How do I know when a name needs a Roman numeral?

Use a Roman numeral whenever the metal can form more than one ion — that means most transition metals plus tin and lead. Zinc, silver, cadmium, and the Group 1, 2, and aluminum cations have a single common charge, so they take no numeral. To find the numeral, compute the total negative charge from the anions and divide it by the number of metal atoms.

Do I have to memorize every polyatomic ion?

No. Memorize a core list of about fifteen, then use patterns: -ite has one fewer oxygen than -ate with the same charge, per- adds one oxygen above -ate, hypo- removes one below -ite, and adding an H reduces the negative charge by one. Learning nitrate, sulfate, carbonate, phosphate, hydroxide, and ammonium first unlocks most of the rest.

Why is H_2O not called dihydrogen monoxide in class?

Technically that name follows the covalent prefix rules, but water, ammonia (NH_3), and methane (CH_4) are so common that their traditional names are the accepted ones. Your teacher will expect the common names for those few compounds and systematic prefix names for everything else.

How do I tell an acid from an ordinary hydrogen compound?

Acids are written with hydrogen first and are understood to be dissolved in water, usually shown as (aq). HCl(g) is hydrogen chloride, a gas; HCl(aq) is hydrochloric acid. If your problem shows the aqueous state or the context is a solution, use the acid naming rules.

CHEM-4.3

Naming Compounds & Writing Formulas

Learn to translate between chemical names and formulas for ionic, covalent, and acidic compounds using charge balance, Roman numerals, polyatomic ions, and prefixes.

What you'll do in this lesson

A voice-first session with the Crimsora tutor on Naming Compounds & Writing Formulas, then targeted practice and FRQs — with the tutor adapting to where you get stuck.

Start Naming Compounds & Writing Formulas free

What this lesson covers

Chemistry has its own two-way dictionary: every compound has a name and a formula, and you need to move between them in either direction without guessing. The good news is that the system is almost entirely rule-based. The catch is that the rules are different for different families of compounds, so the very first decision you make — is this ionic, covalent, or an acid? — determines every step that follows.

In this lesson you will sort compounds into those three families, balance charges to build correct ionic formulas, memorize the small set of polyatomic ions that show up constantly, decide when a Roman numeral is required, apply Greek prefixes to molecular compounds, and handle the hydro-/-ic/-ous naming of acids. By the end you should be able to look at

\text{Cu}_3(\text{PO}_4)_2

and say its name out loud, and hear "dinitrogen pentoxide" and write it down.

Step Zero: Identify the Compound Type

Every naming question starts with the same question: what kind of substance is this? Get this wrong and every rule you apply afterward will be the wrong rule.

Clue	Compound type	Naming system
Metal + nonmetal (or metal + polyatomic ion)	Ionic	Cation name, then anion name; no prefixes
Nonmetal + nonmetal	Covalent (molecular)	Greek prefixes on both elements
Formula begins with H and the substance is aqueous	Acid	hydro- prefix or -ic/-ous ending
Ionic compound containing a transition metal, Sn, or Pb	Ionic, variable charge	Roman numeral required

The metal/nonmetal split is a periodic-table skill: metals sit to the left of the staircase, nonmetals to the right. Ammonium,

\text{NH}_4^{+}

, is the important exception — it contains no metal but behaves as a cation, so

\text{NH}_4\text{Cl}

is named as an ionic compound (ammonium chloride), not with prefixes.

A frequent mistake is applying prefixes to ionic compounds.

\text{CaCl}_2

is calcium chloride, never "calcium dichloride." Ionic formulas do not need prefixes because the charges lock in only one possible ratio: calcium is always

2+

and chloride is always

1-

, so the ratio must be 1:2. Molecular compounds have no such constraint — nitrogen and oxygen form

\text{NO}

\text{NO}_2

\text{N}_2\text{O}

, and

\text{N}_2\text{O}_4

— which is exactly why prefixes exist there and not in ionic naming.

Ionic Compounds: Charge Balance and Roman Numerals

An ionic formula must be electrically neutral overall. Find the charge on the cation and the charge on the anion, then choose the smallest whole-number ratio that makes the total charge zero. Main-group charges come straight from the group number: Group 1 is

1+

, Group 2 is

2+

, Group 13 is usually

3+

, Group 15 is

3-

, Group 16 is

2-

, Group 17 is

1-

.

The criss-cross shortcut works: magnesium

\text{Mg}^{2+}

with nitride

\text{N}^{3-}

gives

\text{Mg}_3\text{N}_2

. But always reduce.

\text{Mg}^{2+}

with

\text{O}^{2-}

criss-crosses to "

\text{Mg}_2\text{O}_2

," which must be reduced to

\text{MgO}

. Forgetting to reduce is one of the most common errors on this topic.

Monatomic anions take the -ide ending: chlorine becomes chloride, oxygen becomes oxide, phosphorus becomes phosphide. Polyatomic ions keep their own names and usually end in -ate or -ite. When more than one polyatomic ion is needed, put it in parentheses: aluminum sulfate is

\text{Al}_2(\text{SO}_4)_3

, not "

\text{Al}_2\text{SO}_{43}

."

Transition metals, plus tin and lead, form more than one possible ion, so their names include a Roman numeral giving the charge on the metal — not the subscript. In

\text{Fe}_2\text{O}_3

, three oxides carry

6-

total, so two irons must carry

6+

, making each iron

3+

: iron(III) oxide. Working backward, copper(II) phosphate needs

\text{Cu}^{2+}

and

\text{PO}_4^{3-}

, giving

\text{Cu}_3(\text{PO}_4)_2

. Silver (

\text{Ag}^{+}

), zinc (

\text{Zn}^{2+}

), and cadmium (

\text{Cd}^{2+}

) are transition metals with only one common charge, so most courses omit the Roman numeral for them.

Polyatomic Ions You Actually Need

Polyatomic ions are charged groups of covalently bonded atoms that travel together through reactions. There is no way around memorizing a core set, but patterns cut the workload roughly in half.

Ion	Formula	Ion	Formula
ammonium	$\text{NH}_4^{+}$	hydroxide	$\text{OH}^{-}$
nitrate	$\text{NO}_3^{-}$	nitrite	$\text{NO}_2^{-}$
sulfate	$\text{SO}_4^{2-}$	sulfite	$\text{SO}_3^{2-}$
carbonate	$\text{CO}_3^{2-}$	bicarbonate	$\text{HCO}_3^{-}$
phosphate	$\text{PO}_4^{3-}$	acetate	$\text{C}_2\text{H}_3\text{O}_2^{-}$
chlorate	$\text{ClO}_3^{-}$	permanganate	$\text{MnO}_4^{-}$
chromate	$\text{CrO}_4^{2-}$	dichromate	$\text{Cr}_2\text{O}_7^{2-}$
cyanide	$\text{CN}^{-}$	peroxide	$\text{O}_2^{2-}$

The -ate/-ite pattern is the key: within a pair, the -ite ion has one fewer oxygen but the same charge. Sulfate is

\text{SO}_4^{2-}

and sulfite is

\text{SO}_3^{2-}

; nitrate is

\text{NO}_3^{-}

and nitrite is

\text{NO}_2^{-}

. In the halogen family the pattern extends outward: perchlorate

\text{ClO}_4^{-}

, chlorate

\text{ClO}_3^{-}

, chlorite

\text{ClO}_2^{-}

, hypochlorite

\text{ClO}^{-}

— per- adds one oxygen above -ate, hypo- removes one below -ite.

Adding an

\text{H}

to a polyatomic ion raises the charge by one:

\text{CO}_3^{2-}

becomes hydrogen carbonate (bicarbonate)

\text{HCO}_3^{-}

;

\text{PO}_4^{3-}

becomes hydrogen phosphate

\text{HPO}_4^{2-}

and then dihydrogen phosphate

\text{H}_2\text{PO}_4^{-}

. Notice that cyanide and hydroxide end in -ide even though they are polyatomic — those two break the usual ending pattern and are worth flagging.

Covalent Compounds and Acids

For a compound made of two nonmetals, name the first element with a Greek prefix (dropped if the prefix would be mono-), then the second element with a prefix and the -ide ending. The prefixes are mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-. So

\text{CO}

is carbon monoxide,

\text{CO}_2

is carbon dioxide,

\text{P}_4\text{S}_{10}

is tetraphosphorus decasulfide, and

\text{SF}_6

is sulfur hexafluoride. Vowel clashes are smoothed out: mono- plus oxide becomes monoxide, penta- plus oxide becomes pentoxide. Water, ammonia (

\text{NH}_3

), and methane (

\text{CH}_4

) keep their common names.

The leading "mono-" is genuinely dropped, so "monocarbon dioxide" is wrong. Going the other direction, prefixes tell you the subscripts directly with no charge math at all: dinitrogen pentoxide is

\text{N}_2\text{O}_5

.

Acids are hydrogen compounds in water, and the ending of the acid name comes from the anion.

Anion ending	Acid name pattern	Example
-ide	hydro- + stem + -ic acid	$\text{HCl}$ = hydrochloric acid
-ate	stem + -ic acid	$\text{H}_2\text{SO}_4$ = sulfuric acid
-ite	stem + -ous acid	$\text{H}_2\text{SO}_3$ = sulfurous acid

A memory hook: "-ate becomes -ic, -ite becomes -ous," and only -ide anions get hydro-. Nitrate gives nitric acid (

\text{HNO}_3

); nitrite gives nitrous acid (

\text{HNO}_2

); phosphate gives phosphoric acid (

\text{H}_3\text{PO}_4

), where the three hydrogens come from balancing the

3-

charge. The number of hydrogens is set by the anion charge, exactly as in ionic charge balance.

Where Students Actually Go Wrong

Most naming errors trace back to a small number of habits, and knowing them in advance saves a lot of rework.

Misreading subscripts as charges is error number one. In

\text{Cu}_2\text{O}

, the Roman numeral is not II because of the subscript 2 — one oxide is

2-

, so two coppers share that, making each copper

1+

: copper(I) oxide. Always compute the metal charge from the anion total.

Forgetting to reduce is error number two. Calcium and oxygen criss-cross to

\text{Ca}_2\text{O}_2

, which reduces to

\text{CaO}

. Peroxides are the deliberate exception;

\text{H}_2\text{O}_2

stays as written because

\text{O}_2^{2-}

is a single ion.

Dropping parentheses is error number three. Magnesium hydroxide is

\text{Mg}(\text{OH})_2

; writing

\text{MgOH}_2

says something chemically different. Parentheses are required whenever a polyatomic ion appears more than once.

Mixing systems is error number four: prefixes on ionic compounds, or Roman numerals on molecular compounds. "Nitrogen(IV) oxide" is not standard;

\text{NO}_2

is nitrogen dioxide.

Finally, watch the transition metals with only one common charge. Zinc is essentially always

2+

and silver

1+

, so

\text{ZnS}

is zinc sulfide, not "zinc(II) sulfide," in most textbooks.

A reliable self-check when writing a formula: add up the total positive charge and the total negative charge and confirm they cancel. For

\text{Al}_2(\text{SO}_4)_3

, that is

2(3+) = 6+

and

3(2-) = 6-

. Zero net charge means the formula is at least charge-consistent, which catches the majority of mistakes before you hand the work in.

Key terms

Ionic compound.: A compound formed from a cation and an anion, typically a metal with a nonmetal or polyatomic ion, whose formula is the smallest whole-number ratio that makes the net charge zero.
Molecular (covalent) compound.: A compound formed from two or more nonmetals sharing electrons; named using Greek prefixes because multiple atom ratios are possible for the same pair of elements.
Polyatomic ion.: A group of covalently bonded atoms carrying an overall charge, such as $\text{SO}_4^{2-}$ or $\text{NH}_4^{+}$ , that stays intact as a unit in a formula.
Roman numeral.: A numeral in parentheses in an ionic name that states the charge on a variable-charge metal cation, as in iron(III) chloride, $\text{FeCl}_3$ .
Charge balance.: The requirement that total positive charge equals total negative charge in a neutral compound, used to determine subscripts in ionic formulas.
Binary acid.: An aqueous acid containing hydrogen and one other element, named with the hydro- prefix and the -ic acid ending, such as hydrobromic acid, $\text{HBr}$ .
Oxyacid.: An acid containing hydrogen, oxygen, and a third element; an -ate anion gives an -ic acid and an -ite anion gives an -ous acid.
-ide ending.: The suffix given to a monatomic anion in an ionic name, as chlorine becomes chloride and nitrogen becomes nitride.

Worked example

Complete the translation for each: (a) name

\text{Fe}(\text{NO}_3)_2

, (b) write the formula for chromium(III) sulfide, (c) name

\text{P}_2\text{O}_5

, (d) write the formula for chlorous acid.

Part (a). Iron is a metal, so this is ionic. The anion in parentheses is nitrate,

\text{NO}_3^{-}

, and there are two of them for a total of

2-

. To balance, the single iron must be

2+

, so the Roman numeral is II. The name is iron(II) nitrate. Do not write "iron(II) dinitrate" — no prefixes in ionic names.

Part (b). Chromium(III) means

\text{Cr}^{3+}

. Sulfide is

\text{S}^{2-}

. The least common multiple of 3 and 2 is 6, so you need two chromiums (

6+

) and three sulfides (

6-

). The formula is

\text{Cr}_2\text{S}_3

. Check:

2(3+) + 3(2-) = 0

.

Part (c). Phosphorus and oxygen are both nonmetals, so this is molecular and takes prefixes. Two phosphorus atoms give di-, five oxygens give penta-, and penta- plus oxide contracts to pentoxide. The name is diphosphorus pentoxide.

Part (d). The -ous ending signals an -ite anion, and the chlorine -ite ion is chlorite,

\text{ClO}_2^{-}

. One hydrogen ion balances the

1-

charge, so the formula is

\text{HClO}_2

. Compare with chloric acid,

\text{HClO}_3

, which comes from chlorate.

Practice questions

Which name correctly matches the formula

\text{Cu}_2\text{CO}_3

copper(I) carbonate
copper(II) carbonate
dicopper carbonate
copper(I) carbonite

Answer: copper(I) carbonate

Carbonate is

\text{CO}_3^{2-}

, a total of

2-

. Two copper ions must supply

2+

altogether, so each copper is

1+

and the Roman numeral is I. Copper(II) carbonate would be

\text{CuCO}_3

. Prefixes such as di- are not used in ionic names, and "carbonite" is not a real ion — the subscript 3 already belongs to carbonate.

Explain why

\text{CaBr}_2

is named calcium bromide while

\text{CBr}_4

is named carbon tetrabromide. Why does one name use a prefix and the other does not?

Answer: Calcium is a metal, so $\text{CaBr}_2$ is ionic and its ratio is fixed by charge balance ( $\text{Ca}^{2+}$ with two $\text{Br}^{-}$ ), meaning the name calcium bromide can only describe one formula. Carbon and bromine are both nonmetals, so $\text{CBr}_4$ is molecular; nonmetals can combine in several ratios, so prefixes are required to specify that there are four bromine atoms.

The rule is not arbitrary. Prefixes exist to remove ambiguity. Because ions have fixed charges, the neutral ratio for a given cation-anion pair is unique, so a prefix would add no information. Molecular compounds have no charge constraint, so nitrogen and oxygen can form

\text{NO}

\text{NO}_2

\text{N}_2\text{O}

, and more — names would be ambiguous without prefixes.

Write the correct chemical formula for aluminum hydroxide, and show the charge-balance check.

Answer: $\text{Al}(\text{OH})_3$ ; check: $1(3+) + 3(1-) = 0$ .

Aluminum is

\text{Al}^{3+}

and hydroxide is

\text{OH}^{-}

, so three hydroxides are needed. Because the polyatomic ion appears more than once, it must be enclosed in parentheses with the subscript outside. Writing

\text{AlOH}_3

would incorrectly suggest one oxygen and three hydrogens rather than three OH units.

FAQ

How do I know when a name needs a Roman numeral?: Use a Roman numeral whenever the metal can form more than one ion — that means most transition metals plus tin and lead. Zinc, silver, cadmium, and the Group 1, 2, and aluminum cations have a single common charge, so they take no numeral. To find the numeral, compute the total negative charge from the anions and divide it by the number of metal atoms.
Do I have to memorize every polyatomic ion?: No. Memorize a core list of about fifteen, then use patterns: -ite has one fewer oxygen than -ate with the same charge, per- adds one oxygen above -ate, hypo- removes one below -ite, and adding an H reduces the negative charge by one. Learning nitrate, sulfate, carbonate, phosphate, hydroxide, and ammonium first unlocks most of the rest.
Why is $\text{H}_2\text{O}$ not called dihydrogen monoxide in class?: Technically that name follows the covalent prefix rules, but water, ammonia ( $\text{NH}_3$ ), and methane ( $\text{CH}_4$ ) are so common that their traditional names are the accepted ones. Your teacher will expect the common names for those few compounds and systematic prefix names for everything else.
How do I tell an acid from an ordinary hydrogen compound?: Acids are written with hydrogen first and are understood to be dissolved in water, usually shown as (aq). $\text{HCl}(g)$ is hydrogen chloride, a gas; $\text{HCl}(aq)$ is hydrochloric acid. If your problem shows the aqueous state or the context is a solution, use the acid naming rules.

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