Naming and Interpreting Compounds: From Symbols to Substances

Why Naming Matters: Turning Formulas into Real Substances

Chemistry uses a compact “code” to describe substances. A formula like NaCl or C6H12O6 is a set of symbols and numbers, but it also implies what particles are present, how many of each, and often what kind of substance you have (a salt, a molecular compound, an acid, a hydrate, and so on). Naming is the reverse process: it turns that code into a standardized label so that anyone can identify the same substance.

This chapter focuses on how to interpret a chemical formula and how to name compounds using common rules. You will practice identifying the type of compound first, then applying a step-by-step naming method. You will also learn how to read formulas for useful information such as composition, ratio of atoms, and sometimes charge balance.

Step 1: Classify the Compound Before You Name It

Most naming mistakes happen because the compound type is misidentified. Before you try to name anything, decide which category it belongs to. Use these quick checks.

Quick classification checklist

• Starts with H (hydrogen) and is written like HCl, H2SO4, HNO3: often an acid (especially when discussed in water). Some hydrogen-containing compounds are not acids (like CH4), so look at what follows H.
• Contains a metal (or NH4+) combined with a nonmetal or a polyatomic ion: typically an ionic compound.
• Only nonmetals (including hydrogen) and no obvious ionic pieces: typically a molecular (covalent) compound.
• Has a dot in the formula like CuSO4·5H2O: a hydrate (an ionic compound with water molecules trapped in the crystal).
• Looks like a base (often ends with OH and begins with a metal or NH4): typically an ionic compound named as a hydroxide (e.g., NaOH).

Once you classify it, the naming rules become much more predictable.

Interpreting Chemical Formulas: What the Symbols and Numbers Tell You

A chemical formula communicates composition. You should be able to answer: Which elements are present? How many of each? Are there groups repeated? Is there water of hydration?

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Subscripts and coefficients

• Subscripts (small numbers in the formula) tell how many atoms of that element are in one formula unit or molecule. In CO2, there are 2 oxygen atoms per carbon atom.
• Coefficients (numbers in front, like 3CO2) multiply the entire formula. 3CO2 means 3 molecules of CO2, containing 3 carbon atoms and 6 oxygen atoms total.

Parentheses: grouped units

Parentheses indicate a group that repeats. For example, Ca(OH)2 contains 1 Ca, and the OH group appears twice, so it contains 2 O and 2 H. Parentheses are common with polyatomic ions and with repeated groups in molecular formulas.

Hydrates: the dot means “plus water”

In a hydrate, the dot separates the ionic compound from water molecules that are part of the crystal structure. For example, MgSO4·7H2O means one formula unit of magnesium sulfate is associated with 7 water molecules.

From formula to atom counts: a step-by-step method

Use this method whenever you need the total number of each atom.

• Step 1: Write each element symbol you see.
• Step 2: Apply subscripts to each element. If no subscript, it is 1.
• Step 3: If parentheses exist, multiply the subscripts inside by the subscript outside the parentheses.
• Step 4: If a coefficient exists, multiply every atom count by the coefficient.

Example: 2Al2(SO4)3

• Inside parentheses: SO4 occurs 3 times.
• One Al2(SO4)3 contains Al: 2, S: 3, O: 12.
• With coefficient 2: Al: 4, S: 6, O: 24.

Naming Ionic Compounds: The Core Pattern

Ionic compound names usually have two parts: the name of the positive ion (cation) followed by the name of the negative ion (anion). The formula itself is not named with prefixes like “di-” or “tri-” in most ionic cases; instead, the charge balance determines the ratio.

Step-by-step: naming a simple ionic compound (metal + nonmetal)

• Step 1: Name the cation (metal) as the element name.
• Step 2: Name the anion (nonmetal) by changing the ending to -ide.

Example: KBr → potassium bromide

Example: CaO → calcium oxide

When the metal can have more than one charge: Roman numerals

Many metals (especially transition metals) can form more than one cation. In that case, the name must specify the charge using a Roman numeral in parentheses right after the metal name.

Step-by-step: naming ionic compounds with variable-charge metals

• Step 1: Identify the anion and its charge (from memorized common ions or from the formula context).
• Step 2: Use the formula to determine the metal’s charge so the total charge is zero.
• Step 3: Name the metal with a Roman numeral equal to its charge.
• Step 4: Name the anion (ending -ide for single-element anions, or the polyatomic ion name).

Example: FeCl3

• Each chloride is Cl−. Three chlorides total charge = −3.
• So Fe must be +3.
• Name: iron(III) chloride.

Example: Cu2O

• Oxide is O2−.
• Two Cu must total +2, so each Cu is +1.
• Name: copper(I) oxide.

Polyatomic ions: keep the ion name unchanged

Polyatomic ions are groups of atoms that act as a unit with a specific charge. When naming ionic compounds containing them, you do not change their endings to -ide (except a few special cases like hydroxide already has its own name). You use the polyatomic ion’s name as-is.

Example: NaNO3 → sodium nitrate

Example: CaCO3 → calcium carbonate

Example: (NH4)2SO4 → ammonium sulfate

Step-by-step: naming an ionic compound with a polyatomic ion and a variable-charge metal

Example: Co(NO3)2

• Nitrate is NO3−. Two nitrates total charge = −2.
• So Co must be +2.
• Name: cobalt(II) nitrate.

Common “-ate” and “-ite” patterns (oxygen-containing ions)

Many polyatomic ions contain oxygen and come in related pairs. The naming pattern helps you interpret formulas and names consistently.

• -ate usually indicates the form with more oxygen.
• -ite usually indicates the form with less oxygen.

Example pair: sulfate (SO4 2−) vs sulfite (SO3 2−)

Example pair: nitrate (NO3 −) vs nitrite (NO2 −)

This matters because a single syllable change in the name corresponds to a different formula.

Naming Molecular (Covalent) Compounds: Prefixes Tell the Counts

Molecular compounds (typically nonmetal + nonmetal) are named using prefixes that indicate how many atoms of each element are present. This is where the formula-to-name connection is very direct: the subscripts become prefixes.

Common prefixes

• 1: mono-
• 2: di-
• 3: tri-
• 4: tetra-
• 5: penta-
• 6: hexa-
• 7: hepta-
• 8: octa-
• 9: nona-
• 10: deca-

Step-by-step: naming a molecular compound

• Step 1: Name the first element as the element name.
• Step 2: Add a prefix to the first element if it has a subscript greater than 1 (often you omit mono- on the first element).
• Step 3: Name the second element with an -ide ending.
• Step 4: Always use a prefix for the second element, including mono- if needed.
• Step 5: If a prefix ends in “a” or “o” and the next element name begins with a vowel, the final vowel is often dropped for smoother pronunciation (e.g., “monoxide” not “monooxide”).

Example: CO → carbon monoxide

Example: CO2 → carbon dioxide

Example: N2O4 → dinitrogen tetroxide

Example: PCl5 → phosphorus pentachloride

Interpreting a molecular name back into a formula

Because prefixes encode numbers, you can reverse the process.

Example: sulfur trioxide

• “sulfur” (no prefix) implies 1 S.
• “trioxide” implies 3 O.
• Formula: SO3.

Example: dinitrogen monoxide

• di- nitrogen → N2
• mono- oxide → O1
• Formula: N2O.

Acids: Naming Compounds That Start with Hydrogen

Acid names are a special system. In many introductory contexts, an “acid” refers to a substance that produces H+ in water, and the name depends strongly on the anion attached to hydrogen. The same formula can be discussed as a gas or as an aqueous acid, and the name can change based on context (for example, HCl(g) vs HCl(aq)).

Binary acids (hydrogen + single nonmetal)

These are typically written as HX (or H2X for some). When treated as acids in water, the naming pattern is:

• hydro- + nonmetal root + -ic acid

Example: HCl(aq) → hydrochloric acid

Example: HBr(aq) → hydrobromic acid

Example: H2S(aq) → hydrosulfuric acid

Oxyacids (hydrogen + oxygen-containing anion)

For acids based on oxygen-containing anions, the name depends on whether the anion ends in -ate or -ite.

• Anion ending -ate → acid ending -ic
• Anion ending -ite → acid ending -ous
• No “hydro-” prefix for oxyacids

Example: HNO3 → nitric acid (nitrate → nitric)

Example: HNO2 → nitrous acid (nitrite → nitrous)

Example: H2SO4 → sulfuric acid (sulfate → sulfuric)

Example: H2SO3 → sulfurous acid (sulfite → sulfurous)

Step-by-step: naming an acid from its formula

• Step 1: Confirm it is an acid context (often written with (aq) or discussed as an acid).
• Step 2: Identify the anion part (what remains after removing H).
• Step 3: If the anion is a single element ending in -ide, use hydro- + root + -ic acid.
• Step 4: If the anion ends in -ate, change to -ic acid; if -ite, change to -ous acid.

Example: HClO4

• Anion is ClO4−, called perchlorate.
• -ate → -ic acid.
• Name: perchloric acid.

Hydrates: Naming Compounds with Water Attached

Hydrates are named as an ionic compound followed by a prefix + “hydrate” to show how many water molecules are included per formula unit.

Step-by-step: naming a hydrate

• Step 1: Name the ionic compound part normally.
• Step 2: Count the number of water molecules after the dot.
• Step 3: Use a prefix for that number + the word hydrate.

Example: CuSO4·5H2O

• CuSO4 is copper(II) sulfate.
• 5 waters → penta-.
• Name: copper(II) sulfate pentahydrate.

Example: Na2CO3·10H2O → sodium carbonate decahydrate

Interpreting Names: What a Name Lets You Predict

A correct chemical name is not just a label; it hints at composition and sometimes at the kind of particles present.

From an ionic name to a formula (practical method)

When given an ionic compound name, you can often write the formula by combining ion charges so the total is zero.

Step-by-step method

• Step 1: Write the cation symbol and charge.
• Step 2: Write the anion symbol and charge.
• Step 3: Choose subscripts so total positive charge equals total negative charge.
• Step 4: Use parentheses if more than one polyatomic ion is needed.

Example: aluminum sulfate

• Al3+ and SO4 2−.
• Least common multiple of 3 and 2 is 6 → need 2 Al (total +6) and 3 sulfate (total −6).
• Formula: Al2(SO4)3.

Example: iron(III) oxide

• Fe3+ and O2−.
• LCM is 6 → 2 Fe and 3 O.
• Formula: Fe2O3.

From a molecular name to a formula (prefix decoding)

Example: dinitrogen pentoxide

• di- nitrogen → N2
• penta- oxide → O5
• Formula: N2O5.

Example: sulfur hexafluoride

• 1 sulfur → S
• hexa- fluoride → F6
• Formula: SF6.

Common Pitfalls and How to Avoid Them

Mistaking ionic for molecular (and using prefixes incorrectly)

Do not use prefixes like “di-” or “tri-” for typical ionic compounds. For example, CaCl2 is not “calcium dichloride”; it is calcium chloride. The “2” is implied by charge balance, not by naming prefixes.

Forgetting Roman numerals for variable-charge metals

If the metal can have more than one common charge, include the Roman numeral. FeCl2 and FeCl3 are different substances, so “iron chloride” is incomplete. They are iron(II) chloride and iron(III) chloride.

Confusing -ate and -ite

Because -ate and -ite correspond to different oxygen counts, mixing them changes the formula. Nitrate (NO3−) and nitrite (NO2−) lead to different compounds and different acids.

Misreading parentheses in formulas

In Al2(SO4)3, the 3 multiplies both S and O inside the parentheses. A quick check is to expand it: (SO4)3 means S3O12.

Acid naming without context

HCl can be called hydrogen chloride (as a molecular compound, often a gas) or hydrochloric acid (when acting as an acid in water). In many chemistry problems, (aq) signals the acid name is expected.

Practice Set: Name These Formulas (with Worked Steps)

1) SnCl2

Step 1: Metal + nonmetal → ionic.

Step 2: Chloride is Cl−, two chlorides total −2.

Step 3: Tin must be +2 → tin(II).

Name: tin(II) chloride.

2) SnCl4

Step 1: Ionic.

Step 2: Four Cl− total −4 → tin is +4.

Name: tin(IV) chloride.

3) N2O3

Step 1: Nonmetal + nonmetal → molecular.

Step 2: di- nitrogen, tri- oxide.

Name: dinitrogen trioxide.

4) (NH4)3PO4

Step 1: Contains NH4+ and a polyatomic anion → ionic.

Step 2: Name cation: ammonium.

Step 3: PO4 3− is phosphate.

Name: ammonium phosphate.

5) H2CO3

Step 1: Starts with H and contains an oxygen-containing anion → oxyacid.

Step 2: Anion is carbonate (CO3 2−), ends in -ate.

Step 3: -ate → -ic acid.

Name: carbonic acid.

6) Co(OH)3

Step 1: Ionic (metal + hydroxide).

Step 2: OH− is −1 each; three hydroxides total −3.

Step 3: Cobalt must be +3.

Name: cobalt(III) hydroxide.

Practice Set: Write the Formula from the Name (with Worked Steps)

1) calcium nitrate

Step 1: Calcium is Ca2+.

Step 2: Nitrate is NO3−.

Step 3: Need two nitrates to balance +2.

Formula: Ca(NO3)2.

2) copper(I) sulfide

Step 1: Copper(I) is Cu+.

Step 2: Sulfide is S2−.

Step 3: Need two Cu+ to balance one S2−.

Formula: Cu2S.

3) diphosphorus tetrafluoride

Step 1: di- phosphorus → P2.

Step 2: tetra- fluoride → F4.

Formula: P2F4.

4) iron(III) carbonate

Step 1: Iron(III) is Fe3+.

Step 2: Carbonate is CO3 2−.

Step 3: LCM of 3 and 2 is 6 → 2 Fe and 3 carbonate.

Formula: Fe2(CO3)3.

Mini-Reference: Fast Naming Templates

Ionic (fixed-charge metal)

Template: metal name + nonmetal root + ide

Example: MgCl2 → magnesium chloride

Ionic (variable-charge metal)

Template: metal name(Roman numeral) + anion name

Example: PbO2 → lead(IV) oxide

Molecular (two nonmetals)

Template: prefix + element 1 + prefix + element 2 root + ide

Example: Cl2O7 → dichlorine heptoxide

Acids

Binary acid: hydro + root + ic acid

Oxyacid: -ate → -ic acid, -ite → -ous acid

Hydrates

Template: ionic compound name + prefix + hydrate

Example: BaCl2·2H2O → barium chloride dihydrate