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Chocolate Challenge Find the 12 differences between the two pictures. Source: http://www.smart-kit.com/wp-content/uploads/2008/03/chocolate-puzzle.jpg The answers are … Source: http://www.smart-kit.com/wp-content/uploads/2008/03/chocolate-puzzle.jpg Atoms and Their Structure Timeline of Development of Current Atomic Model Discovery Discovery of the Neutron of the Proton 450 BC Democritus proposed the idea of atomos 1802 Beginning of Modern Atomic Theory 1897 1911 1913 1930 Discovery of the Nucleus Discovery of the Electron Introduction of the wave mechanical model The Idea of Energy Levels for Electrons was Proposed What is Today’s Atomic Model? Dense, Positively Charged Nucleus Mostly Empty Space Composed of Protons, Neutrons, and Electrons Negatively Charged Electron Cloud Most Probable Location of the Electrons A Summary of Atomic Structure The atom consists of positive, negative, and neutral entities called protons, electrons, and neutrons Protons determine element’s identity # of protons is unique for each element Electrons determine element’s chemical properties Neutrons act as a “glue” for the protons to minimize charge repulsions Protons and neutrons are located in the nucleus of the atom, which is small Therefore, the nucleus is positively-charged Electrons are located outside of the nucleus at an average distance of 10-8 cm Therefore, the electron cloud is negatively-charged Nucleus Size vs. Electron Cloud Size • Most of the volume of the atom is due to electrons Masses of the Proton, Neutron, Electron • Most of the mass of the atom is due to the protons and neutrons within the nucleus Nucleus Size vs. Electron Cloud Size Element Symbols, Atomic Number, and Mass Number Atomic number (Z) Represents the number of protons in the nucleus Mass number (A) Represents the total number of nucleons in the nucleus (i.e., protons and neutrons) Mass Number = # of protons + # of neutrons Mass number is NOT the number on the Periodic Table Although numerically similar, this is the average atomic mass – more on this later! Isotopes There can be a variable number of neutrons for the same number of protons Isotopes have the same number of protons but different numbers of neutrons “Different” versions of the same element Masses are different due to the different number of neutrons So, isotopes have the same Z but different A! Isotopes of Carbon An Element’s Charge All elements on the periodic table are electrically neutral In Positive charge of the protons equals negative charge of the electrons order for an element to be neutral, the number of protons must equal the number of electrons Atomic number equals number of electrons! Calculating # of Protons, Electrons, and Neutrons Atomic # Na Hg Mass # # of P+ # of N # of E- Isotopic Forensics