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atom the smallest particle of an element that retains its identity in a chemical formula Democritus believed that atoms were indivisible and and indestructible Dalton Atomic Theory - Atoms of the same element are identical. atoms can be combined into whole number ratios electrons negatively charged, floating around nucleus. number is the same as protons protons positively charged. inside the nucleus, same number as atomic number neutrons no charge, inside nucleus. atomic mass - proton = # of neutrons nucleus tiny central core of an atom composed of protons and neutrons. positively charged Rutherford gold foil experiment - atoms are made up if mostly empty space with a small dense nucleus in the center Thomson cathode ray, discovered that electrons have a negative charge. Bohr proposed that an electron is found only in orbits, around nucleus period the horizontal rows on a periodic table group each vertical column of the periodic table atomic mass unit (AMU) unit used when finding relative abundance Isotopes atoms that have the same number of protons but different number of neutrons