Question

Give the empirical formulas and names of the compounds formed from the following pairs of ions: (a) \(\mathrm{Rb}^{+}\) and \(\mathrm{I}^{-}\), (b) \(\mathrm{Cs}^{+}\) and \(\mathrm{SO}_{4}^{2-}\), (c) \(\mathrm{Sr}^{2+}\) and \(\mathrm{N}^{3-}\), (d) \(\mathrm{Al}^{3+}\) and \(\mathrm{S}^{2-}\).

Short answer

(a) \( \mathrm{RbI} \), Rubidium Iodide; (b) \( \mathrm{Cs_{2}SO_{4}} \), Cesium Sulfate; (c) \( \mathrm{Sr_{3}N_{2}} \), Strontium Nitride; (d) \( \mathrm{Al_{2}S_{3}} \), Aluminum Sulfide.

Step-by-step solution

Combining Ions for Compound (a)

The ions given are \( \mathrm{Rb}^{+} \) and \( \mathrm{I}^{-} \). To form a neutral compound, combine 1 \( \mathrm{Rb}^{+} \) ion with 1 \( \mathrm{I}^{-} \) ion. This gives the empirical formula \( \mathrm{RbI} \). The name of the compound is Rubidium Iodide.

Combining Ions for Compound (b)

The ions are \( \mathrm{Cs}^{+} \) and \( \mathrm{SO}_{4}^{2-} \). To balance the charges, 2 \( \mathrm{Cs}^{+} \) ions are needed to neutralize 1 \( \mathrm{SO}_{4}^{2-} \) ion. This results in the empirical formula \( \mathrm{Cs_{2}SO_{4}} \). The name of the compound is Cesium Sulfate.

Combining Ions for Compound (c)

The ions are \( \mathrm{Sr}^{2+} \) and \( \mathrm{N}^{3-} \). To find the smallest number of each ion that results in a neutral compound, use 3 \( \mathrm{Sr}^{2+} \) ions and 2 \( \mathrm{N}^{3-} \) ions. This forms the empirical formula \( \mathrm{Sr_{3}N_{2}} \). The compound is named Strontium Nitride.

Combining Ions for Compound (d)

The ions are \( \mathrm{Al}^{3+} \) and \( \mathrm{S}^{2-} \). For neutrality, 2 \( \mathrm{Al}^{3+} \) ions and 3 \( \mathrm{S}^{2-} \) ions are needed, yielding the formula \( \mathrm{Al_{2}S_{3}} \). The compound is called Aluminum Sulfide.

Key concepts

Ionic Compounds

Ionic compounds are chemical compounds composed of ions held together by ionic bonds. In these compounds, positively charged ions, known as cations, and negatively charged ions, known as anions, attract each other to form a stable compound. The formation of ionic compounds usually involves a metal donating electrons to a non-metal.

• The metal forms the cation, such as in the exercise's examples:
  • Rubidium (\( \mathrm{Rb}^{+} \) ) forms rubidium ions.
  • Cesium (\( \mathrm{Cs}^{+} \) ) forms cesium ions.
  • Strontium (\( \mathrm{Sr}^{2+} \) ) forms strontium ions.
  • Aluminum (\( \mathrm{Al}^{3+} \) ) forms aluminum ions.
• The non-metal forms the anion, for example:
  • Iodine (\( \mathrm{I}^{-} \) ) forms iodide ions.
  • Sulfate (\( \mathrm{SO}_{4}^{2-} \) ) forms sulfate ions.
  • Nitrogen (\( \mathrm{N}^{3-} \) ) forms nitride ions.
  • Sulfur (\( \mathrm{S}^{2-} \) ) forms sulfide ions.

The main characteristic of ionic compounds is their high melting and boiling points. They are often solid and crystalline at room temperature and are usually soluble in water.

Neutral Compound

A neutral compound is one where the total positive charge from the cations is equal to the total negative charge from the anions. This balance ensures that the compound does not carry an overall charge. In forming neutral compounds, the number of each ion present is adjusted so that their charges cancel each other out.
For example, in the rubidium iodide (\( \mathrm{RbI} \) ), there is one rubidium ion (\( \mathrm{Rb}^{+} \) ) and one iodide ion (\( \mathrm{I}^{-} \) ). The positive and negative charges are equal, so they form a neutral compound.
In cesium sulfate (\( \mathrm{Cs_{2}SO_{4}} \) ), two cesium ions (\( \mathrm{Cs}^{+} \) ) each with a charge of +1 are needed to balance the charge of -2 from one sulfate ion (\( \mathrm{SO}_{4}^{2-} \) ). Hence, the overall charge of the compound is zero.
The concept of neutrality applies to all compounds in chemistry, ensuring they are stable and reactive without an excess electric charge.

Charge Balance

Charge balance in chemistry means ensuring that the total charge of an ionic compound is zero. When creating formulas for ionic compounds, it is vital to consider the charges of the ions involved. This is often a straightforward process of ensuring that the number of cations and anions combine to yield a neutral compound.

• For example, in strontium nitride (\( \mathrm{Sr_{3}N_{2}} \) ), three strontium ions (\( \mathrm{Sr}^{2+} \) ) each carry a charge of +2, totaling +6. Two nitride ions (\( \mathrm{N}^{3-} \) ) each carry a -3 charge, totaling -6. The charges balance to zero, making a neutral compound.
• Similarly, in aluminum sulfide (\( \mathrm{Al_{2}S_{3}} \) ), two aluminum ions (\( \mathrm{Al}^{3+} \) ) carry a charge of +6, balanced by three sulfide ions (\( \mathrm{S}^{2-} \) ) totaling -6, achieving neutrality.

When determining the formula of an ionic compound, you might follow a cross-multiplication rule or simply factor in the smallest whole number ratio needed for charge balancing, ensuring the compound remains neutral.

Chemical Nomenclature

Chemical nomenclature is the system of naming chemical compounds. The names reflect the composition and sometimes the properties of the compounds. In the case of ionic compounds, the naming system is quite straightforward and follows specific rules:

• The name of the cation (positive ion) is written first in the compound name. This name is the same as the element's name, such as "rubidium" from \( \mathrm{Rb}^{+} \).
• The anion (negative ion) follows, and its name is often derived from the element’s name. For monatomic anions, typically, the element's name ends in "-ide," as in "iodide" from \( \mathrm{I}^{-} \) or "sulfide" from \( \mathrm{S}^{2-} \). For polyatomic ions like \( \mathrm{SO}_{4}^{2-} \) , the name like "sulfate" is used.

Let's see how this works:

• Rubidium iodide (\( \mathrm{RbI} \) ) consists of the cation "rubidium" and the anion "iodide."
• Cesium sulfate (\( \mathrm{Cs_{2}SO_{4}} \) ) includes the cation "cesium" and the polyatomic anion "sulfate."
• Strontium nitride (\( \mathrm{Sr_{3}N_{2}} \) ) is named after the cation "strontium" and the anion "nitride."
• Aluminum sulfide (\( \mathrm{Al_{2}S_{3}} \) ) includes "aluminum" and "sulfide."

The simplicity of naming ionic compounds makes it easier to communicate chemical information clearly and effectively. This system highlights the components of the compound while ensuring that all chemists understand what's being described.