• Greek definition of an atom
• Dalton’s model of the atom
• Examples of pure elements that we see in everyday life.
• The chemical composition of life, the earth, and the stars
• Thomson’s discovery of the electron, the “raisin bun” model of the atom
• Rutherford’s experiment and his model of the atom
• What’s wrong with the Rutherford model?
• Terms: element, atom, molecule, electron, nucleus, proton, neutron, ion

• What is the smallest particle known? The Greeks gave this some thought. They imagined that if you kept cutting an object up into small pieces, say keep cutting it in half, then eventually you will not be able to cut it any smaller. The leader of the Greek discussion group (530 BC) was Democritus. He coined the phrase for the smallest indivisible piece as “the atom”. This means “that which cannot be divided further”.
• This was not really science that they were conducting, rather it was philosophy. The Greeks did not follow the interplay of observation and hypothesis that characterizes science.

• Modern atomic theory began with the work of the Englishman, John Dalton. In 1808, he published a book that introduced the modern concept of atoms. His argument went like this:
• Most materials can be broken down into simpler compounds. For instance, burning wood results in carbon dioxide, water, and ash. As another example, if you use an electric current to break down water, then you end up with two gases, hydrogen, and oxygen.
• Furthermore, no matter how much of the original material you broke down, you always ended up with the same stuff. E.g., water always ended up with H and O and always in the same proportions (8 O and 1 H by weight). (How would you weigh hydrogen?)
• Some materials could not be broken down. For instance, charcoal (pure carbon) could not be broken down further.
• The materials that could not be broken down were called chemical elements.
• Dalton suggested that for each chemical element there is a corresponding species of very small, indivisible objects called atoms. He borrowed the word from the Greeks. Dalton's atoms were envisioned to be small, hard spheres, like small bowling balls.
• Two or more atoms that are stuck together to form an isolated group was to be called a molecule. For instance, water, H₂O, is a molecule.
• This model was highly successful in describing what was currently known, and was accepted immediately by almost everyone. It explained

1) why chemical elements could not be broken down. They were made of only one type of indivisible atom.
2) The regular proportion of elements in compounds. E.g. the O and H in water.
• We still use the word “atom” for historical reasons, although we now know that the atom is made of even smaller parts.
• Scientists now recognize 92 naturally occuring elements. Furthermore, we have also artificially created elements through to 112, as well as 114, 116, and 118. Elements 113, 115, and 117 have not yet been found.

• At the time of Dalton’s work, only about 30 elements were known. However, the invention of the battery by Volta allowed the use of “electrolysis” (splitting by electricity) to break down molecules into elements. This quickly led to a doubling of the number of known elements.
• This is an example of how theory and experiment work together.
• In 1869, Dimitri Mendeleev invented the periodic table. He organized the elements in terms of their atomic weights, and put them into columns and rows. The columns were organized by common and distinctive chemical properties.The table had holes in it where no elements were known to exist. The holes provided a guide for finding new elements.
• Most objects that we see are compounds, made of molecules or crystalline materials. But there are several examples of pure elemental materials that we see often.

1) Helium: a gas that is lighter than air. It is used to fill balloons etc. It is the only compound that does not form a solid upon cooling.
2) Carbon: pencil lead, charcoal, diamonds are all examples of pure carbon. The differences between them are related to the different ways that the atoms are bonded together. We will look into this in a couple of weeks.
3) Aluminum: a lightweight, silvery metal. The surface is usually oxidized to aluminum oxide, or corundum, which is the same thing as ruby or sapphire. Aluminum was so rare at the turn of the century that at the world’s fair in 1908, it was worth more than 10 times its weight in gold.
4) Copper: reddish metal of pennies and pots, and many wires. It is cheap to purify.
5) Gold: yellow, soft metal.
• The earth is made mainly of 4 elements, O, Si, Mg, Fe. Most of the surface of the earth includes these 4 elements as well as Ca and Al. Together they account for about 99% of the mass of the earth. So most inanimate things that we see are made of these 6 elements.
• As far as the living world goes, the important elements are C, O, H, N, P and S.
• Most stars are formed of H and He.

• Dalton’s concept of the atom was not to last.
• In 1897, John Thomson identified the electron, an object with negative charge and much smaller and lighter than an atom. He did this while studying cathode rays. If a tungsten filament (the cathode) is heated, by application of voltage in a vacuum, then a stream of electrons is emitted that flies to a positively charged plate. Thomson figured out that the "cathode rays" were charged particles and, in fact, were electrons. He won the Nobel Prize for this. “If one accepts the hypothesis that elementary substances are made up of atoms, one cannot escape the conclusion that electricity itself is divided into elementary parts which behave as atoms of electricity”.

• All electrons are alike, and can be obtained from any material, even from pure elements. Therefore they are part of the atom and the atom cannot be indivisible.
• The atoms must look like a raisin bun, with the bun as the atom, and the raisins as the electrons.

1) element: A chemical substance that cannot be broken down
2) atom: the smallest particle that retains its chemical identity
3) molecule: any collection of two or more atoms bound together
4) electron: an atomic particle with negative charge and low mass
5) nucleus: the small, massive, central part of an atom
6) proton: positively charged nuclear particle
7) neutron: electrically neutral nuclear particle
8) ion: an electrically charged atom

• In 1911, a New Zealander, Rutherford, made the most important discovery about the nature of the atom. He started out with a radioactive substance that emitted alpha particles (He nuclei). They behave like small bullets. He made a stream of alpha particles that intercepted a piece of gold foil.

• The existing theory of the atom was that of the raisin-bun model. Rutherford was shooting atomic bullets into the bun to see what would happen. The results were quite surprising and it took him a couple of years to make sense out of them. Most of the alpha particles, say 999/1000 passed right through the gold foil like it wasn’t there. However, 1/1000 were deflected, some by a large angle, some by a small angle, and some deflected right back.
• His model was that the atom is a lot of empty space with a very small, but dense nucleus. The nucleus is about 1/10,000 of the size of the atom. The nucleus is positively charged with the charge due to particles called protons. For every proton in the nucleus there is an electron that is whirling around in orbit, like a planet around the sun. The result is a neutral atom.
• The electrons are not held rigidly to the nucleus but are somewhat weakly bound, and can be detached. In such a case, we have an atom with a charge, called an ion.
• In 1932, when he discovered how to split the atom, it was determined that the nucleus is made of both protons and neutrons.
• Protons are small and positively charged, neutrons are neutral. The neutrons are the “glue” that hold the nucleus together.

• The image of the Rutherford atom is elegant and it is the most common one that people think of, but it does not really work. It seems analogous to the solar system with a massive central part (sun vs. nucleus) and a lot of empty space that contains electrons (planets) whirling about in their orbits.

• However, the electrons, unlike the planets, are electrically charged and a charged particle in circular orbit is accelerating and thus must give off radiation. The radiation has energy and so the energy must come from somewhere (conservation of energy). It must come from the atom. Thus, the energy of the atoms must decrease. Calculations show that the lifetime of the Rutherford atom is less than 1 sec.
• Therefore, the Rutherford model cannot be true.