Ionic Compounds

Core Concept

An ionic compound is a neutral compound formed by the attraction between oppositely charged ions:

Cations: Positively charged ions (usually metals that have lost electrons).
Anions: Negatively charged ions (usually nonmetals that have gained electrons).

The Golden Rule: The total positive charge must exactly equal the total negative charge. The overall compound is always neutral (zero net charge).

Practice Tips

The "Criss-Cross" Method: Take the numerical value of the charge of the cation and make it the subscript of the anion, and vice versa.
Reduce to Simplest Ratio: Ionic formulas are empirical formulas. If you get $Mg_2O_2$, you must simplify it to $MgO$.
Roman Numerals for Transition Metals: If the metal can have multiple charges (like Iron, $Fe^{2+}$ or $Fe^{3+}$), you must include a Roman numeral in the name to specify the charge—e.g., Iron(III) chloride.
Parentheses Rule: Only use parentheses around a polyatomic ion if there is a subscript of 2 or greater outside of it. For example, $NaNO_3$ needs none, but $Mg(NO_3)_2$ does.

Test Yourself

Assorted Multiple Choice

When magnesium, which forms a two plus ion, reacts with chlorine, which forms a one minus ion, what is the correct chemical formula for the resulting ionic compound?

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Ionic Nomenclature

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01

Naming Binary Ionic Compounds (Fixed-Charge Metals)

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Writing Formulas for Binary Ionic Compounds (Fixed-Charge Metals)

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Naming Ionic Compounds with Transition Metals (Variable Charge)

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Writing Formulas for Ionic Compounds with Transition Metals (Variable Charge)

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Naming Ionic Compounds with Polyatomic Ions

→ 06

Writing Formulas for Ionic Compounds with Polyatomic Ions

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Determining Ion Charges and Interpreting Formulas

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Error Identification and Correction in Nomenclature

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Mixed Nomenclature

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Other / Uncategorized

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Assorted Multiple Choice

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Key Rules and Patterns

Unlike covalent compounds (like carbon dioxide), ionic compound names do not use Greek prefixes (di-, tri-, tetra-) to tell you how many atoms are there. Instead, the ratios are implied by the charges of the ions.

The Two Types of Metals

Fixed-Charge Metals (Groups 1, 2, and a few others): These metals always form the same charge (e.g., $\text{Na}^+$ is always $+1$, $\text{Mg}^{2+}$ is always $+2$).
Variable-Charge Metals (Transition Metals): These metals can form multiple different charges (e.g., Iron can be $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$). We use Roman numerals in parentheses to specify their charge.

The Two Types of Anions

Monatomic Anions: Single nonmetal atoms. Drop the ending of the element name and add "-ide" (e.g., Chlorine becomes Chloride).
Polyatomic Ions: Groups of atoms glued together that carry a collective charge. Never change their names. Keep them exactly as they appear on your polyatomic ion chart (e.g., $\text{SO}_4^{2-}$ is always Sulfate).

Step-by-Step Process: Formula to Name

When handed a chemical formula, follow this mental checklist:

Step 1: Identify and name the cation (the metal).
- Is it a transition metal? If yes, calculate its charge and include a Roman numeral.
Step 2: Identify and name the anion (the nonmetal or polyatomic ion).
- Is it a single atom? Change the ending to "-ide".
- Is it polyatomic? Use its specific name.
Step 3: Put the two names together (Cation + Anion).

  
      Formula: NaCl
    
      Ions:
      Cation: Sodium (Na+)
Anion: Chloride (Cl-)
    
      Final Name:
      Sodium chloride
    
      Why?
      Fixed charge metal; simple anion ends in -ide.
    
      Formula: Mg(NO3)2
    
      Ions:
      Cation: Magnesium (Mg2+)
Anion: Nitrate (NO3-)
    
      Final Name:
      Magnesium nitrate
    
      Why?
      Polyatomic ion retains its exact name. No prefixes used!
    
      Formula: FeCl2
    
      Ions:
      Cation: Iron(II) (Fe2+)
Anion: Chloride (Cl-)
    
      Final Name:
      Iron(II) chloride
    
      Why?
      Iron is a transition metal. Because there are two -1 chlorides, Iron must be +2.
    
      Formula: FeCl3
    
      Ions:
      Cation: Iron(III) (Fe3+)
Anion: Chloride (Cl-)
    
      Final Name:
      Iron(III) chloride
    
      Why?
      Same elements as above, but a different charge requires a different Roman numeral.

Troubleshooting & Common Mistakes

Mistake: Forgetting Roman Numerals on Transition Metals.

Incorrect: $\text{Fe}_2\text{O}_3$ = Iron oxide.
Correct: Iron(III) oxide. Without the Roman numeral, it's impossible to know which iron ion you are using.

Mistake: Putting Roman numerals on standard metals.

Incorrect: $\text{NaCl}$ = Sodium(I) chloride.
Correct: Sodium chloride. Group 1 and 2 metals never get Roman numerals because their charges never change.

Mistake: Confusing the Roman numeral with the subscript.

The Roman numeral represents the charge, not how many atoms are in the formula. For example, in $\text{Fe}_2\text{O}_3$, the iron has a $3+$ charge, so it is Iron(III), even though the subscript next to $\text{Fe}$ is 2.

Mistake: Changing polyatomic endings.

$\text{NaNO}_3$ is sodium nitrate. Do not change it to "sodium nitride" (which is $\text{Na}_3\text{N}$). One letter completely changes the chemical!

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