Ionic compounds form when electrons are transferred between a metallic element and a nonmetallic element. This transfer results in the formation of charged particles called ions, which are held together by powerful electrostatic forces known as ionic bonds. The atom that loses electrons becomes a positively charged cation, while the atom that gains electrons becomes a negatively charged anion. This bonding is driven by the atoms’ tendency to achieve a stable electron configuration.
Identifying the Ions: Barium and Bromine
To determine the formula for a compound involving Barium and Bromine, the first step is to identify the charge each element will adopt when forming an ion. Barium (Ba) is classified as an alkaline earth metal, residing in Group 2 of the periodic table. Atoms in this group characteristically possess two valence electrons, which they readily lose to achieve a stable electron configuration. By surrendering these two electrons, Barium forms a cation with a positive two charge, represented as Ba\(^{2+}\).
Conversely, Bromine (Br) is a halogen, a nonmetal found in Group 17 of the periodic table. Halogens have seven valence electrons and require only one more electron to complete their outer shell and achieve stability. Bromine achieves this stable state by gaining a single electron, thereby forming an anion with a negative one charge, written as Br\(^{-}\). The distinct positive and negative charges of Ba\(^{2+}\) and Br\(^{-}\) confirm that these two elements will react to form an ionic compound.
Achieving Charge Neutrality
The fundamental principle governing the formation of all ionic compounds is the requirement for electrical neutrality, meaning the net charge of the final compound must be zero. This condition dictates the precise ratio in which the individual cations and anions must combine. Since the Barium ion carries a 2+ charge and the Bromide ion carries a 1- charge, a single Barium ion cannot be balanced by a single Bromide ion. The charges must perfectly cancel out to achieve a neutral structure.
To achieve this necessary balance, the 2+ charge of the one Barium ion requires two separate 1- charges from the Bromide ions. This 1:2 ratio—one Barium ion for every two Bromide ions—is the smallest whole-number ratio that satisfies the condition of charge neutrality. When writing the chemical formula, subscripts are used to indicate this required ratio of ions within the compound.
The Final Chemical Formula
The stoichiometric requirement of one Ba\(^{2+}\) ion for every two Br\(^{-}\) ions directly determines the chemical formula for Barium and Bromine. The convention for writing ionic formulas places the symbol for the metal cation first, followed by the symbol for the nonmetal anion. Using the determined ratio, the chemical formula is written as BaBr\(_{2}\), which represents the smallest repeating unit of the compound. This compound is systematically named Barium Bromide, with the name of the anion (Bromine) taking the suffix “-ide”.
The formula BaBr\(_{2}\) is known as the empirical formula, which provides the simplest whole-number ratio of the elements present in the compound. This formula succinctly encapsulates the electrical balance achieved through the transfer of Barium’s two valence electrons, one going to each of the two Bromine atoms. The resulting Barium Bromide is a white, crystalline solid.