Question

If an element has one electron in its outermost energy level then it is likely to be a metal or a non metal

Answer 1

Its a metal as metals have tendency to lose electrons.

Answer 2

It is likely to be a metal, as it has one electron to lose to form a positive ion.
For example,
Sodium (Na) - 2,8,1
Na+ -----> Na + e-

Elements with valencies from 1 to 3 are generally metals