Atomic Theory Timeline

Democritus Everything made of small particles Called atmos Earth, air, fire, and water

Antoine Lavoisier Law of Conservation of Mass Mass cannot be created or destroyed Mass of reactant= mass products

Joseph Proust Law of Constant Composition or Law of Definite Proportions a chemical compound always contains the same elements in exactly the same proportions by mass.

John Dalton Atomic Theory of Matter Each element is composed of extremely small particles called atoms All atoms of a given element are identical, but they differ from atoms of other elements Atoms are neither created nor destroyed in a chemical rxn A given compound always has the same relative numbers and kinds of atoms

J. J. Thompson Led to the Plum- Pudding model of the atom Atoms are neutral so have both positive and negative charges Positively charged sphere with negative charges imbedded in it. Used cathode ray- tube experiments to discover the electron Small negatively charged particles that are part of every atom

Rutherford Discovers the nucleus Gold foil experiment Nucleus is tiny, dense, and positively charged Almost all mass is in nucleus Surrounded by mostly empty space where negatively charged particles are.

Atomic structure

Parts of Atoms Nucleus- small- holds protons and neutrons Protons (p+)- positively charged Nuetrons (n0)- no charge Mass about equal in size but neutron just bigger p+ =1.00727647amu n0 =1.00866491600amu Electron cloud- surrounds nucleus holds electrons Electrons (e-) – negatively charged Mass =5.4857990946×10−4 amu So small add almost no mass to atom

Atomic number = # p+ Atoms of a single element always have the same # p+ # p+ = # e- When atom is neutral If not neutral, have more or less e-

Mass Number Mass of atom Number of particles in the nucleus Mass number = # p+ + # n0

Isotopes Atoms of the same element with different masses Must have different numbers of neutrons Show which isotope you are talking about using isotopic notation Lithium-8 83Li

Determining # of Particles in Atoms Name Symbol Atomic # Mass # # p+ # n0 # e- Carbon-12   6 7 2411Na 47 22 3416S-2