2Models or TheoriesAllow scientists to communicate their ideas to others.A model should be able to explain previous observations and predict future outcomes.
3(greek for indivisible) HISTORY OF THE ATOMDemocritus develops the idea of atoms460 BChe pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he calledATOMA(greek for indivisible)
4HISTORY OF THE ATOM ATOMS John Dalton 1808 suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called themATOMS
5Dalton’s Atomic Theory 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, but they are different in mass and size from the atoms of other elements.Compounds are created when atoms of different elements link together in definite proportions.
6Thomson’s Experiment: Cathode Rays Passing an electric current makes a beam appear to move from the negative to the positive end. (By adding an electric field, he was able to conclude that the particles were negative.)
7HISTORY OF THE ATOM ELECTRON Joseph John Thompson 1898 found that atoms could sometimes eject a far smaller negative particle which he called anELECTRON
8HISTORY OF THE ATOM PLUM PUDDING MODEL 1904 Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's chargelike plums surrounded by pudding.PLUM PUDDINGMODEL
9HISTORY OF THE ATOM Ernest Rutherford 1910 fired + particles (alpha particles) at a thin sheet of gold foil, he expected the particles to go straight though
10HISTORY OF THE ATOMgold foilhelium nucleihelium nucleiThey found that while most of the positive particles passed through the foil, a small number were deflected and, to their surprise, some positive particles bounced straight back.
11HISTORY OF THE ATOM However, this was not the end of the story. Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus.He suggested that the there was a small, dense, positive charge in a central nucleus. With this holding the large, negative electron cloud in place by electrical attraction.However, this was not the end of the story.
12HISTORY OF THE ATOM Niels Bohr 1913 studied under Rutherford at the Victoria University in Manchester.Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons.
14HELIUM ATOM + - + - Shell proton neutron electron What do these particles consist of?
15ATOMIC STRUCTURE Particle Charge Mass Found Proton +ve heavy nucleus NeutronneutralElectron-velightoutside nucleus
16number of electrons = number of protons ATOMIC STRUCTURE4HeAtomic massthe number of protons andneutrons in an atom2Atomic numberthe number of protons in an atomnumber of electrons = number of protons
17ATOMIC STRUCTUREElectrons are arranged in Energy Levels or Shells around the nucleus of an atom.first shell a maximum of 2 electronssecond shell a maximum of 8 electronsthird shell a maximum of 8 electronsfourth shell a maximum of 2 electrons
18SUMMARY The Atomic Number of an atom = number of protons in the nucleus.The Atomic Mass of an atom = number ofProtons + Neutrons in the nucleus.The number of Protons = Number of Electrons.Electrons orbit the nucleus in shells.Each shell can only carry a set number of electrons.
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