# Atomic Models

## Summary

- Rutherford model (planetary model)
- Gold foil experiment
- Alpha particles from radioactive polonium where directed at a piece of gold leaf.
- Alpha particles are helium nuclei, which are relatively heavy and double positively charged (
^{4}_{2}He^{2+}).

- Alpha particles are helium nuclei, which are relatively heavy and double positively charged (
- Most of the alpha particles went nearly straight through with relatively little deflection.
- A small fraction (1 in 100,000) were deflected nearly 180°.

- Alpha particles from radioactive polonium where directed at a piece of gold leaf.
- Planetary model
- The atom is mostly empty space.
- The mass of the atom is concentrated in a small, central,fixed, positive nucleus.
- The electrons are free to orbit the nucleus in a manner similar to the way planets orbit the Sun.

- Problem
- Electrons orbiting the nucleus would experience a centripetal acceleration.
- Accelerating electrons would emit electromagnetic waves.
- Since waves transfer energy, the electrons would lose energy and spiral into the nucleus.
- A planetary atom would collapse.

- Gold foil experiment
- Bohr model
- The orbits of the electrons in an atom…
- exist as stationary states called orbitals
- The electron is a standing wave that fits in the circumference of the orbit.

- can only possess discrete values of angular momentum

where…*L*=*n*ℏ*L*=orbital angular momentum of the electron *n*=a positive integer greater than zero ℏ = the reduced Planck constant (ℏ = *h*/2π) - are associated with an energy level
- The
*n*= 1 energy level is known as the ground state. - The
*n*> 1 energy levels are known as excited states.

- The
- can only have specific radii

- exist as stationary states called orbitals
- The discrete spectra of the elements are associated with energy level transitions.
- A photon is emitted when an electron "falls" from a higher to lower energy level.
- The energy of the emitted photon equals the absolute value of the difference in the energy levels.

- A photon is absorbed when an electron "jumps" from a lower to higher energy level.
- The electron can only jump up if the energy of the absorbed photon matches the energy of the transition.

- The equation for both processes is…
*E*= |_{photon}*E*−_{f}*E*|_{i}

- A photon is emitted when an electron "falls" from a higher to lower energy level.
- Problems
- The model does not work for atoms with more than one electron.
- The model is two-dimensional (is flat), while atoms are three-dimensional (occupy volume)

- The orbits of the electrons in an atom…
- Schrödinger model
- The orbital of an electron is a "cloud" that describe the probability of finding the electron in that location.
- The electron is a standing wave in three dimensions that occupies the potential well of the nucleus.