Problem 41 - Chemistry and Chemical Reactivity

Question

What are the charges on the ions in an ionic compound containing the elements barium and bromine? Write the formula for the compound.

Step-by-Step Solution

Answer

The charges are Ba: +2 and Br: -1; the formula is \( \text{BaBr}_2 \).

Step 1: Determine the Valence Electrons of Barium

Barium (Ba) is located in group 2 of the periodic table, which means it has 2 valence electrons. To achieve a stable electron configuration similar to the noble gases, barium will lose these 2 valence electrons.

Step 2: Determine the Charge of Barium Ion

When barium loses its 2 valence electrons, it becomes a positively charged ion, known as a cation. The charge on the barium ion is +2, thus it is written as \( \text{Ba}^{2+} \).

Step 3: Determine the Valence Electrons of Bromine

Bromine (Br) is located in group 17 of the periodic table, which is also known as the halogens group. Halogens need one more electron to fill their outer shell, meaning bromine has 7 valence electrons and tends to gain 1 electron to become stable.

Step 4: Determine the Charge of Bromine Ion

When bromine gains an electron, it becomes a negatively charged ion, known as an anion. The charge on the bromine ion is -1, thus it is written as \( \text{Br}^{-} \).

Step 5: Combine Ions to Form a Neutral Compound

To form a neutral ionic compound, the charges must balance. One \( \text{Ba}^{2+} \) ion will pair with two \( \text{Br}^{-} \) ions to balance out the total charge (\( 2+ \) from barium and \( 2 \times (-1) = -2 \) from bromine).

Step 6: Write the Chemical Formula for the Compound

The chemical formula of the compound is derived from the combination of one barium ion and two bromide ions, resulting in \( \text{BaBr}_2 \).

Key Concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. These electrons play a critical role in how atoms interact and bond with each other. Recognizing the number of valence electrons can help predict an element's chemical behavior.

• In the case of barium (Ba), it belongs to group 2 of the periodic table. This position means barium has 2 valence electrons. To achieve a stable electron configuration akin to noble gases, barium tends to lose these valence electrons.
• On the other hand, bromine (Br) is in group 17 and has 7 valence electrons. Bromine seeks to gain 1 electron to complete its valence shell and become stable.

Understanding the valence electrons is the first step towards knowing how different atoms come together to form compounds.

Cation and Anion Charges

Ions are formed when atoms either lose or gain electrons, resulting in a charge. Atoms that lose electrons become positively charged, while those that gain electrons become negatively charged. This is how cations and anions are formed respectively.

• Barium, losing its 2 valence electrons, forms a cation with a charge of +2, represented as \( \text{Ba}^{2+} \).
• Bromine, upon gaining an electron, forms an anion with a charge of -1, written as \( \text{Br}^{-} \).

The charges of these ions (cation and anion) are vital in determining how they will pair up to form an ionic compound.

Chemical Formula

A chemical formula represents the proportions of atoms in a compound. For ionic compounds, the formula arises from balancing the total positive and negative charges to ensure neutrality.

• In our example, a single \( \text{Ba}^{2+} \) cation with a charge of +2 must balance with two \( \text{Br}^{-} \) anions, each with a charge of -1.
• Thus, the resulting ionic compound has the chemical formula \( \text{BaBr}_2 \), demonstrating a balance of charge: 1 barium ion intertwining with 2 bromide ions.

This formula, \( \text{BaBr}_2 \), encapsulates the whole compound's structure, showing not only which elements are present but in what ratio they exist.