Physics P4 atomic structure

Physics P4 atomic structure

1. In ancient Greece, most people thought that matter was made up of combinations of four elements: earth, air, fire and water. There were a small number of Greeks who had a different idea. They believed that if there was a piece of wood for example, it could be cut into smaller and smaller pieces until it ended up as a piece of wood that was so small it couldn't be cut anymore.

   • The ancient Greek philosopher Demokritos (460-370 BCE) thought that matter was made up of millions of tiny, uncuttable pieces of that same matter. In fact, the word atom comes from the word 'atomos', which means uncuttable.

2. After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding. To explain the two types of static electricity, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time: solids cannot be squashed, therefore the atoms which make them up must be solid throughout rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide.

3. The idea of the atom as the building block of matter has developed over time. What was thought of as a single particle about 1 × 10‾¹º m across is now known to be a collection of smaller particles.

   • Rutherford and the nucleus In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum. (Alpha particles are a form of nuclear radiation with a large positive charge.) The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else. Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking.
     • out to be very, very thin without cracking. Alpha radiation beamed to a thin piece of gold foil which is in a scintillation screen.
     • It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it. If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected.
     • The scientists were very surprised when other things happened: most of the alpha particles did pass straight through the foil a small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil a very small number of alpha particles came straight back off the foil Rutherford considered these observations and he concluded: the fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space a small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom - like charges repel, so the positive alpha particles were being repelled by positive charges the very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - the tiny number doing this means the chance of being on that exact collision course was very small, and so the 'target' bei
     • tiny Alpha raditaion beamed through gold nuclei, with most passing through unaffected, some deflected at small angle and very few rays being almost reflected back.
     • Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom. The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.

4. Further developments to the atomic model Bohr and energy levels Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model. In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus. By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.

   • Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom. Bohr's 'solar system' model of the atom is the way that most people think about atoms today. When atoms absorb energy, the electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus). In time, they jump back down to a lower level releasing light of definite frequencies.
   • es. Two images, one with an electron jumping an energy level as it absorbs radiation, the second image has an electron falling between energy levels as it emits radiation.
   • Chadwick and the neutron The proton had been proposed as being the nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass. However, there was still a difference between the atomic number of the atom and the atomic mass. Bohr and other scientists knew that there also had to be a neutral particle the same size as a proton to keep the nucleus stable and to make up the mass. This was accepted and scientists used models that included the neutron for a further 20 years before it was actually discovered. It wasn't until 1932 that James Chadwick was able to prove its existence. Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil. He was able to prove that a proton-sized neutral particle - now known as the neutron - existed.