Atomic Theory

Antoine Lavoisier (1785)

Law of Conservation of Mass - matter is not created/destroyed during chemical reaction; reactants equals products

Joseph Proust (1799)

Law of Definite Proportions - elements in substances always combine in definite proportions by mass

John Dalton (1803)

Atomic Theory - all matter is made of atoms, all atoms in an element are the same but different with different elements, atoms combine in fixed proportions to make compounds, chemical reaction involves rearrangement of atoms, not creating/destroying

J.J. Thomson (1897)

Cathode ray has electrons, calculated ratio of electron mass to it charge

Plum Pudding Model

Thomson - negatively charged electrons scattered in positive sponge-like structure

Robert Millikan (1909)

Oil drop experiment - determined charge of electron

Ernest Rutherford (1911)

Gold foil experiment; devised Nuclear Theory of the Atom - all positive charge and nearly all mass of an atom are in a tiny nucleus, electron has almost no mass but occupy nearly all the volume of atom; discovered proton

Neils Bohr (1922)

Proposed Planetary model of atom - similar to solar system, sun is nucleus, electrons are planets

Relative masses and charges

Proton - 1 amu +1, Neutron - 1amu 0, Electron 5×10^-4 = 0 -1

Atomic Number

number of protons (same as electrons)

Mass Number

number of protons + neutrons (neutrons = mass # - atomic #)

Ions

charged particles formed when atom loses/gains an electron

Cations

positive, loss of electron

Anion

negative, gains electron

Isotopes

different atoms of same element with different mass numbers