Early Atomic Models

Early Atomic Models Dalton, Thomson, Bohr, Rutherford

Thomson Model • JJ Thomson obviously knew about Dalton’s atomic theory when he discovered the electron and characterized the proton that was first observed by Goldstein. • Thomson took this information and developed a model of the atom

Thomson Model • Cathode Ray Experiment Designed to test emissions from metal Cathode emits rays Anode receives rays Click the picture to run movie

Thomson Model • This model became known as the plum-pudding model • This model said that the atom was a diffuse positive charge and the electrons were randomly scattered throughout the atom.

Thomson Model • In the picture, the blue region is the diffuse positive charge and the red spheres represent the electrons randomly placed in the atom. Positively charged space Electrons e e e e e

Rutherford Au Foil Expt • Ernest Rutherford developed an experiment that would help to reveal the structure and location of the subatomic particles in the atom. • This experiment was proposed by Rutherford and carried out by two of his graduate students: Geiger and Marsden. • Became known as Rutherford Gold Foil Expt.

http://www.sci.tamucc.edu/pals/morvant/genchem/atomic/page7.htmhttp://www.sci.tamucc.edu/pals/morvant/genchem/atomic/page7.htm

Two different models…Which one is right? Thomson Model • Au foil experiment should either allow all of the alpha particles (He2+) to pass through untouched (inconsistent with the nature of solids—should be tightly packed positively charged spheres) or allow no alpha particles to pass through which would be consistent w/ a solid. Rutherford Model • Unsure of what to expect. Need to determine the internal structure of the atom. The scattering pattern of the alpha particles would help indicate the atomic structure.

Rutherford Au Foil Expt Applet • This applet illustrates what they saw. • http://micro.magnet.fsu.edu/electromag/java/rutherford/

Rutherford Atom • All positive charge of atom was in nucleus • Essentially all mass was in the nucleus

Rutherford Model The Au Foil Expt proved several things about the structure of the atom: • All of the positive charge of the atom resides in a very small part of the atom—we call this the nucleus. • The nucleus occupies a very small part of the atom’s total volume (only <1% of alpha particles deflected by Au nuclei). • Electrons must be located outside of the nucleus. • Most of mass of the atom is in the nucleus.

Niels Bohr • Bohr was aware of different spectra for different elements • Hydrogen was postulated to be the simplest of all substances because it had the lightest mass. • He concentrated on hydrogen’s spectrum

Bohr Atom • Bohr postulated that the light that was seen in the spectrum was due to electron transitions. • We call this model of the atom the solar system model of the atom.

Bohr Atom In order to explain discrete spectra, Bohr found that atoms obey three basic rules: • Electrons have only certain energies corresponding to particular distances from nucleus. As long as the electron is in one of those energy orbits, it will not lose or absorb any energy. The energy orbits are analogous to rungs on a ladder: electrons can be only on rungs of the ladder and not in between rungs. • The orbits closer to the nucleus have lower energy. • Atoms want to be in the lowest possible energy state called the ground state (all electrons as close to the nucleus as possible).

Bohr Atom Nucleus Energy Levels

Erwin Schrödinger • Added orbitals (s, p, d, f) to Bohr’s energy levels • The orbital paths created an electron cloud that looks fuzzy, rather that distinct orbits precisely on the surface of spheres • The orbitals allow the electrons to avoid each other, which is necessary as they are all negatively charged • He did this through some very rigorous mathematical description of the electrons in the atom.

Early Atomic Models