Notes: Ionic and Covalent Compounds, and Stoichiometry

Core Chemistry Topics

Overview of Key Topics

For effective note organization, consider these major topics covered so far:

• Periodic Table

• Periodic Trends

• Electron Configurations (often combined with orbital filling diagrams)

• Ionic Compounds

• Covalent Compounds

• Stoichiometry and Calculations

Ionic Compounds

Ionic compounds involve the transfer of electrons, where one atom loses electrons (forming a cation) and another gains electrons (forming an anion).

Naming and Formulas for Transition Metal Compounds

Transition metals can have multiple charges, requiring a Roman numeral in their name to indicate the charge.

• General Naming Convention: Cation name (as it appears on the periodic table) + Roman numeral (indicating charge) + Anion name (with an "-ide" ending for single elements). E.g., Chloride, Phosphide.

• Determining Roman Numeral - Cautions with Reverse Drop and Swap:

  1. Initial Reverse Drop and Swap: For a formula like $M<em>{a}X</em>{b}$, assume the charges are $M^{b+}$ and $X^{a-}$. This gives an initial set of hypothetical charges.

  2. Check Anion's Actual Charge: Consult the periodic table to verify the actual charge of the anion (the non-metal).

     • Example 1: $MnCl_{3}$ (Manganese chloride)

       • Reverse drop and swap: $Mn^{3+}$ and $Cl^{1-}$. (The subscript $3$ from Cl goes to Mn as $3+$; the implied subscript $1$ from Mn goes to Cl as $1-$).

       • Check Anion: Chlorine (Cl) is indeed in Group 17, typically forming a $1-$ ion. This matches.

       • Conclusion: The charge for manganese is $3+$. Name: Manganese(III) chloride.

     • Example 2: Manganese phosphide (hypothetical case where reverse drop and swap might be misleading)

       • Assume an initial reverse drop and swap yielded $Mn^{2+}$ and $P^{1-}$. (This implies a formula like MnP, where charges are mistakenly simplified).

       • Check Anion: Phosphorus (P) is in Group 15, so it typically forms a $3-$ ion, not $1-$.

       • Correction Process (Ratio Method):

         • The derived charge for phosphorus was $1-$, but it should be $3-$. The scaling factor is $3-/1- = 3$.

         • Apply this scaling factor to the derived cation charge: $2+ imes 3 = 6+$.

         • Conclusion: The actual charge for manganese is $6+$. Name: Manganese(VI) phosphide.

Polyatomic Ions

• Recognition: The absence of the "-ide" ending (e.g., chromate, hydroxide) is a strong hint that a polyatomic ion is present, as single elements usually end in "-ide". However, some polyatomic ions (like hydroxide) do end in "-ide".

• Formulas: When using polyatomic ions, enclose the polyatomic ion in parentheses if its subscript is greater than $1$.

  • Example: Lead(III) chromate

    • Lead (Pb) is $3+$ (from Roman numeral).

    • Chromate ($CrO_{4}$) is a polyatomic ion with a charge of $2-$.

    • Drop and swap the charges: $Pb^{3+}$ and $(CrO_{4})^{2-}$.

    • Formula: $Pb<em>{2}(CrO</em>{4})_{3}$.

Hydrates

• Definition: Hydrates are ionic compounds that contain a specific number of water molecules associated with each formula unit.

• Naming: Name the ionic compound first, then add a prefix indicating the number of water molecules, followed by "-hydrate."

  • Prefixes: For example, hepta- for $7$ (similar to covalent naming).

  • Example: $MgSO<em>{4} \bullet 7H</em>{2}O$ is Magnesium sulfate heptahydrate (commonly known as Epsom salts).

• Significance: The presence and number of water molecules in a hydrate significantly alter the compound's physical and chemical properties.

  • Real-world Example: Concrete: Concrete is primarily composed of carbonate compounds and is itself a hydrate. When concrete