ATOMIC STRUCTURE
Models or Theories (C1,3,A2) Allow scientists to communicate their ideas to others. A model should be able to explain previous observations and predict future outcomes.
(greek for indivisible) HISTORY OF THE ATOM Democritus develops the idea of atoms 460 BC he pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called ATOMA (greek for indivisible)
HISTORY 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 them ATOMS
Dalton’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.
Thomson’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.) https://www.youtube.com/watch?v=kBgIMRV895w (15min)
HISTORY OF THE ATOM ELECTRON Joseph John Thompson 1898 found that atoms could sometimes eject a far smaller negative particle which he called an ELECTRON
HISTORY 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 charge like plums surrounded by pudding. PLUM PUDDING MODEL
HISTORY OF THE ATOM Ernest Rutherford 1910 fired (+ve) particles (alpha particles) at a thin sheet of gold foil, he expected the particles to go straight through
Rutherford’s Gold Foil Experiment helium nuclei helium nuclei His team 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.
Another view of Rutherford’s Gold Foil Experiment
HISTORY 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 there was a small, dense, positive charge in a central nucleus and a large, negative electron cloud held in place by electrical attraction to the nucleus. However, this was not the end of the story.
HISTORY 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.
Bohr’s Atom electrons in orbits nucleus
HELIUM ATOM Shell proton N + - + N - neutron electron
ATOMIC STRUCTURE Particle Charge Mass Found Proton +ve heavy nucleus Neutron neutral Electron -ve light outside nucleus
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