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4.2 Structure at Atom Subatomic Particles: protons, electrons, and neutrons Protons o Rutherford concluded that amount of positive charge varies with elements o Positively charged (1+) o Found in nucleus o Every atom of an element has the same number of protons (“Atomic Number”) Electrons o Particles that Thomson detected o Negatively charged (1– ) o Found outside of nucleus o 1/1836 of mass of proton or neutron (has mass, but very little) o Every atom of an element has the same number of electrons (equal to the number of protons). Neutrons o 1932 Chadwick discovered o Neutral charge (0) o Found in the nucleus o Mass (almost exactly) equal to proton o Most (but NOT ALL) atom of an element has the same number of neutrons. Particle Proton Electron Neutron Symbol Charge Relative Mass (Proton = 1) Location p+ Positive 1 In nucleus e- Negative 1 1836 Outside nucleus n Neutral 1 In nucleus The Atomic Number of an element equals the number of protons in an atom of that element. o Each element has a unique one o Also equals the number of electrons The Mass number equals the sum of protons and neutrons o Number of neutrons equals mass number minus atomic number Isotopes o Atoms of same element with different numbers of neutrons (so also different mass number). o Same atomic number o Different mass number o For most elements it is hard to notice any differences in the physical or chemical properties of isotopes. (Hydrogen is the exception.)