in ‘orbits’ around it. Rutherford’s Atomic Model

Niels Bohr (1885 – 1962) In 1913, not long after Rutherford’s discovery a Danish Physicist called Niels Bohr carried out some measurements on the energy emitted and absorbed by atoms and came up with the theory that the electrons inhabited fixed orbits or shells. This became known as the Bohr model of the atom and is the one we use today. The Bohr model. A central, positive nucleus and the electrons in fixed orbits or shells around it. Larger atoms have more shells.

James Chadwick (1891 – 1974) In 1920 Rutherford came up with the idea that atoms must contain a third particle. He thought this because the masses of atoms that were being measured were heavier than you would get from just the masses of protons and electrons added together. He said that this particle would have no charge and that this would make it very difficult to detect. Following work in Germany and France the Neutron was finally identified by James Chadwick working in Cambridge. He realised that an unusual radiation that had been observed in some experiments was in fact a neutral, sub-atomic particle.

Back to Rutherford Rutherford’s Alpha Scattering Experiment The is one of the most important experiments in the history of science. Professor Rutherford wanted to see what happened to alpha particles when they collided with atoms. He asked two of his students (Hans Geiger and Ernest Marsden) to carry out the experiment.

The Alpha Scattering Experiment You are going to work in groups of 3 or 4 to figure out the details of the experiments and its results: Some information about the experiment is stuck up outside the room. You will have 4 opportunities to collect information. Taking it in turn, one person, a runner, will leave the room and look at the information. The first time, the runner will have 40 s. The second time 30s, third time 20 s, and final time 10 s. You will have 2 min in between each run to compile your information.

The Experimental Set-up Geiger and Marsden placed the microscope as shown. As expected, most of the alpha particles went straight through the foil. A few alpha particles were scattered by angles less than 90º, also as expected.

The Experimental Set-up Geiger and Marsden counted the tiny green flashes in the microscope produced when alpha particles hit the screen. Both Geiger and Marsden described this as one of the most difficult and boring experiments they’d ever had to do. Moveable Microscope Zinc sulphide screen (glows (scinntilates) when hit by alpha particles) Lead shield Source of alpha particles Vacuum (air pumped out) Very thin gold foil

The Experimental Set-up It would be a complete and utter waste of time! Why didn’t you put the microscope behind the gold foil? There’s no point! There is absolutely nothing inside an atom that could reflect an alpha particle! Marsden Geiger Rutherford

The Experimental Set-up Much to their surprise, a very small number of alpha particles (about 1 in 8000) bounced off the gold atoms! Under protest, Geiger and Marsden placed the microscope behind the gold leaf. They handed the results to Professor Rutherford who now had to explain what was going on.

Alpha () Particle Scattering Rutherford suggested that most of the mass of the atom is in a tiny positively charged centre called the nucleus and the electrons form a cloud within the ‘space’ of the atom. He also knew that the negatively charged electrons had too little mass to deflect the alpha particles. Therefore, the deflection must be due to the positive charges. Due to the small number of alpha particles that were deflected through these large angles, this meant that there must be large distances between these positive charges. The diameter of a gold atom is 0.3x10-9 m which is 0.3 nanometres (0.3nm). The diameter of the nucleus is less than 1x10-14 m. Air was pumped out of the apparatus to create a vacuum. This was to done to ensure that the alpha particles were not absorbed by any air. If an atom were the size of this screen, the nucleus would be 100 times smaller than a full stop. Rutherford knew that alpha particles had masses equal to those of light atoms (e.g. helium) so whatever deflected the alpha particles must have more mass. It has since been discovered that the nucleus is over 10,000 times smaller than the size of an atom and most of an atom (over 99.9%) is empty space. An experiment by Ernest Rutherford discovered alpha particle scattering. This gave a clearer insight into the structure of the atom. The actual work was done by two of his students: Hans Geiger and Ernest Marsden, who were later to make significant contributions in nuclear physics. However, a tiny minority, about one in eight thousand, were deflected through large angles (greater than 900). At the start of the 20th century, physicists knew that the atom contained positive and negative charges but the structure was still not understood. They fired alpha particles (from a radioactive source) at sheet of gold foil (gold leaf) that was one thousandth of a millimetre thick. At this width it is about 3,000 atoms across. They observed that the vast majority of the alpha particles passed straight through the foil with a few undergoing small deflections. A fluorescent screen was placed around the foil, that would produce a flash of light whenever it was hit by an alpha particle. gold foil positively charged nucleus alpha () particle fluorescent screen negatively charged electron 0.001mm gold foil (0.001mm thick) radioactive source NOT TO SCALE

Before Vs After Up until Rutherford’s experiment, it was thought that atoms were like a ‘plum pudding’ – the electrons were negative “plums” embedded in a ball of positive pudding. But, this is what he found. What do you think he deduced about the structure of an atom from these results?

Nobel Prize All science is either Physics or stamp collecting. The top scientific honour in the world is the Nobel Prize. Ernest Rutherford was awarded the Nobel Prize in 1908 for discovering the atomic nucleus. (He deserved it – it was his calculations based on the data that measured the size of the nucleus. Geiger and Marsden were given full credit in the published scientific paper.) All science is either Physics or stamp collecting. However, he was slightly disappointed because he was given the Nobel Prize for Chemistry instead of Physics Rutherford