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History of Atoms Democritus was 1st to suggest existence of atoms Said matter could be divided to a certain point and then no more Dalton devised a theory based on Democritus’s ideas in the early 1800s Theory had five points History continued 1. All matter is made of atoms 2. Atoms can be distinguished by their mass 3. Atoms of the same element have the same mass and properties 4. Atoms of different elements have different masses and properties 5. Atoms can’t be created, destroyed or SUBDIVIDED Atomic Structure Atoms are tiny units that determine the properties of all matter. Atoms are the smallest unit of matter, though they can be broken into smaller pieces. These pieces are the protons, neutrons, and electrons. Subatomic Particles Protons (p+) have a charge of +1. They have a mass of 1.673 * 10 –24 g. Electrons (e-) have a charge of –1. They have a mass of 9.11 * 10-28 g. Neutrons (n) have no charge, but have the same mass as a proton. Protons and neutrons are located in the nucleus of an atom. Electrons orbit the nucleus, in energy levels. Within an energy level, the electrons occupy orbitals. Electrons Discovered in 1897 by Thomson using CRT CRT = cathode ray tube Ran electricity through a gas, causing glowing beam in tube Glow was due to motion of electrons among energy levels Protons Evidence for existence of protons seen in 1886, Eugen Goldstein observed CRT with rays moving toward CATHODE (- end) of tube Concluded must be +, since he knew cathode was – and opposite charges attracted Was actually seeing movement of ions Existence of protons finally proven by Rutherford in 1919 Neutrons Non charged particles found in nucleus Existence proven in 1932 by James Chadwick He repeated experiments done by daughter & son-in-law of Marie Curie, but with the goal of finding a neutral particle He won Nobel Prize in 1935 for his discovery Nucleus Discovered by Rutherford in “gold foil experiment” Thomson had proposed p+ and e- were evenly mixed throughout atom Rutherford shot α particles (He nuclei) at gold foil, most passed through, but some DEFLECTED Proposed model of atom containing mostly empty space, with + charge and mass concentrated in central region called NUCLEUS Gold Foil Experiment Rutherford expected the α – particles (+ He nuclei) to go through the foil, or to rebound. Some particles were deflected, apparently due to repulsive force of like charges Niels Bohr (1913) Bohr suggested that e- were in orbits around the nucleus and each e- has a certain energy that determines its path around the nucleus. These paths are called energy levels. Electrons can only be in certain energy levels. By 1925, Bohr’s model no longer explained all observations. In Modern theory, it is suggested that e- behave like waves on a vibrating string. Atomic Math # of protons in an element = the element’s atomic number. atomic # determines the element’s position in the PT. All atoms are electrically neutral, so # of p+ must = # of e Mass number = # of protons + # of neutrons. Isotopes result from differing #s of neutrons, thus have different masses. Atomic Mass Refer to relative masses of atoms since 1 atom is EXTREMELY small Reference isotope is Carbon-12 (C atoms with 6 p+ & 6n) 1 a.m.u. (amu) is 1 atomic mass unit, mass or 1 p+ or n Reported atomic masses are weighted averages, based on the relative abundance of natural isotopes Example calculation Potassium has an atomic mass of 39.14 39K has a relative abundance of 93.3%, 40K has a relative abundance of 0.012%, & 41K has a relative abundance of 6.7%. (39*.933)+(40*.00012)+(41*.067) = 39.14 (note #s may be off slightly from PT reported value, due to rounding)