This metal is very reactive, has properties similar to magnesium, and is in the same period as bromine.

What element is this?

Calcium is the element that you are describing.

Calcium.

This question is easy using the periodic table. All elements in the same group as Magnesium will have similar properties, so basically you're looking for whatever is in Group 2, Period 4. Groups are the columns and Periods are the rows.