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Chapter 4: Atomic Structure. Subatomic Particles Symbol Notation Isotopes Average Atomic Mass. Democritus. Atom : smallest part of an element that retains the identity of that element.

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## Chapter 4: Atomic Structure

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**Chapter 4: Atomic Structure**Subatomic Particles Symbol Notation Isotopes Average Atomic Mass**Democritus**• Atom: smallest part of an element that retains the identity of that element. • Democritus (460-370 B.C.): believed in tiny, indivisible, indestructible particles he called “atomos.” • He had NO experimental proof!**John Dalton’s Atomic Theory (1803-1807)**• All elements are composed of tiny indivisible particles called atoms. • Atoms of the same element are identical; atoms of different elements are different. • Atoms of different elements can mix in whole number ratios to form compounds. • Chemical reactions cause atoms to separate and recombine, NOT change into different atoms.**Comparing Subatomic Particles**The proton gives identity to an atom, the electron gives personality.**Mass Number & Atomic Number**Mass Number = Number of protons + neutrons Atomic Number = Number of protons**Mass of Atoms**• Mass of atom is very small! • Heaviest atom is 4 x 10-22g. • Diameters of atoms: 100-500 pm (1-5 angstroms) • 1 A = 10-10m • Both pm and A are commonly used for diameters.**Isotopes Have the Same…**• Number of protons • Atomic Number • Number of electrons**Isotopes Have Different…**n0 • Number of neutrons • Mass Number • Mass**Isotopes of Hydrogen**Hydrogen-3 Hydrogen-1 Hydrogen-2 Protium Deuterium Tritium**Atomic Mass Unit (amu)**• carbon-12 isotope used as standard • 1 atom of carbon-12 = 12 amu • carbon-12 has 12 particles with mass • mass of proton = 1 amu • mass of neutron = 1 amu 12 C 6**Average Atomic Mass**• Weighted average of all isotopes • Seen as mass number on periodic table • Units in amu**Calculating Average Atomic Mass**Isotope A Isotope B

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