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From Democritus to Chadwick Ancient History Democritus An Ancient Greek Philosopher Claimed that all things were made up of tiny particles called atoms Aristotle Claimed that all matter is made up of 4 elements Earth Water Fire /Heat Air John Dalton: Billiard Ball Model Claimed that all matter was made up of tiny particles that could not be broken down into anything smaller -- called atoms Model: Billiard Ball Hard, indestructible ball Dalton’s 5 Postulates (Ideas) All matter is made up of small particles called atoms Atoms cannot be created, destroyed or divided into smaller particles All atoms of the same element are identical in mass and size All atoms of different elements differ from each other in mass and size Compounds are created when atoms of different elements link together in different proportions J.J. Thomson: Plum Pudding Model (Raisin Bun Model) JJ discovered that matter had separate negative particles (electrons) Claimed that an atom could be broken down into positive and negative parts. The tiny electrons (negatively charged particles) were embedded within a “cloud” of protons (positively charged particles) Model: Raisin Bun Raisins= electrons Bun = protons Dominant model for many years. Ernest Rutherford A Scientist working for JJ Thomson Famous for “Gold Foil Experiment” Fired alpha particles (positively charged helium nuclei) at a thin piece of gold foil Expected that the heavy, positively charged particles would pass straight through the atoms of gold in the foil (based on Raisin bun model). NOT WHAT HE FOUND…WATCH… Rutherford’s Results Alpha particles Went straight through Went through but on angles, Even bounced back (straight/on angles) Conclusions The positive charge and mass of an atom must be concentrated in a very small area at the centre of an atom -- called the “nucleus” Electrons were a problem: Rutherford left them in clouds around the nucleus which can’t work as they would fall into the nucleus. Rutherford Model Electron Clouds Nucleus Niels Bohr A Scientist who used the solar system to model the atom! Claimed that the electrons must orbit the nucleus in fixed energy levels -- these levels are called shells The nucleus contains protons Chadwick A Scientist working for Rutherford Discovered that the nucleus contains protons and ALSO NEUTRONS Neutrons are neutral (do not carry a charge) These are what we call “Bohr-Rutherford diagrams” Bohr-Rutherford Diagrams Atom Parts http://www.youtube.com/watch?v=O5iaw5WNuB0 Protons Positively charged particles Located in the nucleus (centre) of an atom Using the Periodic Table: the Atomic Number (Z) tells how many protons exist in an atom Symbol: p+ Electrons Negatively charged particles Smallest Sub-atomic Particle Located outside of the nucleus (centre) Orbit around the nucleus in rings (shells) Using the Periodic Table: # of Electrons are equal to # of Protons in atoms (not ions) For atoms the # of electrons is equal to the Atomic Number (Z) Symbol: e- Neutrons Have a neutral charge or “no charge” Located in the nucleus (centre) of an atom Atomic Mass = protons and neutrons together Using the Periodic Table: neutrons are equal to Atomic Mass (A) - Atomic Number (Z) Symbol: no Atomic Info Atom quick representation A Z X Example: sodium is element #11 and has a mass of 23. It can be represented as 23 11 Na It has 11 protons, 11 electrons and 12 neutrons! (23-11 =12) Class/Home work Pg 233 #3, 4, 5 AND pg 240 #1, 3, 6