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Protons neutrons electrons Charge Positive neutral negative Mass 1amu 1amu 0 amu Volume 0 0 all Location Nucleus Nucleus Cloud Symbol p+ n0 e- Democritus • Atomos = uncuttable/indivisible • Smallest part of matter • No real evidence Dalton • Gathered evidence supporting Democritus' hypothesis • Small tiny sphere = atom • Atoms of the same element are all alike • Atoms of different elements are different from each other • Atoms of different elements can combine to form compounds • Billiard ball model Thomson • UPDATE: not a solid sphere, there are parts that are negatively charged electrons. Subatomic particles. • Plum pudding model - negative electrons embedded in positive sphere Rutherford • UPDATE: electrons aren't embedded in positive mass. Atoms' masses exist within the nucleus of the atom - densely packed positively charged center of atoms. Electrons orbit outside nucleus, taking up space • Gold foil experiment • Planetary model; nuclear atom Now, atoms can be seen and manipulated using inventions such as the scanning electron microscope. Atomic number = number of protons • Identifies one element from another • Also gives number of electrons on a neutral atom Mass number = total mass of the atom (amu) • Protons + neutrons Isotopes - different # neutrons but same protons Isotope symbol: Upper left - mass # Lower left - atomic # (smaller #, only protons) Name. Symbol. Atomic #. Mass #. Protons. Neutrons. Electrons. Isotope Symbol Carbon. Hg. 37. W. 54. 14 200 86 138. 183 131 56 Atomic mass = weighted average of the naturally occurring isotopes • Atomic mass IS NOT THE SAME AS mass number!!! • Mass numbers are whole numbers, sum of protons and neutrons • Atomic numbers are averages so can be decimals and are found on the periodic table underneath element symbol • Atomic mass is usually closest to the most abundant natural isotope