Balancing charges: Magnesium and bromine

Suppose you want to know the formula, or composition, of a compound that results from reacting a metal and a nonmetal. You start by putting the two atoms side by side, with the metal on the left. Then you add their charges. Figure 1.1 shows this process for magnesium and bromine. (Forget about the crisscrossing lines for now. I explain them in the upcoming section “Using the crisscross rule.”)

Figure 1.1: Figuring the formula of magnesium bromide.

The electron configurations for magnesium and bromine are

Magnesium (Mg): 1s²2s²2p⁶3s²

Bromine (Br): 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁵

Magnesium, an alkaline earth metal, has two valence electrons that it loses to form a cation with a 2+ charge. The electron configuration for the magnesium cation is

Mg²⁺: 1s²2s²2p⁶

Bromine, a halogen, has seven valence electrons, so it gains one electron to complete its octet (eight valence electrons) and form the bromide anion with a 1– charge. The electron configuration for the bromide anion is

Br^1–: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶

When writing the formula of a compound, the compound must be neutral. That is, it needs to have equal numbers of positive and negative charges. So after writing the atoms, you need to balance their charges.

The magnesium ion has a 2+, so it requires two bromide anions, each with a single negative charge, to balance the two positive charges of magnesium. So the formula of the compound that results from reacting magnesium with bromine is MgBr₂.