Magnesium bromide is a binary ionic compound. from its formula, mgbr2, how do you know that
mg is the metal?

Magnesium loses electrons to form

Explanation:

An ionic bond is formed when an element completely transfers its valence electron to another element. Metals donate the electron and forms a positively charged ion called as cation. Non metals accept the electrons and forms a negatively charged ion called as anion.

Electronic configuration of magnesium:

Magnesium atom will loose one electron to gain noble gas configuration and form magnesium cation with +2 charge.

Electronic configuration of bromine

Bromine atom will gain one electron to gain noble gas configuration and form bromide ion with -1 charge.

Thus as magnesium forms a cation , it is the metal.

that tool would be a compass