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The Earliest Models Atomic Theory The Structure of the Atom Democritus (440 BC) “Atomos” meaning indivisible All matter is made up of indivisible particles called “atoms” and the void, which is empty space between atoms This theory was abandoned due to the presence of the church and Aristotle Time John Dalton JJ Thomson – Atoms are indivisible and can’t be broken down All matter is made up of tiny particles All atoms of one element are identical Atoms of different elements have different properties Atoms create compounds by combining atoms of different elements Discovered Cathode Rays consist of negative particles through a series of three experiments The Rutherford Model Rutherford’s Nuclear Atom Model Alpha Particle Scattering Experiment Rutherford shot a beam of alpha particles (+) at a sheet of gold foil Most of the alpha particles went through without any significant deflection A small fraction had a large deflection A minute fraction rebounded The above results could not be explained by Thomson’s atom model The atom is mostly empty space All the positive charge is concentrated in a small volume called the NUCLEUS of the atom Electrons revolve around the nucleus like the planets of the solar system was added by Bohr Raisin Bun He did this by measuring how much the cathode rays were deflected by a magnetic field and how much energy they carried Based on Crookes and Thomson’s research, Thomson concluded that the atom is really a positive field with negatives embedded within its matrix Crash course atomic history 1 Plank/ Einstein - Quantum The Bohr Model Max Plank determined that energy is released and absorbed by atoms in certain fixed amounts known as quanta Plank’s theory Einstein extended Plank’s theory by determining that radiant energy is also quantized—he called the discrete energy packets photons. Einstein’s theory was that electromagnetic radiation has characteristics of both a wave and a stream of particles. Electrons revolve around the nucleus in certain allowed orbits; each orbit corresponds to a specific amt of EN The Bohr Model The Bohr Model Free electrons collide with orbiting electrons and knock the electrons out of orbit excited state As long as the electron remains in the same orbit, it neither emits nor absorbs EN As the electron jumps from one orbit to another, EN is absorbed or emitted 65 4 3 2 ENERGY IN PHOTON OUT ground state A specific quanta of energy is needed EN difference between orbits corresponds to specific wave lengths 1 Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom The Bohr Model Wave Mechanical Model Each element has a unique bright-line emission spectrum. If a wave (light) can behave like a particle than why can’t a particle behave like a wave? “Atomic Fingerprint” Helium Bohr’s calculations only worked for hydrogen! deBroglie’s understanding that any small moving particle, such as an electron in motion, has an associated wave behaviour This allowed Schrodinger to consider the behaviour of the inside the atom 2 Wave Mechanical Model Wave Mechanical Model The positive nucleus is surrounded by a cloud of electron waves Electrons can only have quantized EN levels because of the requirement for whole # of wavelengths for electron waves Heisenberg Uncertainty Principle Wave Mechanical Model Wave Mechanical Model Orbital (“electron cloud”) Atomic Orbitals Impossible to know both the velocity and position of an electron at the same time Region in space where there is 90% probability of finding an e- Orbital S orbital Spherical in shape Size increases as the principle quantum number increases There is only one s orbital in a sublevel s Radial Distribution Curve Wave Mechanical Model Wave Mechanical Model Atomic Orbitals Atomic Orbitals D orbital Has four lobes per orbital Aligned according to x, y, z axis Only 5 d orbitals in a sublevel P orbital p Dumb-bell shaped Aligned along x, y, z axis Only 3 p orbitals in a sublevel Size increases as the principle quantum number increases d F orbital you don’t need to know shapes orbitals f 3
