The Physics
Hypertextbook
Opus in profectus

# 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 (42He2+).
• Most of the alpha particles went nearly straight through with relatively little deflection.
• A small fraction (1 in 100,000) were deflected nearly 180°.
• 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.
• 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

L = n

where… 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.
• can only have specific radii
• 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…

Ephoton = |Ef −Ei|

• 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)
• 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.