Question

Write the formulas for and give the names of the compounds formed by the following ions: a. \({Cr}^{2+}\) and \({F}^{-}\) b. \({Ni}^{2+}\) and \({O}^{2-}\) c. \({Fe}^{3+}\) and \({O}^{2-}\)

Short answer

Chromium(II) fluoride: \(CrF_2\)Nickel(II) oxide: \(NiO\)Iron(III) oxide: \(Fe_2O_3\)

Step-by-step solution

Identify the Charges of the Ions

First, identify the charges of each ion given in the exercise: a. \({Cr}^{2+}\) and \({F}^{-}\) b. \({Ni}^{2+}\) and \({O}^{2-}\) c. \({Fe}^{3+}\) and \({O}^{2-}\)

Determine the Ratio of Ions in Each Compound

For each set of ions, the compound must be electrically neutral. Thus, the total positive charge must equal the total negative charge: a. One \({Cr}^{2+}\) ion pairs with two \({F}^{-}\) ions to balance the charges. b. One \({Ni}^{2+}\) ion pairs with one \({O}^{2-}\) ion to balance the charges. c. Two \({Fe}^{3+}\) ions pair with three \({O}^{2-}\) ions to balance the charges.

Write the Chemical Formulas

Use the ratios determined in Step 2 to write the formulas for the compounds: a. \({Cr}^{2+}\) and \({F}^{-}\): \({CrF_2}\) b. \({Ni}^{2+}\) and \({O}^{2-}\): \({NiO}\) c. \({Fe}^{3+}\) and \({O}^{2-}\): \({Fe_2O_3}\)

Name the Compounds

Finally, name each of the compounds formed: a. \({CrF_2}\): Chromium(II) fluoride b. \({NiO}\): Nickel(II) oxide c. \({Fe_2O_3}\): Iron(III) oxide

Key concepts

chemical formula

The chemical formula of a compound tells us which elements are present and how many atoms of each are in the compound. For ionic compounds, the formula reflects the ratio of cations (positively charged ions) to anions (negatively charged ions) that results in an electrically neutral compound.
For example, in the compound formed by \({Cr}^{2+}\) and \({F}^{-}\), the balance of charges is obtained by having one \({Cr}^{2+}\) ion and two \({F}^{-}\) ions, leading to the chemical formula \({CrF_2}\).
The formula indicates that each unit of chromium(II) fluoride contains one chromium ion and two fluorine ions.

ion charges

Ions are atoms or molecules that have gained or lost one or more of their electrons, thus acquiring an electrical charge. Cations are positively charged ions, while anions are negatively charged. The charge on an ion is typically indicated by a superscript after the element symbol (e.g., \({Fe}^{3+}\) for iron with a 3+ charge).
Understanding ion charges is crucial when forming compounds because these charges dictate how ions combine to create a neutral compound.

• For instance, Chromium ion \({Cr}^{2+}\) is a cation with a 2+ charge, while fluorine ion \({F}^{-}\) is an anion with a 1- charge.
• To form a neutral compound with these ions, the total positive charge must balance the total negative charge.

compound naming

Naming ionic compounds involves combining the names of the cation and the anion. For transition metals like chromium and iron, which can form ions with different charges, the charge is indicated by a Roman numeral in parentheses.

• For instance, \({CrF_2}\) is called Chromium(II) fluoride because the chromium ion has a 2+ charge.
• Similarly, \({NiO}\) is called Nickel(II) oxide since nickel has a 2+ charge in this compound.
• The compound \({Fe_2O_3}\), involving iron ions with 3+ charge, is named Iron(III) oxide.

neutral charge balance

In any ionic compound, the total charge from the cations must equal the total charge from the anions, resulting in a neutral charge balance. This is what keeps ionic compounds stable.
For instance, in \({CrF_2}\), one \({Cr}^{2+}\) ion (2+ charge) balances with two \({F}^{-}\) ions (each 1- charge) for a total neutral charge.
Similarly, \({NiO}\) has a \({Ni}^{2+}\) ion balancing with \({O}^{2-}\) ion.
Lastly, \({Fe_2O_3}\) balances two \({Fe}^{3+}\) ions (each with 3+ charge) with three \({O}^{2-}\) ions (each with 2- charge), maintaining overall neutrality.