Early Atomic ModelsEarly Atomic Models

Dalton, Thomson, Bohr, Dalton, Thomson, Bohr, RutherfordRutherford

Thomson ModelThomson Model

JJ Thomson obviously knew about JJ Thomson obviously knew about Dalton’s atomic theory when he Dalton’s atomic theory when he discovered the electron and characterized discovered the electron and characterized the proton that was first observed by the proton that was first observed by Goldstein. Goldstein.

Thomson took this information and Thomson took this information and developed a model of the atomdeveloped a model of the atom

Thomson ModelThomson Model

Cathode Ray ExperimentCathode Ray ExperimentDesigned to test emissions from metalDesigned to test emissions from metalCathode emits raysCathode emits raysAnode receives raysAnode receives rays

Click the picture to run movie

Thomson ModelThomson Model

This model became known as the This model became known as the plum-plum-pudding modelpudding model

This model said that the atom was a This model said that the atom was a diffuse positive charge and the electrons diffuse positive charge and the electrons were randomly scattered throughout the were randomly scattered throughout the atom.atom.

Thomson ModelThomson Model

In the picture, the blue region is the diffuse In the picture, the blue region is the diffuse positive charge and the red spheres positive charge and the red spheres represent the electrons randomly placed in represent the electrons randomly placed in the atom.the atom.

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Positively charged space

Electrons

Rutherford Au Foil ExptRutherford Au Foil Expt

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

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

page7.htmpage7.htm

Two different models…Two different models…Which one is right?Which one is right?

Thomson ModelThomson ModelAu foil experiment should Au foil experiment should either allow all of the either allow all of the alpha particles (Healpha particles (He2+2+) to ) to pass through untouched pass through untouched (inconsistent with the (inconsistent with the nature of solids—should nature of solids—should be tightly packed be tightly packed positively charged positively charged spheres) or allow no spheres) or allow no alpha particles to pass alpha particles to pass through which would be through which would be consistent w/ a solid.consistent w/ a solid.

Rutherford ModelRutherford ModelUnsure of what to expect. Unsure of what to expect. Need to determine the Need to determine the internal structure of the internal structure of the atom. The scattering atom. The scattering pattern of the alpha pattern of the alpha particles would help particles would help indicate the atomic indicate the atomic structure.structure.

Rutherford Au Foil Expt AppletRutherford Au Foil Expt Applet

This applet illustrates what they saw.This applet illustrates what they saw.

http://micro.magnet.fsu.edu/electromag/javhttp://micro.magnet.fsu.edu/electromag/java/rutherford/a/rutherford/

Rutherford AtomRutherford Atom

All positive charge of atom was in nucleusAll positive charge of atom was in nucleus

Essentially all mass was in the nucleusEssentially all mass was in the nucleus

Rutherford ModelRutherford Model

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

Niels BohrNiels Bohr

Bohr was aware of Bohr was aware of different spectra for different spectra for different elementsdifferent elements

Hydrogen was Hydrogen was postulated to be the postulated to be the simplest of all simplest of all substances because it substances because it had the lightest mass.had the lightest mass.

He concentrated on He concentrated on hydrogen’s spectrum hydrogen’s spectrum

Bohr AtomBohr Atom

Bohr postulated that the light that was Bohr postulated that the light that was seen in the spectrum was due to electron seen in the spectrum was due to electron transitions.transitions.

We call this model of the atom We call this model of the atom the solar the solar system modelsystem model of the atom. of the atom.

Bohr AtomBohr Atom

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

Bohr AtomBohr Atom

Energy LevelsNucleus

Erwin SchrErwin Schröödingerdinger

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

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