The History of the Atom

The History of the Atom
The History of the Atom

Democritus first proposed an atomic model. He said that all matter could be divided into smaller and smaller pieces until a single indivisible particle was reached. He called this particle an atom after the Greek word ‘atomos’ meaning not cuttable. He believed that different atoms are of different sizes, have regular geometric shapes and are always in motion. He also said that there is empty space between atoms.
Aristotle severely criticized Democritus’ theory, arguing that the idea of atoms in continuous motion is a void is not logical. He supported the ‘four element’ theory of matter (proposed by Empedocles a century earlier. This theory is based on the idea that all matter is made up of four basic substances; earth, water, air and fire. He said each of these basic substances had different combinations of four qualities: dry, moist, cold and hot.
Dalton expanded on the atomic theory proposed by Democritus. It consisted of the following statements: all matter is composed of tiny, indivisible particles called atoms, all atoms of an element have identical properties, atoms of different elements have different properties, atoms of two or more elements can combine in constant ratios to form new substances, in chemical reactions, atoms join together or separate from each other but are not destroyed. This theory explains the law of Conservation of Mass. He discovered that atoms are only rearranged during chemical reactions, atoms are neither created nor destroyed in chemical reaction and therefore there will be no change in mass during chemical reactions. His theory explains the Law of Constant Composition, which is that atoms combine to form molecules in a fixed ratio in a given chemical reaction. He also said that since the atoms of an element have identical properties, such as mass, and combine I constant ratios, every compound must have a fixed, definite composition.
Thomson did experiments using Cathode ray tubes. He proposed that cathode rays were subatomic (particles smaller than an atom). These were called electrons. He said that electrons are negatively charged and distributed inside the atom, which is a positively charged sphere containing mostly empty space.
Nagaoka represented the atom as a large, positively charged sphere surrounded by a ring of negative electrons.
Rutherford was famous for his gold foil or alpha particle experiment. He ‘shot’ alpha particles (small, positively charged particles produced by radioactive decay) through very thin pieces of gold foil. Jos prediction was that all the alpha particles would travel through (in a straight line) the foil not really affected by the atoms of gold. (This was based on Thomson’s model of the atom and the idea that the atom was composed mostly of empty space). The result was that most of the alpha particles did pass through the foil, a small percentage of particles were deflected at large angles. The conclusion was that an atom must contain a positively charged core, which is surrounded mostly by empty space containing negative electrons. He called the core the nucleus. He invented the word proton for the smallest unit of positive charge in the nucleus.
Chadwick then showed that the atom nucleus must contain neutral articles as well as positive and negative particles. He called these neutral particles neutrons.
By: Dominic Aston 3DAG