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Dalton’s Atomic Theory Democritus - Greek philosopher who first suggested atoms John Dalton (1766-1844) Studied ratios in which elements combine Dalton put together the first atomic theory Dalton’s Atomic Theory All elements are composed of tiny indivisible particles called atoms Atoms of the same element are identical. The atoms of any one element are different from those of any other element Atoms of different elements can physically mix together or can chemically combine with one another in simple-whole number ratios to form compounds Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction Subatomic particles Electrons (e-) - Negatively charged subatomic particle Discovered by J.J. Thomson in 1897 Used a Crook’s tube to run current through gases Beam was attracted to a magnet Conclusion - beam is made up of small particles he called electrons Robert Millikan (1868-1953) first to calculate electron mass - 9.11 x 10-28 g QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Subatomic particles If a negative charged particle exist; therefore, a positive particle must exist as well. By 1920, the positive particle had been confirmed. Proton (p+) - positively charged subatomic particle. Mass of 1.67 x 10-24 g Subatomic particles Neutron (no) - subatomic particles with no charge Discovered by James Chadwick (1891 1974) in 1932 Nearly same mass as a proton Finding how many subatomic particles for each atom Atomic Number - whole number on p.t. Gives the number of protons Atoms are electrically neutral; there, positives equal negatives. Atomic number also equals number of electrons Finding how many subatomic particles for each atom Mass number = protons plus neutrons Mass number = p+ + no Mass number is not found on the periodic table So, nO = mass number - p+ If carbon has a mass number of 14, how many e-’s, p+’s, and no’s does it have? Symbols Mass number is in top left and atomic number is in bottom left 9Be 16O 8 4 How many subatomic particles in each? Oxygen - 8 p+’s, 8 e-’s, 8 no’s Beryllium - 4 p+’s, 4 e-’s, 5 no’s