Question

Predict the chemical formula for each of the following binary ionic compounds given the formula of calcium oxide, \(\mathrm{CaO}\) : (a) beryllium oxide (b) calcium selenide

Short answer

(a) Beryllium oxide is BeO; (b) Calcium selenide is CaSe.

Step-by-step solution

Identify Ions Involved

First, note the ions involved. Beryllium oxide involves beryllium (\( \mathrm{Be} \)) and oxygen (\( \mathrm{O} \)). Calcium selenide involves calcium (\( \mathrm{Ca} \)) and selenium (\( \mathrm{Se} \)). Beryllium forms \( \mathrm{Be}^{2+} \) ions. Oxygen typically forms \( \mathrm{O}^{2-} \) ions. Calcium forms \( \mathrm{Ca}^{2+} \) ions, and selenium forms \( \mathrm{Se}^{2-} \) ions.

Determine the Charge Balance

In ionic compounds, charges must balance. Both BeO and CaSe have metal ions with \(2+\) charge and nonmetal ions with \(2-\) charge, achieving a neutral compound when one of each ion is paired together.

Write the Chemical Formula

Since both compounds achieve neutrality with one-to-one ratios, the chemical formula for beryllium oxide is \( \mathrm{BeO} \), similar to calcium oxide's \( \mathrm{CaO} \). The chemical formula for calcium selenide is \( \mathrm{CaSe} \).

Key concepts

Binary Ionic Compounds

Binary ionic compounds consist of two elements: a metal and a nonmetal. The formation of these compounds involves the transfer of electrons from the metal to the nonmetal. The metal donates electrons because it wants to achieve a stable electron configuration, resembling the nearest noble gas. The nonmetal, on the other hand, accepts the electrons to also reach a stable electron configuration.
A classic example of binary ionic compounds is calcium oxide (CaO). In this compound, calcium (Ca) is the metal and oxygen (O) is the nonmetal. Calcium loses two electrons to become calcium ions (\(\mathrm{Ca}^{2+}\)), while oxygen gains those two electrons to become oxide ions (\(\mathrm{O}^{2-}\)). These ions bond together due to their opposite charges, forming the compound CaO. The principle holds true for other binary ionic compounds, such as beryllium oxide (BeO) and calcium selenide (CaSe).
Understanding how metals and nonmetals form binary ionic compounds is crucial when predicting chemical formulas. This concept gives insight into why certain elements behave the way they do when forming compounds.

Ions Charge Balance

For any ionic compound to exist, the total charge from the positively charged ions (cations) and negatively charged ions (anions) must balance each other, resulting in a net charge of zero. This principle is known as the charge balance principle. It's crucial for forming stable compounds. If charges do not balance, the compound is unstable and does not form easily.
Take the example of beryllium oxide (BeO): Beryllium forms a \(\mathrm{Be}^{2+}\) ion, while oxygen forms a \(\mathrm{O}^{2-}\) ion. The positive two charge from beryllium and the negative two charge from oxygen cancel each other perfectly. This results in a stable compound with no overall charge.
Similarly, in calcium selenide (CaSe), calcium and selenium ions achieve a similar charge balance. Each calcium ion \(\mathrm{Ca}^{2+}\) pairs with one selenium ion \(\mathrm{Se}^{2-}\), creating a neutral compound. Maintaining this balance is fundamental to forming any binary ionic compound.

Chemical Formulas

Chemical formulas are shorthand representations that display the elements present in a compound and their relative proportions. For binary ionic compounds, the chemical formula shows the ratio of ions needed to achieve charge balance.
To determine the chemical formula for binary ionic compounds, such as beryllium oxide or calcium selenide, one examines the charges of the ions involved. For example:

• Beryllium (\(\mathrm{Be}^{2+}\)) and oxygen (\(\mathrm{O}^{2-}\)) form a 1:1 ratio because their charges are equal and opposite. Thus, the chemical formula for beryllium oxide is \(\mathrm{BeO}\).
• Calcium (\(\mathrm{Ca}^{2+}\)) and selenium (\(\mathrm{Se}^{2-}\)) also form a 1:1 ratio. So, the formula for calcium selenide is \(\mathrm{CaSe}\).

It's essential to use the simplest whole number ratio of ions, as seen in these examples, to express the formula of any ionic compound.

Neutral Compounds

A neutral compound is one where the sum of all charges from the ions results in a net charge of zero. Achieving neutrality is essential because it ensures the compound's stability. In binary ionic compounds, neutrality comes from the combination of a cation and an anion with matching magnitudes but opposite charges.
Consider calcium oxide (CaO) as a reference. Both calcium \(\mathrm{Ca}^{2+}\) and oxide \(\mathrm{O}^{2-}\) cancel out each other's charges, resulting in a neutral compound. This same concept applies to beryllium oxide (BeO) and calcium selenide (CaSe).
For a compound to remain neutral, the chemical formula showcases the correct ratio of cations to anions, ensuring no excess charge is present. This focus on neutrality is what allows ionic compounds to form the solid structures that are so common in the natural world.