Problem 108 - Chemistry The Molecular Nature of Matter

Question

Identify each of the following as molecular or ionic, and give its name: (a) \(\mathrm{V}\left(\mathrm{NO}_{3}\right)_{3}\) (e) \(\mathrm{GeBr}_{4}\) (h) \(\mathrm{I}_{2} \mathrm{O}_{4}\) (b) \(\mathrm{Au}_{2} \mathrm{~S}\) (f) \(\mathrm{K}_{2} \mathrm{CrO}_{4}\) (i) \(\mathrm{I}_{4} \mathrm{O}_{9}\) (c) \(\mathrm{Au}_{2} \mathrm{~S}_{3}\) (g) \(\mathrm{Fe}(\mathrm{OH})_{2}\) (j) \(\mathrm{P}_{4} \mathrm{Se}_{3}\) (d) \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}\)

Step-by-Step Solution

Answer

The compounds are named as follows: (a) Vanadium(III) Nitrate, (e) Germanium Tetrabromide, (h) Diiodine Tetroxide, (b) Gold(I) Sulfide, (f) Potassium Chromate, (i) Tetraiodine Nonoxide, (c) Gold(III) Sulfide, (g) Iron(II) Hydroxide, (j) Tetraphosphorus Triselenide, (d) Cobalt(II) Acetate.

Step 1 - Identify Compound Types

Determine if the compound is ionic or molecular. Ionic compounds are generally formed between metals and nonmetals or involve polyatomic ions. Molecular compounds are usually composed of nonmetals only.

Step 2 - Determine the Naming System

Use the appropriate naming system for each compound. For ionic compounds, name the metal (cation) first, followed by the nonmetal (anion). Use Roman numerals to denote the charge on transition metals, if necessary. For molecular compounds, use numerical prefixes to denote the numbers of each atom and end the name of the second element with the suffix '-ide'.

Step 3 - Name each Compound Individually

Apply the rules of each naming system to the given compounds and name them accordingly. For polyatomic ions, use their established names.

Step 4: Substep 3a - Name \(\mathrm{V}\left(\mathrm{NO}_{3}\right)_{3}\)

This is an ionic compound with a transition metal and a polyatomic ion. Vanadium (V) is the transition metal and nitrate \(\mathrm{NO}_{3}\) is the polyatomic ion. The name is Vanadium(III) Nitrate.

Step 5: Substep 3b - Name \(\mathrm{GeBr}_{4}\)

This is a molecular compound, composed of a metalloid (germanium) and a nonmetal (bromine). Use prefixes to denote number of atoms. The name is Germanium Tetrabromide.

Step 6: Substep 3c - Name \(\mathrm{I}_{2} \mathrm{O}_{4}\)

This is a molecular compound. Using prefixes, the name is Diiodine Tetroxide.

Step 7: Substep 3d - Name \(\mathrm{Au}_{2} \mathrm{~S}\)

This is an ionic compound with gold and sulfur. The name is Gold(I) Sulfide.

Step 8: Substep 3e - Name \(\mathrm{K}_{2} \mathrm{CrO}_{4}\)

This is an ionic compound with the metal potassium and the polyatomic ion chromate. The name is Potassium Chromate.

Step 9: Substep 3f - Name \(\mathrm{I}_{4} \mathrm{O}_{9}\)

This is a molecular compound. The name is Tetraiodine Nonoxide.

Step 10: Substep 3g - Name \(\mathrm{Au}_{2} \mathrm{~S}_{3}\)

This is an ionic compound with gold and sulfur. The name is Gold(III) Sulfide.

Step 11: Substep 3h - Name \(\mathrm{Fe}(\mathrm{OH})_{2}\)

This is an ionic compound with the metal iron and the polyatomic ion hydroxide. Iron has a charge of +2, hence the name Iron(II) Hydroxide.

Step 12: Substep 3i - Name \(\mathrm{P}_{4} \mathrm{Se}_{3}\)

This is a molecular compound with two nonmetals. The name is Tetraphosphorus Triselenide.

Step 13: Substep 3j - Name \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}\)

This is an ionic compound with the metal cobalt and the polyatomic ion acetate. The name is Cobalt(II) Acetate.

Key Concepts

Ionic and Molecular Compounds

Understanding the difference between ionic and molecular compounds is a fundamental aspect of chemistry. Ionic compounds consist of a metal and a nonmetal, or they include polyatomic ions. They are held together by the strong electrostatic forces between positively charged cations and negatively charged anions. An example from our exercise is potassium chromate \( \mathrm{K}_{2} \mathrm{CrO}_{4} \), where potassium is the metal cation and chromate is the polyatomic anion.

Molecular compounds, on the other hand, are made up of nonmetals, and they share electrons to form covalent bonds. A clear example is diiodine tetroxide \( \mathrm{I}_{2} \mathrm{O}_{4} \), where iodine and oxygen share electrons to form a molecule. Shorter sentences and everyday language help to clarify that in ionic bonding, atoms 'transfer' electrons, whereas in covalent bonding, atoms 'share' electrons.

Chemical Nomenclature

Chemical nomenclature, the process of naming compounds, helps scientists communicate clearly. For ionic compounds, the metal (cation) is named first followed by the nonmetal (anion), with the end of the nonmetal's name typically changing to '-ide'. Polyatomic ions have specific names that must be memorized, such as nitrate in vanadium(III) nitrate \( \mathrm{V}\left(\mathrm{NO}_{3}\right)_{3} \). When dealing with transition metals, it's important to indicate the metal's ionic charge using Roman numerals, as seen in gold(I) sulfide \( \mathrm{Au}_{2} \mathrm{S} \).

Conversely, molecular compounds involve prefixes to indicate the number of atoms present, applied to both elements with the second element's name ending in '-ide', such as in germanium tetrabromide \( \mathrm{GeBr}_{4} \). By simplifying the rules and providing frequent examples, we can make this complex topic more approachable for learners.

Transition Metals

Transition metals can be tricky to understand due to their ability to take on multiple charges. They are the elements found in the center of the periodic table and include metals like iron, copper, and gold. These metals often form ionic compounds and necessitate the use of Roman numerals in their names to indicate their specific charge. For example, iron can have a charge of +2 or +3, leading to different compounds such as iron(II) hydroxide \( \mathrm{Fe}(\mathrm{OH})_{2} \) versus iron(III) oxide.

Emphasizing the variability of charges with transition metals and demonstrating through examples, like cobalt(II) acetate \( \mathrm{Co}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2} \), aids in cementing this concept for students.

Polyatomic Ions

Polyatomic ions are ions comprising multiple atoms that act as a single charged entity. Familiarizing oneself with common polyatomic ions is key to understanding many chemical compounds. For example, nitrate \( \mathrm{NO}_{3}^{-} \) and hydroxide \( \mathrm{OH}^{-} \) are polyatomic ions. Compounds containing polyatomic ions follow standard ionic naming rules, but instead of changing the nonmetal's suffix to '-ide', the name of the polyatomic ion is used as it is. Take potassium chromate \( \mathrm{K}_{2} \mathrm{CrO}_{4} \) as an example, where 'chromate' signifies the presence of the \( \mathrm{CrO}_{4}^{2-} \) ion. An exercise improvement would be to provide a list of common polyatomic ions to enhance memory retention.

By recognizing these ions, students will have an easier time naming compounds such as sodium sulfate or ammonium phosphate.