January 11th, 2008Elements & AtomsQ: When a chemical bond is broken, the sign of the energy is positive. To break a bond, it always costs you energy => energy is positiveTo create a bond, it always needs/takes away energy => energy is negativeAtom: smallest particle of an element that can exist (Democritus came up with this)John Dalton’s Atomic Theory1.All atoms of a given element are identicala.This is wrong because of isotopes, which are atoms of an element that have the same # of protons, but different # of neutrons. b.However, atoms are identical in their chemical compounds and behave the same chemicallyi.The atomic # is represented by “z” which is the # of protonsii.Nuclide # is represented by A, which is the # of protons andneutrons2.Atoms of DIFFERENT elements have different properties, most notably mass3.Compounds are formed by specific combinations of two or more elements (whole # ratios)a.Constant composition: compound is made of the same elements (water is also H20)4.
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