Vacant Available
There really is not an opposite. What is the opposite of friend? Maybe 'enemy', but not really that, either. What is the opposite of 'une' or 'one'? That is like asking what the opposite of a piece of wood is . . . there is none.
The opposite would be winner, or victor. In the phrase "Finders keepers; losers weepers" the opposite is finder.
The usual opposite of facil (easy) is dificil(difficult, hard).
The opposite of the verb to show is to hide, or to conceal.
"Step-brother" is a noun and does not have an opposite.
occupied
vacancy
The opposite could be idle, inactive, or quiet.
occupiedFull, occupied, overflowing...
The opposite of "libre" is "ocupado" in Spanish, which means "busy" or "occupied". It refers to when something is not available or free.
An atom's energy levels are orbitals which can contain 2 electrons each, assuming that they are traveling in opposite directions from each other.
unoccupied (really not sure just trying to help)
The past participle of "occupied" is "occupied." For example, "The room was occupied by other guests."
The past tense is occupied.
Each orbital can hold a maximum of 2 electrons, with opposite spins according to the Pauli exclusion principle.
occupied
It was occupied by the Philistines.