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History of The Atom Early Ideas of The Atom • Video: Democritus to Dalton History Of Atom • 300 B.C. – Democritus: smallest piece of matter could not be divided any further “Indivisible” • 1805 – Dalton: “Atomic Theory” – Pure substances/elements are made up of atoms – Atoms of the same element are exactly alike. – Atoms could not be created or destroyed or divided into smaller particles. – Compounds formed by joining 2 or more elements. 1805: Dalton’s Billiard Ball Model Atom is “indivisible” • Video: Introduction to particle accelerators 1875: Crookes - Crookes used gas discharge tubes, a screen and a bar magnet to study atoms - discovered: the magnet could deflect the beam Video: Cathode tube gas discharge tube Closed tube with low pressure electrode 1 gas-filled tube electrode 2 cathode ray Place a paddle wheel in the path of the ray and it moves! This means the ray must have some mass! 1897: Thomson’s Discovery of the Electron • Video: Thomson’s particle accelerator - Used a particle accelerator and screen to “see” effects - varied the voltage and measured the bending of the beam - Deduced the mass was 2000X lighter than the atom….this was the ELECTRON! - The rest of the atom is a sphere of positive charge Thomson’s Raisin Bun Model Let’s Review: 1909 Millikan’s Oil Drop Experiment • Determined the CHARGE of an e- by balancing an electrically charged oil drop and later the MASS of the e- was determined 1909: Rutherford’s Gold Foil Experiment • Video: Gold Foil experiment What happens when α particles hit the atoms in the gold foil? radioactive source gold foil Something dense and very small must cause the particles to change direction…the NUCLEUS! Rutherford’s findings: 1. The atom contains an extremely small, dense NUCLEUS (positive) 2. The atom is mostly EMPTY SPACE! (because only some particles (1/8000) were deflected) Rutherford’s Nuclear Model -Electrons orbit the nucleus like planets orbit the Sun What is the problem with the Rutherford model? • Rutherford’s nuclear model does not obey classical laws of physics….as electrons orbit around the nucleus they continuously lose energy and therefore they should spiral into the nucleus! Atoms can’t exist!?!? 1913: Bohr • Rescued Rutherford’s model with his studies of spectroscopy • Spectroscopy is the analysis of light The Hydrogen Line Spectrum violet blue green red Bohr determined that e- are not allowed to orbit “anywhere” they must only occupy certain well defined orbits -e- exist in defined/fixed orbits -“nothing in between” Bohr’s model: similar to planets orbiting around the Sun sun If electrons can be found in discrete places can we know precisely where an electron is? The Hydrogen Line Spectrum violet blue green red Where does the red line come from? Where does the green line come from? Where does the blue line come from? Where does the violet line come from? • Electrons exist only at discreet energy levels • When excited electrons in H fall back down to lower energy levels, they emit a photon of light 1932: Rutherford and Chadwick • Video: Rutherford discovers the nucleus - Many years later: 1932 Why? - Discovered that the mass of the nucleus does not equal the mass of protons only - This means the nucleus is made up of 2 parts: NEUTRONS and PROTONS! Let’s Review • Song: Atomic Theories 1900: Planck - Determined that electromagnetic energy could only be emitted in quantized amounts - Quantum = a unit or packet of energy∴ - There are no partial amounts - These quanta are now called PHOTONS! (Einstein) 1905: Einstein - A “photon” is a unit of light Heisenberg – not this guy! Heisenberg - Why? Because trying to observe the electron changes its direction! Video: Uncertainty Principle Quantum Mechanics • Since the mere act of observing an echanges it’s direction or momentum, we can only measure the PROBABILITY of finding an electron in a specific location. • Quantum tells us the statistical probability of finding an electron at a given location derived from wave equations and used to determine the shape of ORBITALS (not orbits!) The Wave Nature of Light • 1926 – Schrödinger – Wave Mechanics – The Electron is a particle but it’s behaviour is predicted by a wave – Electron wave permeated space and time Video: Quantum Mechanics Electrons are found in orbitals not orbits! Let’s Review • Song: Atomic Theories Summary • Video: The Space Between