Polyatomic Ions

Polyatomic Ions

High School

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Learning Objectives

• Define polyatomic ions and differentiate them from binary ions.

• Identify common polyatomic ions and their charges.

• Write the chemical names for ionic compounds containing polyatomic ions.

• Write the chemical formulas for ionic compounds containing polyatomic ions.

Key Vocabulary

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Polyatomic Ion

An ion composed of two or more covalently bonded atoms that acts as a single charged unit.

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Ternary Ionic Compound

A compound made of three or more elements, typically including at least one polyatomic ion.

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Cation

A positively charged ion, which can be a metal or a polyatomic ion like ammonium (NH₄⁺).

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Anion

A negatively charged ion, which can be a non-metal element or a polyatomic ion like nitrate (NO₃^-).

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Oxidation Number

A number assigned to an element in a compound that represents its degree of oxidation or charge.

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What are Polyatomic Ions?

Polyatomic Ions

• A group of two or more atoms joined by covalent bonds, acting as a single unit with a net charge.

• The prefix 'poly-' means many, so the name literally means 'many-atom ion'.

• Its formula, like K₂SO₄, usually has more than two capital letters representing different elements.

Binary Ions

• A binary ion, also called a monatomic ion, is a single atom that carries a positive or negative charge.

• Polyatomic ions like sulfate (SO₄^2-) are different from single-atom binary ions.

• Common examples of binary ions include the sodium ion (Na⁺) and the chloride ion (Cl^-).

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Solved Example 1
What is the chemical formula for calcium phosphate, an ionic compound formed from calcium ions (Ca²⁺) and phosphate ions (PO₄³⁻)?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the chemical formula for calcium phosphate.

• Knowns: The cation is calcium (Ca²⁺) and the anion is the polyatomic ion phosphate (PO₄^3-).

• Unknown: The subscripts for each ion in the final formula.

• Method: Use the 'criss-cross' method to balance the positive and negative charges to zero.

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Solved Example 1
What is the chemical formula for calcium phosphate, an ionic compound formed from calcium ions (Ca²⁺) and phosphate ions (PO₄^3-)?

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Step 2: Solve for the Unknown

• Write the cation symbol first, followed by the anion: Ca²⁺ PO₄^3-.

• The numerical value of the anion's charge (3) becomes the subscript for the cation (Ca).

• The numerical value of the cation's charge (2) becomes the subscript for the anion (PO₄).

• This gives the formula Ca₃(PO₄)₂. Parentheses are required because there is more than one polyatomic ion.

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Solved Example 1
What is the chemical formula for calcium phosphate, an ionic compound formed from calcium ions (Ca²⁺) and phosphate ions (PO₄^3-)?

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Step 3: Evaluate the Answer

• Verify that the total charge of the compound is neutral.

• Total positive charge: 3 Ca ions × (+2 charge) = +6.

• Total negative charge: 2 PO₄ ions × (-3 charge) = -6.

• The net charge is (+6) + (-6) = 0. The formula is correct.

Multiple Choice

Based on the definition provided, which statement accurately describes a key characteristic of a polyatomic ion?

• The atoms within it are held together by ionic bonds.

• It is always composed of only two atoms.

• It is a group of atoms acting as a single unit with a charge.

• Its formula will only ever contain two capital letters.

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Naming Ternary Ionic Compounds

• To name, state the cation first, followed by the polyatomic anion.

• ​Anion names often end in -ite or -ate; MgCO₃ is Magnesium Carbonate.

• A common polyatomic cation is ammonium (NH₄⁺), as in Ammonium Chloride (NH₄Cl).

• For transition metals, use Roman numerals for charge, like Copper(I) Carbonate (Cu₂CO₃).

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Solved Example 2
Determine the systematic name for the ionic compound Fe(NO₃)₃. This compound contains the transition metal iron and the polyatomic nitrate ion. Use the Stock system to indicate the charge.

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the systematic name for Fe(NO₃)₃.

• Knowns: The compound is neutral. The anion is the polyatomic ion nitrate (NO₃), which has a charge of -1. There are 3 nitrate ions. The cation is iron (Fe), a transition metal.

• Unknown: The charge of the iron cation, which is needed for the Stock system name.

• Principle: The total positive charge from the cation must balance the total negative charge from the anions.

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Solved Example 2
Determine the systematic name for the ionic compound Fe(NO₃)₃. This compound contains the transition metal iron and the polyatomic nitrate ion. Use the Stock system to indicate the charge.

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Step 2: Solve for the Unknown

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Solved Example 2

Determine the systematic name for the ionic compound Fe(NO₃)₃. This compound contains the transition metal iron and the polyatomic nitrate ion. Use the Stock system to indicate the charge.

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Step 3: Evaluate the Answer

• Verify the charges balance to zero. One iron(III) ion has a +3 charge, and three nitrate ions have a total charge of 3 × (-1) = -3. The sum of the charges is (+3) + (-3) = 0, so the compound is neutral.

• The name Iron(III) Nitrate correctly identifies the cation (Iron), its specific charge (III), and the polyatomic anion (Nitrate). The answer is reasonable and correctly derived.

Multiple Choice

According to the rules for naming compounds with transition metals, what is the crucial second step when naming a compound like Cu₂CO₃?

• Determine the charge of the cation based on the total charge of the anion(s).

• Change the ending of the cation's name to '-ide'.

• Write the name of the anion first, followed by the cation.

• Always use the polyatomic ion name as the first part of the compound's name.

Decoding Anion Names: -ide, -ite, and -ate

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-ide Suffix

• ​This is for monatomic anions, or single atoms with a charge.

• ​​It usually means no oxygen atoms are in the anion.

• ​Potassium Nitride (K₃N) contains the nitride ion.

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-ite Suffix

• ​Indicates a polyatomic ion that contains one or more oxygen atoms.

• ​​This suffix is given to the ion with fewer oxygen atoms.

• ​For example, the Nitrite ion has the chemical formula NO₂^-.

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-ate Suffix

• ​Also indicates a polyatomic ion that contains one or more oxygen atoms.

• ​​This suffix is given to the ion with more oxygen atoms.

• ​For example, the Nitrate ion has the chemical formula NO₃^-.

Multiple Choice

Which anion ending should be used for a polyatomic ion that contains fewer oxygen atoms compared to a related ion?

• -ite

• -ous

• -ide

• -ate

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Writing Formulas With Polyatomic Ions

• Write the symbol and charge for the cation (Mg²⁺) and anion (NO₃^-).

• ​Use the criss-cross rule to make charge numbers the subscript of the other ion.

• Put parentheses around a polyatomic ion if its new subscript is greater than 1.

• The final formula for Magnesium Nitrate is correctly written as Mg(NO₃)₂.

Multiple Choice

When writing the chemical formula for Magnesium Nitrate (Mg²⁺ and NO₃^-), why are parentheses required around the nitrate ion?

• Because magnesium is a metal and nitrate is a non-metal.

• To show that two nitrate ions are needed to balance the charge of one magnesium ion.

• To indicate that nitrate has a negative charge.

• Because the criss-cross rule only applies to polyatomic ions.

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Common Misconceptions about Polyatomic Ions

Misconception	Correction
Subscripts inside an ion (like the 4 in SO42-) can be changed.	Subscripts within an ion are fixed and cannot be changed to balance charges.
Compounds ending in -ate or -ite are entirely ionic.	Atoms within the ion have covalent bonds; the overall compound is ionic.
Parentheses must always be used with polyatomic ions in formulas.	Parentheses are only used when more than one polyatomic ion is present.
All polyatomic ions are negatively charged anions.	Most are anions, but some are positive cations, like ammonium (NH4+).

Multiple Choice

What is the key difference in the naming process for a compound like K₂S versus K₂SO₃?

• The presence of a metal does not affect the naming of K₂SO₃.

• The charge of potassium must be specified for K₂SO₃ but not for K₂S.

• The name for K₂SO₃ uses the specific name of the polyatomic ion ending in '-ite', while K₂S uses the element name with an '-ide' suffix.

• K₂S is named anion first, while K₂SO₃ is named cation first.

Multiple Choice

Why is the correct formula for Ammonium Phosphate, formed from (NH₄)⁺ and (PO₄)^3-, written as (NH₄)₃PO₄?

• Phosphate is a transition metal that requires special balancing.

• The subscript '4' from ammonium is criss-crossed to the phosphate ion.

• Three +1 ammonium ions are required to create a total +3 charge, which balances the single -3 phosphate ion.

• The formula must contain three nitrogen atoms and four hydrogen atoms.

Multiple Choice

A compound has the formula Sn(CO₃)₂. Given that the carbonate ion (CO₃) has a charge of 2-, analyze this formula to determine the correct name for the compound.

• Tin Carbide

• Tin(I) Carbonate

• Tin(IV) Carbonate

• Tin(II) Carbonate

Multiple Choice

Predict the chemical formula for the compound formed between the Aluminum ion (Al³⁺) and the Sulfate ion (SO₄^2-).

• Al₂(SO₄)₃

• Al₃(SO₄)₂

• Al(SO₄)

• Al₂SO₄

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Summary

• Polyatomic ions are groups of atoms that act as a single unit with a charge.

• Identify polyatomic compounds by looking for more than two element symbols in the formula.

• When naming compounds, the cation is always named first, followed by the anion.

• The anion suffix -ite indicates fewer oxygen atoms, while -ate indicates more oxygen atoms.

• Use the Criss-Cross rule to write the correct chemical formula for a neutral compound.

• Parentheses are used in a formula only when there is more than one polyatomic ion.

Poll

On a scale of 1-4, how confident are you about writing names and formulas for compounds with polyatomic ions?

• 1

• 2

• 3

• 4