B2.7 Write formulas and IUPAC names for binary and polyatomic compounds

Ontario Grade 11 Chemistry

Matter, Chemical Trends, and Chemical Bonding

Ontario Grade 11 Chemistry — B2.7

A white crystal of table salt and a gas such as carbon dioxide are both compounds, but their names and formulas are built in different ways. A formula tells which elements or ions are present and their number ratio. A name tells how to identify the compound. To move reliably between them, first identify whether the compound is ionic or molecular. Then apply the matching rules.

What you will learn

Use the type of elements in a compound to choose a naming method.
Write and name binary ionic compounds, including compounds with metals that can form more than one ion.
Write and name binary molecular compounds using prefixes.
Write formulas and IUPAC names for compounds that contain polyatomic ions.

1. From elements and particles to a naming method

A compound is a substance made from two or more different elements joined in a fixed ratio. An element is a substance made of one kind of atom. A chemical formula uses element symbols and subscripts to show the ratio. For example, in

\mathrm{MgCl_2}

, the subscript 2 means two chlorine atoms or ions for each magnesium atom or ion.

The periodic table helps identify likely ions. Metals, found mainly on the left and centre of the table, commonly form positive ions by losing electrons. Non-metals, found mainly on the right, commonly form negative ions by gaining electrons. An ion is an atom or group of atoms with a charge. In an ionic compound, positive and negative ions combine in a ratio that makes the overall compound neutral.

A molecular compound is made of non-metal atoms joined in groups called molecules. Its formula shows the number of each kind of atom in one molecule. Unlike ionic formulas, molecular formulas use prefixes in the name to state the number of atoms.

A binary compound contains exactly two different elements. It may be ionic, such as a metal with a non-metal, or molecular, usually two non-metals. A polyatomic ion is a charged group of atoms that stays together when it is part of an ionic compound. Compounds with these ions can contain more than two elements.

Identify the elements before naming a compound.
Metal plus non-metal usually signals an ionic compound; two non-metals usually signal a molecular compound.
A neutral ionic formula must have a total charge of zero.

2. Binary ionic compounds

For a binary ionic compound, name the positive ion first. Its name is usually the metal's element name. Name the negative ion second, and change the ending of the non-metal name to -ide. For example, chlorine becomes chloride. This gives magnesium chloride for

\mathrm{MgCl_2}

.

To write the formula, use the ion charges to find the smallest whole-number ratio that makes the total charge zero. The charges are a planning tool; they are not written as charges in the final neutral formula. Do not change the small numbers within an ion, and do not write a subscript of 1.

Some metals can form ions with different charges. For these metals, the name includes a Roman numeral in parentheses to show the charge of the metal ion. The numeral is not the number of atoms. For example,

\mathrm{FeCl_3}

is iron(III) chloride because three chloride ions, each with charge

-1

, balance one iron ion with charge

+3

. Use the course reference table or the formula's charge balance to identify the metal's charge.

\mathrm{Mg^{2+} + 2Cl^- \rightarrow MgCl_2}

Name the metal first and the non-metal second with an -ide ending.
Use charge balance to determine subscripts.
Use a Roman numeral for a metal that can form ions with more than one charge.

3. Binary molecular compounds

For a binary molecular compound, use prefixes to show how many atoms of each element are in the formula. Common prefixes are mono- for one, di- for two, tri- for three, tetra- for four, and penta- for five. The first element keeps its element name. The second element ends in -ide.

The prefix mono- is usually left off the first element's name. It is kept when needed on the second element. Thus,

\mathrm{CO}

is carbon monoxide, not monocarbon monoxide, and

\mathrm{CO_2}

is carbon dioxide. The prefix tells the atom count; do not use ion charges to set the subscripts in a molecular formula.

Read the formula's subscripts to name the compound, and read the prefixes in the name to write the subscripts. If the first element has no subscript, it represents one atom. For instance, the name dinitrogen tetroxide indicates two nitrogen atoms and four oxygen atoms, so its formula is

\mathrm{N_2O_4}

.

\mathrm{CO_2}

Prefixes describe atom counts in molecular compounds.
Use -ide for the second element.
Do not apply ionic charge balancing to compounds made from two non-metals.

4. Compounds with polyatomic ions

Learn the name, formula, and charge of the polyatomic ions listed in your course reference table. Common examples include nitrate,

\mathrm{NO_3^-}

; sulfate,

\mathrm{SO_4^{2-}}

; hydroxide,

\mathrm{OH^-}

; and ammonium,

\mathrm{NH_4^+}

. The atoms inside each ion stay together as a unit when you write a compound formula.

Name an ionic compound with a polyatomic ion by naming the positive ion first and the negative ion second. Keep the polyatomic ion's name unchanged. For example,

\mathrm{Ca(NO_3)_2}

is calcium nitrate. The parentheses show that the subscript 2 applies to the whole nitrate ion, not only to oxygen.

To write a formula, balance the charges of the positive and negative ions, just as for a binary ionic compound. If more than one of a polyatomic ion is needed, place its formula in parentheses and put the count outside. Do not use parentheses when only one of that ion is needed. Preserve the ion's internal subscripts and charge while working; omit the charges from the final neutral compound formula.

\mathrm{Ca^{2+} + 2NO_3^- \rightarrow Ca(NO_3)_2}

Keep each polyatomic ion's atoms together and preserve its formula.
Use parentheses when a formula needs more than one of a polyatomic ion.
Name the ions; do not add an -ide ending to a polyatomic ion's name.

Useful molecular prefixes

Number of atoms	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-

Worked example

Write and name a compound from its ions

A compound contains aluminium ions,

\mathrm{Al^{3+}}

, and sulfate ions,

\mathrm{SO_4^{2-}}

. Determine its neutral formula and IUPAC name.

Identify the ions
Aluminium is the positive ion, and sulfate is the negative polyatomic ion. The sulfate group remains together as $\mathrm{SO_4^{2-}}$ .
Find a neutral ratio
The smallest total positive and negative charges that match are six. Two aluminium ions provide a total charge of $+6$ , while three sulfate ions provide a total charge of $-6$ .
$2(+3)+3(-2)=0$
Write the formula
Use two aluminium ions and three sulfate ions. Since there is more than one sulfate ion, put its formula in parentheses before adding the outside subscript.
$\mathrm{Al_2(SO_4)_3}$
Name the compound
Name the positive ion first, then the polyatomic ion. Aluminium has a fixed charge in this naming pattern, so no Roman numeral is needed.

Answer: The formula is

\mathrm{Al_2(SO_4)_3}

, and the name is aluminium sulfate.

Check: The formula has two aluminium ions with a total charge of

+6

and three sulfate ions with a total charge of

-6

. The net charge is zero.

Common mistakes and how to avoid them

Writing the ion charges as part of the final compound formula.

Correction: Use charges to choose the ratio, then write the neutral compound formula without ion charges.

Using prefixes to name an ionic compound such as magnesium chloride.

Correction: Use ion names and charge rules for ionic compounds. Reserve prefixes for binary compounds made from non-metals.

Reading the Roman numeral in iron(III) chloride as three iron atoms.

Correction: The Roman numeral gives the charge of the iron ion. Subscripts show the number of particles in the formula.

Writing

\mathrm{CaNO_3{}_2}

when two nitrate ions are needed.

Correction: Use parentheses around the complete polyatomic ion:

\mathrm{Ca(NO_3)_2}

. The outside subscript applies to every atom in that ion.

Changing a polyatomic ion's internal subscripts while balancing charges.

Correction: Keep the ion formula intact. Add an outside subscript, using parentheses when needed, to show how many whole ions are present.

Lesson summary

First decide whether a compound is ionic or molecular.
For binary ionic compounds, name the metal, then the non-metal with an -ide ending; balance charges to write the formula.
For binary molecular compounds, use prefixes to show atom counts and change the second element's ending to -ide.
For compounds with polyatomic ions, keep each ion intact and use parentheses when more than one is needed.
Use Roman numerals to state the charge of a metal that can form more than one ion.

Check your understanding

Question 1

What is the correct name for

\mathrm{K_2S}

?

Dip potassium sulfide
Potassium sulfide
Potassium sulfur
Potassium(II) sulfide

Show answer and explanation

Potassium sulfide

This is an ionic compound. Name the metal first and change sulfur to sulfide. Potassium has a fixed ion charge, so no prefix or Roman numeral is needed.

Question 2

What formula matches dinitrogen trioxide?

$\mathrm{N_2O_3}$
$\mathrm{NO_3}$
$\mathrm{N_3O_2}$
$\mathrm{N_2O_5}$

Show answer and explanation

\mathrm{N_2O_3}

Di- means two nitrogen atoms, and tri- means three oxygen atoms. The formula is

\mathrm{N_2O_3}

.

Question 3

What is the correct formula for sodium carbonate, using carbonate as

\mathrm{CO_3^{2-}}

?

$\mathrm{NaCO_3}$
$\mathrm{Na_2CO_3}$
$\mathrm{Na(CO_3)_2}$
$\mathrm{Na_2(CO_3)_2}$

Show answer and explanation

\mathrm{Na_2CO_3}

Each sodium ion has charge

+1

. Two sodium ions balance one carbonate ion with charge

-2

. Only one carbonate ion is needed, so parentheses are not required.

Key terms

Binary compound: A compound containing two different elements.
Ion: An atom or group of atoms with an electrical charge.
Molecular compound: A compound made of non-metal atoms joined in molecules.
Polyatomic ion: A charged group of atoms that stays together as a unit in a compound.
Subscript: A small number in a formula that shows how many atoms or ions are present.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation B2.7. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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B2.7 · Write formulas and IUPAC names for binary and polyatomic compounds

Ontario Grade 11 Chemistry

Ontario Grade 11 Chemistry — B2.7

What you will learn

1. From elements and particles to a naming method

2. Binary ionic compounds

3. Binary molecular compounds

4. Compounds with polyatomic ions

Useful molecular prefixes

Write and name a compound from its ions

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SCH3U

About this lesson and its review