Problem 32 - Introductory Chemistry Atoms First

Question

Predict the chemical formula of the compound that results from the reaction between the elements Ba and \(\mathrm{F}\).

Step-by-Step Solution

Answer

The chemical formula of the compound that results from the reaction between the elements Ba and F is \(BaF_2\).

Step 1: Determine the charges of the elements.

To predict the chemical formula for the compound formed from Ba and F, we need to know their charges when they form ions. Barium (Ba) is a Group 2 element, which means it has two valence electrons. When it loses those two valence electrons, it forms a +2 cation (Ba²⁺). Fluorine (F) is a Group 17 element, which means it has 7 valence electrons. When it gains one electron, it forms a -1 anion (F⁻).

Step 2: Use the formula for ionic compound formation.

The formula for the ionic compound between two ions is based on the simple rule that the charges (or oxidation states) of cations and anions must balance out, meaning the positive charges from the cations will be equal to the negative charges from the anions.

Step 3: Create the compound by balancing charges.

In this case, we have the Ba²⁺ cation and F⁻ anion. To balance the charges, we need two F⁻ anions to balance out one Ba²⁺ cation. So, the resulting compound will have one Ba²⁺ ion and two F⁻ ions.

Step 4: Write the chemical formula.

The chemical formula for the compound formed between Ba and F is BaF₂. This is because there is one Ba²⁺ ion and two F⁻ ions in the compound, giving us a balanced charge as required for an ionic compound.

Key Concepts

Ionic Compound Formation

Understanding the process of ionic compound formation is fundamental in predicting chemical formulas. An ionic compound is a substance composed of oppositely charged ions that are bound together by strong electrostatic forces known as ionic bonds. These compounds typically form when a metal reacts with a nonmetal. The metal atoms lose electrons to become positively charged cations, while the nonmetal atoms gain electrons to become negatively charged anions.

Ionic compounds are electrically neutral, which means the total positive charge of the cations equals the total negative charge of the anions. To predict the formula of an ionic compound, you should:

1. Determine the charges of the ions that the reacting elements will form.
2. Combine the ions in a ratio that makes the sum of all charges in the compound zero.

This often leads to a formula that doesn't necessarily have the same number of atoms of each element as indicated by the question but shows the smallest whole-number ratio of ions that achieves neutrality.

Exercise Improvement Advice

For more clarity, it could be helpful to students to explain that the subscript numbers in a chemical formula indicate the number of each type of ion needed to achieve a neutral charge. For instance, in the compound BaF₂, the subscript '2' after the fluorine (F) signifies that two fluoride ions are needed to balance the charge of one barium (Ba²⁺) ion.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are critical in determining how an element will react with others. They are responsible for an atom's chemical properties and its ability to form bonds with other atoms. Elements in the same group of the periodic table typically have the same number of valence electrons, which is why they often exhibit similar chemical behaviors.

For example, barium (Ba), as a Group 2 element, has two valence electrons that it can lose to form a cation with a charge of +2. Conversely, fluorine (F), which is in Group 17, has seven valence electrons and needs to gain one additional electron to achieve a stable configuration similar to the noble gases. When fluorine gains that electron, it becomes an anion with a charge of -1.

Visualizing Electron Transfer

To make this concept easier to understand, imagine a scenario where barium, eager to lose its two valence electrons, encounters fluorine, which is just one electron shy of stability. The transfer takes place, and as a result, ionic bonds are formed between the now positively charged barium and the two negatively charged fluorine atoms, giving rise to the compound BaF₂.

Oxidation States

The oxidation state, often called oxidation number, reflects an atom's degree of oxidation or reduction, essentially denoting the number of electrons an atom has gained, lost, or shared when bonding with other atoms. It's an essential concept in chemistry because it helps to determine the stoichiometry in compounds and reactions.

The rules for assigning oxidation states include:

• For an atom in its elemental form, the oxidation state is always zero.
• For simple monoatomic ions, the oxidation state is equal to the ion's charge.
• In covalent compounds, the more electronegative element is assigned a negative oxidation state.

When predicting the formula of an ionic compound such as that formed from barium and fluorine, the oxidation states guide us in balancing the total charges. Here, barium has an oxidation state of +2, and fluorine has an oxidation state of -1. To neutralize the overall charge, two fluorine atoms with an oxidation state of -1 each are needed to balance the +2 charge of a single barium atom, leading to the formation of BaF₂.

Rule of Thumb

A practical tip for remembering oxidation states is that they must always add up to zero for a neutral compound. This simple rule often aids in the swift prediction of chemical formulas based on the known charges of the involved ions.