In ionic compounds, such as aluminium fluoride (AlF3), the Lewis dot structure is composed as such:
x.
Al3+[:F :]-3
..
The pairs of dots in the diagram represent
lone pairs, while the single
dot represent a
bond pair, and the "x" represents the
electron taken from the
aluminium. The
negative ion in the compound is always the one inside the brackets, while the
positive ion is the one outside.