First, a little about definitions.

Orbital states can be described with several terms:

**Filled**- An orbital that contains the maximum number of electrons it can hold.**Empty**- An orbital that contains no electrons.**Occupied**- An orbital that contains at least one electron.**Unoccupied**- An orbital that contains at least one open space for an electron.

Filled/Occupied | |

Occupied/Unoccupied | |

Unoccupied/Empty |

With these definitions in mind, we can see that a filled orbital is also occupied, but an occupied orbital is not necessarily filled. Also, an orbital can be both occupied and unoccupied. A good way to remember this is that occupied means that one space is occupied by an electron, and unoccupied measn at least one space is free to accept an electron. Thus, it is better to think of filled and empty in terms of the entire orbital, but of occupied and unoccupied in terms of the spaces for electrons.

So, the best way to remember the terms would be as follows:

**Filled**- An orbital that is__filled__with the maximum number of electrons it can hold.**Empty**- An orbital that is__empty__of electrons.**Occupied**- An orbital that has at least one space__occupied__by one electron.**Unoccupied**- An orbital that has at least one__unoccupied__space for an electron.

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