Chemical Names, Formulas & Polyatomic Ions · Grade 11 Science
Chapter: Bonding, Formulas & Molecular StructureChemical Names, Formulas & Polyatomic Ions
Learning goals
- I can write neutral ionic formulas with polyatomic ions.
- I can distinguish molecular prefixes, subscripts, and metal-charge numerals.
Readings for this unit
Before this unit
Ions, integer ratios, and counting formula subscripts.
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Start with something to explain
Why can changing a subscript change the substance rather than merely its amount?
Illustrative evidence case. CO and CO₂ have different formulas, while 2CO₂ changes how many CO₂ entities are counted.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can changing a subscript change the substance rather than merely its amount?
Your first model
Separate a coefficient in front from subscripts inside each formula.
Build the language from the idea
- polyatomic ion
- A bonded group of atoms with an overall charge.
- Roman numeral
- A notation specifying charge of a variable-charge metal cation.
- subscript
- A number giving atom counts within a formula.
A chemical formula states relative particle counts, not a recipe for mixing arbitrary amounts. An ionic compound must be electrically neutral overall. Combine charge magnitudes to find the smallest whole-number ratio, then check the total charge. Ca²⁺ and Cl⁻ give CaCl₂ because +2 + 2(−1) = 0.
A polyatomic ion is a bonded group with net charge that acts as a unit in many reactions. Common examples are ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, and phosphate PO₄³⁻. Parentheses preserve a group when more than one is needed: Ca(NO₃)₂ contains two nitrate ions.
Name a simple ionic compound with the cation first and anion second. A monatomic anion ends in -ide; oxygen gives oxide and chlorine chloride. Keep the established polyatomic-ion name. Metals with multiple common charges use Roman numerals to specify the cation charge: FeCl₃ is iron(III) chloride, not three iron atoms.
Binary molecular compounds usually use numerical prefixes: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6. CO is carbon monoxide and CO₂ carbon dioxide. Prefixes specify atom numbers, while Roman numerals specify ionic charge. Do not apply molecular prefixes indiscriminately to salts.
Formula subscripts count atoms inside one formula unit or molecule; a coefficient multiplies the entire formula. In 3Ca(NO₃)₂ there are 3 calcium atoms, 6 nitrogen atoms, and 18 oxygen atoms in the represented particle count. Changing a subscript changes substance identity; changing a coefficient changes amount.
Build a small ion-reference notebook and check neutrality instead of memorizing a criss-cross trick alone. Charge ratios must be reduced: Mg²⁺ and O²⁻ yield MgO, not Mg₂O₂. Naming is a way to communicate structure and composition; household product names do not necessarily identify pure substances.
Build your understanding
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Part 1 · Determine whether the naming system is ionic or molecular
Chemical naming systems differ because formulas describe different kinds of entities. In common ionic nomenclature, ion names and charges determine a neutral ratio; Greek prefixes are not normally used to name sodium oxide as disodium monoxide. For many binary molecular compounds, prefixes communicate atom counts, as in carbon dioxide. A Roman numeral in a variable-charge metal name identifies the metal’s oxidation-state/charge assignment in the ionic model, not the number of metal atoms. Identify the compound type before choosing the naming rules.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why does carbon dioxide use a prefix while iron(III) chloride uses a Roman numeral?
Visual reasoning — after your attempt
The molecular prefix gives oxygen count in CO₂; the Roman numeral assigns iron +3 in the ionic charge account for FeCl₃.
Ionic naming commonly gives a cation followed by an anion, while many binary molecular names use numerical prefixes. Decide from the supplied identities and bonding context rather than blindly applying prefixes to every formula. Element symbols and subscripts carry composition; names should reproduce that information correctly.
NaCl is sodium chloride, not sodium monochloride in standard ionic naming. CO₂ is carbon dioxide in familiar binary molecular naming.
Pause and explain
Why are molecular prefixes inappropriate for ordinary sodium chloride naming?
Check your reasoning after answering
Ionic naming uses ion identities and charge-balanced ratios rather than the binary molecular prefix system.
Part 2 · Use neutrality to identify variable metal charge
For a neutral ionic formula, the sum of ionic charges must be zero. If chloride is −1, three chlorides require an iron assignment of +3 in FeCl₃; two require +2 in FeCl₂. The subscripts and the charge are different quantities. Write an equation rather than copying a subscript into a Roman numeral without checking the anion’s charge. This introductory method applies to the specified ion model; some mixed-valence solids and complex ions require a more detailed account.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Would an oxygen subscript of three by itself prove a metal has charge +3?
Visual reasoning — after your attempt
No. Use the number and charge of every ion in the full neutral formula, including the metal count.
Some metals form more than one common cation. A Roman numeral gives the charge/oxidation state specified in the ionic name, not the number of metal atoms. Infer the metal charge from a neutral formula using known anion charges.
In FeCl₃, three Cl⁻ ions total −3, so Fe is +3 and the name is iron(III) chloride. Iron(II) chloride would have a different formula.
Pause and explain
Does iron(III) mean three iron atoms?
Check your reasoning after answering
No. The numeral specifies the +3 ion/oxidation state in this naming context. The formula determines atom or ion count.
Part 3 · Keep a polyatomic ion intact
A polyatomic ion is a charged group of covalently connected atoms, such as nitrate NO₃⁻ or sulfate SO₄²⁻. Its internal formula is retained when charge balance determines how many groups are needed. Calcium nitrate therefore needs two nitrate ions, written Ca(NO₃)₂, giving one Ca, two N and six O atoms. Parentheses group the ion; they do not change its internal subscript to force neutrality. Learn common ion names and charges as linked information, and distinguish similarly named but chemically different ions.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
How many oxygen atoms are represented by Ca(NO₃)₂, and why is CaNO₆ not an equivalent formula account?
Visual reasoning — after your attempt
Six oxygen atoms occur in two nitrate groups, along with two nitrogen atoms. CaNO₆ would lose a nitrogen and fail to represent the stated ions.
A polyatomic ion has a supplied formula and net charge. Use parentheses when more than one of that group is needed. Do not alter its internal subscripts to balance a compound; doing so changes the ion’s chemical identity. Show the complete charge sum.
Ca²⁺ with NO₃⁻ gives Ca(NO₃)₂. The formula contains one calcium, two nitrogen, and six oxygen atoms per formula unit.
Pause and explain
Why write Ca(NO₃)₂ rather than CaNO₆?
Check your reasoning after answering
Two intact nitrate ions are required. Parentheses preserve nitrate’s N: O ratio and show the full group repeated twice.
Part 4 · Check the name by reconstructing the formula
A reliable nomenclature check reconstructs a formula from the proposed name. Aluminum sulfate combines Al³⁺ and SO₄²⁻ in a 2:3 ratio, while dinitrogen tetroxide specifies molecular counts directly. Reducing a molecular formula such as N₂O₄ to NO₂ would change the named molecule, even though both share an empirical atom ratio. Ionic formula ratios are usually reduced to simplest whole numbers without altering polyatomic groups. Distinguish molecular formula from empirical ratio when checking the result.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why is changing N₂O₄ into NO₂ not merely simplifying the same molecular formula?
Visual reasoning — after your attempt
It changes the molecule’s atom counts. The two formulas share an empirical ratio but denote different molecular entities.
After naming a formula, reconstruct it from the name and supplied ion charges. This round trip catches ambiguous charges and lost parentheses. Molecular names and ionic names encode composition differently, so the check should use the selected naming convention consistently.
From iron(III) oxide, Fe³⁺ and O²⁻ give Fe₂O₃. From dinitrogen tetroxide, prefixes give N₂O₄ without balancing fictional ion charges.
Pause and explain
What is a useful independent check on a chemical name?
Check your reasoning after answering
Rebuild the formula from the name and reference ion charges or prefixes, then compare it with the original composition.
Apply the whole unit · Name and count a nitrate compound
Illustrative reference ions: Fe³⁺ and NO₃⁻. A proposed neutral salt has formula Fe(NO₃)₃. Use standard ionic naming.
Your task
Name the compound, show its charge balance, count each element in one formula unit and in two formula units, and explain the different jobs of the Roman numeral, parentheses, and leading coefficient.
What your response should show
- Choose the appropriate chemical model or relationship and state its conditions.
- Show substitutions, units, particle or charge counts, and intermediate reasoning as relevant.
- Use supplied evidence to justify the conclusion; distinguish observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
Three nitrate ions give −3, balancing one Fe³⁺.
Name: iron(III) nitrate.
One formula unit contains Fe: 1,N: 3,O: 9.
A coefficient 2 multiplies all counts: Fe: 2,N: 6,O: 18.
The Roman numeral denotes iron’s +3 state; the subscript outside parentheses repeats the nitrate group three times; a leading coefficient repeats the entire formula unit. These are not interchangeable. The formula describes composition of a salt, not proof that individual intact neutral Fe(NO₃)₃ molecules exist in an extended solid.
Scientific references and source-model foundations
Parent / teacher teaching notes
Provide a small ion reference list. Require charge sums and formula reconstruction. Watch for changing polyatomic-ion subscripts instead of repeating intact groups. Ask the learner to explain each particle symbol, axis, unit and arrow before calculating. Distinguish hypothetical models and calculated curves from measured records. Public OpenSciEd excerpts here are selected foundation prompts, not complete high-school teacher editions or an endorsement. All required advanced explanations and practice are provided locally.
The key idea
Use charge neutrality for ionic formulas and distinguish ion charge from atom count.
Worked examples
Example 1
- Least common charge magnitude is 6.
- Use 2 aluminum ions and 3 sulfate ions.
- Check +6−6 = 0.
Example 2 · Worked application
- Three oxide ions contribute 3×(−2)=−6.
- Two irons must total +6, giving +3 each.
- Use a Roman numeral for the variable-charge metal.
The Roman numeral gives oxidation state, not number of iron atoms.
Example 3
- Three O²⁻ contribute −6.
- Two iron ions must each be +3.
Example 4 · Find and fix the mistake
- Test the claim against the stated conditions: The Roman numeral gives oxidation state, not number of iron atoms.
- III denotes iron's +3 oxidation state; charge balance gives Fe₂O₃.
The Roman numeral gives oxidation state, not number of iron atoms.
Example 5 · Reason from the visual model
- A polyatomic ion is a charged group of covalently connected atoms, such as nitrate NO₃⁻ or sulfate SO₄²⁻. Its internal formula is retained when charge balance determines how many groups are needed. Calcium nitrate therefore needs two nitrate ions, written Ca(NO₃)₂, giving one Ca, two N and six O atoms. Parentheses group the ion; they do not change its internal subscript to force neutrality. Learn common ion names and charges as linked information, and distinguish similarly named but chemically different ions.
- Six oxygen atoms occur in two nitrate groups, along with two nitrogen atoms. CaNO₆ would lose a nitrogen and fail to represent the stated ions.
Original stoichiometric formula construction; the reaction-style arrow indicates combining counts, not a complete solution reaction.
Avoid these mistakes
- Do not break a polyatomic group apart when balancing charge.
Check your understanding
Compare coefficients, subscripts, and Roman numerals.
Discuss the answer
Coefficients multiply whole particles; subscripts count atoms within a formula; Roman numerals specify a metal ion charge.
Return to the phenomenon
Return to the initial case. Why can changing a subscript change the substance rather than merely its amount? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Subscripts specify composition of an entity; coefficients scale its amount. Naming and equation balancing must preserve that distinction.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, calculate and revise
Keep a first model and an evidence-supported revision. Attempt each task before opening guidance; all required explanations and supplied-data routes are available locally.
Activity 1 · Return to the opening case
Use the opening phenomenon, unit definitions and explained Parts 1–4.
Your task
Return to the initial case. Why can changing a subscript change the substance rather than merely its amount? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use the stated observations and appropriate units.
- Connect a labeled model with the chemical mechanism.
- State assumptions and an evidence limitation.
- Explain a substantive revision or next test.
Hint
Keep species identity, material transfer and energy account distinct.
Response guidance — discuss after your attempt
Subscripts specify composition of an entity; coefficients scale its amount. Naming and equation balancing must preserve that distinction.
Activity 2 · Transfer to a changed case
Cover the worked answer and use the explained models, relationships and notebook.
Your task
Write the formula for iron(III) sulfate and count all atoms per formula unit. Then name N₂O₃ using molecular prefixes.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the chemical mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name known quantities and conditions, show intermediate steps and check conservation.
Response guidance — discuss after your attempt
Fe³⁺ and SO₄²⁻ require Fe₂(SO₄)₃: two Fe, three S and twelve O atoms. N₂O₃ is dinitrogen trioxide. The Roman numeral and prefixes serve different bookkeeping roles.
Words to know
- polyatomic ion
- A bonded group of atoms with an overall charge.
- Roman numeral
- A notation specifying charge of a variable-charge metal cation.
- subscript
- A number giving atom counts within a formula.
In the real world
Use the chapter investigation to connect this unit to measured evidence and a practical decision.
Exit check
Compare coefficients, subscripts, and Roman numerals.
Check your explanation after answering
Coefficients multiply whole particles; subscripts count atoms within a formula; Roman numerals specify a metal ion charge.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.