Beginning of the 20th century:

(1) atoms were electrically neutral - equal amounts of + and - charge

(2) the negative charge is associated with cathode rays (electrons) particles having very small mass

(3) atoms are stable

J. J. Thomson (1900)

discovered the electron - cathode rays

plum-pudding model - electrons and protons evenly spread throughout the atom (diameter = 10-10 meters)

Max Planck (1900)

blackbody radiation

ultraviolet catastrophe

E = hf where f = c/λ

Brownian Motion (1827)

discovered initially by Scottish botanist, Robert Brown, when he witnessed pollen grains "jiggling" when examined under a microscope - later he saw the same agitated behavior with dust particles and grains of soot

"Brownian Motion" is now known to be the result of the collisions between neighboring atoms/molecules (Einstein - 1905)

Rutherford's gold foil experiment (1909) with Geiger and Marsden (repeated with carbon and aluminum)

1 out of every 8000 alpha particles scattered through an angle > 90°

discovered a small positively charged nucleus (diameter ≤ 10-14 meters)

the value of the charge needed from the scattering data would equate to the magnitude of the charge held in the nucleus - which would dictate the number of electrons surrounding the nucleus which closely matched the atomic number in Mendeleev's periodic table (1869)

Bohr model (1913) how were these electrons arranged?

steady orbitals - deBroglie wavelengths

energy levels - light emission, light absorption - excitation and de-excitation

spectral lines

Franck/Hertz (1914) experimentally verified discrete energy levels

bombarded room temperature mercury vapor with electrons of specific KE

absorption peak at 4.9 eV which was then shown to represent the energy of the 254 nm wavelength in mercury's emission spectrum

James Chadwich (1932)

discovered the neutron

Werner Heisenburg (1935)

first proposed the neutron-proton theory of nuclear structure.