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Structure of the Nuclear Atom Electrons Section 5.2 Electrons Remember Dalton? Most of his atomic theory is still accepted. Most !! Atoms are known to be divisible. Atoms are composed of smaller particles. These are the subatomic particles. Like… Electrons An electron is a negatively charged subatomic particle. An English physicist J.J. Thomson “discovered” the electron in 1897. Electrons Electrons He concluded that the rays must have a property. There must be a part of an atom that is negatively charged. He called them electrons. Thompson went on to determine(in 1900) that the mass of an electron is 1/2000 of the mass of a hydrogen atom. Protons and Neutrons So, cathode rays are flowing electrons that are removed from atoms. What is left of the atom? Points to ponder. Atoms have no net charge. Huh? Electric charges are carried by particles of matter(Thompson.) Electrical charges are whole number quantities. Huh? An equal number of (+) and (-) charges result in no charge. So, if you remove one electron(one unit of (-) charge) you should have one excess unit of (+) charge left. Protons and Neutrons Proton One can now assume that there is a particle which carries one unit of positive charge. E. Goldstein noticed a “backward” flowing stream under the cathode ray. It was being attracted to the cathode(-) side of the apparatus. He called them canal rays. Made up of protons. Protons and Neutrons Neutrons Summary Atoms are made up of protons, neutrons and electrons. Protons have a positive charge of 1. Neutrons have no charge. Electrons have a negative charge of -1 and are 1/1840 times smaller than protons and neutrons. Table 5.1 page 111 The Atomic Nucleus How are these subatomic particles put together? Democritus: hard spherical Dalton: like Democritus J.J. Thompson: “Plum pudding” electrons stuck to the nucleus Ernest Rutherford: nucleus surrounded by electrons. Protons and Neutrons Rutherford The results were well… So now, the atom has a nucleus. The protons and neutrons make up the nucleus. Most of the atom is made up of empty space. The chain link fence analogy. Section review page 112 #’s 4,5,6