Models of the Atom© Copyright NewPath Learning. All Rights Reserved. 94-4836Visit www.newpathlearning.com for Online Learning Resources.Atomic ModelsScientists use models to explain things that cannot be seen directly. The first model of the atom was proposed by the Greek philosopher, Democritus, who hypothesized that all matter was made of small particles, called atoms.Dalton's Atomic TheoryIn the early 1800s, John Dalton conducted experiments on gases and hypothesized the following: 1) All matter is made of atoms and atoms are small particles that cannot be created, divided or destroyed. 2) All atoms of the same element are identical & different elements have different types of atoms. 3) Atoms combine with other atoms to form new substances. Thomson's Model of the AtomIn 1904, J.J. Thomson proposed a model of the atom as a solid sphere with equal numbers of positive and negative charges spread throughout, much as raisins might be embedded in the surface of a pudding. +OxygenJ.J. Thomson1856-1940negative particles - "corpuscles"are repelled by the magnet++++++Thomson's atomic model"plum pudding"equally distributedpositive & negative particles"plum pudding" modelRutherford's ModelIn 1911, Ernest Rutherford proposed a new model of the atom which consisted of a positively charged nucleus, containing most of the atomic mass of the atom, and negatively charged electrons orbiting around the nucleus like planets around the Sun.+highest energylevelNiels Bohr1885-1962lowest energylevelphotonelectronQuantum MechanicalmodelErwin Schrödinger1887-1961dense areas -more likely to nd electronsprobability plot of electron density+Bohr's Model of the AtomIn 1913, Niels Bohr proposed electrons move in stable, or stationary, orbits at fixed distances from the nucleus. Each orbit had an energy associated with it. The closest orbit had the lowest energy and the energy increased with the distance from the nucleus. If an electron moves between orbits, then energy in the form of light (or photons), is absorbed or emitted. OxygenErnest Rutherford1871-1937goldfoildetecting screenalpha-particleemieralpha particles(+ charge)+++++++++++++++++++++++++++Rutherford'smodelatoms ingold foilmost alpha particles pass throughbeam ofalpha (+)particles+++++ +protonquarksnucleusuudThe Modern Atomic TheoryIn 1926, Erwin Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the Quantum Mechanical model of the atom. This model can be represented as a nucleus surrounded by an electron cloud. The probability of finding the electron is greatest where the cloud is most dense and least likely in a less dense area of the cloud. In 1932, James Chadwick discovered the neutron, an uncharged particle contained in the nucleus. Through continued experimentation, more particles have been discovered in the atom. Quarks are believed to be even smaller units than protons and neutrons. In turn, quarks are made up of vibrating strings of energy. The search to find even smaller particles that make up an atom continues.John DaltonJ.J. ThompsonNiels BohrErwin Schrödingernucleus
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