Question

Magnesium bromide is a binary ionic compound. From its formula, MgBr2, how do you know that
Mg is the metal?

Answer 1

You that Mg is the metal if in the periodic table it lands between group 1-12. So yeas Mg is a metal because it lands in group 2.

Answer 2

Answer: Magnesium loses electrons to form $Mg^{2+}$

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:

$[Mg]:12: 1s^22s^22p^63s^2$

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

$[Mg^{2+}]:10:1s^22s^22p^63s^0$

Electronic configuration of bromine

$[Br]:35:1s^22s^22p^63s^23p^64s^23d^{10}4p^5$

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

$[Br^-]=1s^22s^22p^63s^23p^64s^23d^{10}4p^6$

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