Every electron on an atom is in a definite state (energy level) with a definite energy.

1
Quantum Theory

1. Every electron on an atom is in a definite state
   (energy level) with a definite energy.
2. In order for an electron to change energy levels,
   a photon must be either absorbed or emitted,
   governed by ?E hc/?
3. The allowed energy levels for an electron on an
   atom are governed by four quantum numbers, n, l,
   m and s.

2
Rules for determining the ground state of an atom.

• The Pauli exclusion principle. No two electrons
  on an atom can have the same four quantum s
• The Aufbau principle. When electrons go on an
  atom, they fill the lowest available energy
  level.
• Hunds rule. When electrons fill a sit of
  degenerate (ie. same energy) orbitals, they do so
  in a way which maximizes the spin.