C3.8 Identify and write formulas for common ionic, molecular, and acid compounds

Ontario Grade 10 Science

Chemistry: Chemical Reactions

Ionic, molecular, and acid compounds | Ontario Grade 10 Science C3.8

A compound contains two or more different elements in a fixed combination. Its chemical formula uses element symbols and small numbers called subscripts. For example, water is written as

\mathrm{H_2O}

. The subscript 2 means two hydrogen atoms. No subscript after O means one oxygen atom. Ionic, molecular, and acid compounds use different clues when you write their formulas. First identify the kind of compound, then use the matching rule.

What you will learn

Identify whether a compound is ionic, molecular, or an acid.
Use ion charges or name prefixes to write common compound formulas.
Check that symbols and subscripts match the compound name.

1. From element symbols to compound type

An element is a pure substance made of one kind of atom. Each element has a symbol. For example, H stands for hydrogen, O for oxygen, and Na for sodium. A formula is a compact way to show which elements are in a compound and how many atoms or groups are present.

A subscript is the small number written after a symbol. It tells how many atoms or groups are present. If there is no subscript, the count is one. A charge is different: it is written above and to the right of a symbol or group. Charges help with some formulas, but they are not written as subscripts in the final formula for a neutral ionic compound.

Ionic compounds are made from positive and negative ions. An ion is an atom or group of atoms with an electric charge. Ionic compounds commonly contain a metal and a non-metal, or a metal and a charged group of atoms. Molecular compounds are made from non-metal elements joined in molecules. A molecule is a group of atoms joined together. Common acid formulas begin with hydrogen, and their names include the word acid.

These descriptions are clues, not substitutes for reading the name carefully. Once you know the compound type, choose the formula rule that fits it. Do not use ionic charge balancing to interpret a molecular name, and do not use molecular prefixes to replace the charge information for an ionic compound.

Symbols identify elements; subscripts show counts.
Charges are not the same as subscripts.
Identify the compound type before writing a formula.

2. Ionic compounds: make the total charge zero

In an ionic compound, positive and negative ions combine. The total charge of the compound must be zero. Write the positive ion first and the negative ion second. Then choose the smallest whole-number ratio of ions that balances their charges.

For example, calcium ions have a charge of

2+

and chloride ions have a charge of

1-

. One calcium ion needs two chloride ions. The resulting formula is

\mathrm{CaCl_2}

. The charge signs are useful while working out the ratio, but they do not appear in this neutral compound formula.

Some ions are groups of atoms that stay together as one charged unit. Such a group is called a polyatomic ion. Sulfate is one example, written as

\mathrm{SO_4^{2-}}

. If more than one whole polyatomic ion is needed, put parentheses around its formula before adding the subscript. For instance, magnesium nitrate is written as

\mathrm{Mg(NO_3)_2}

. The subscript 2 applies to the whole nitrate group.

A metal may form ions with different charges. In a name such as iron(III), the Roman numeral tells the charge of the iron ion:

3+

. Use the charge given by the name when deciding the ratio with the negative ion.

\mathrm{Ca^{2+} + 2Cl^- \rightarrow CaCl_2}

Write the positive ion first and the negative ion second.
Choose the smallest ratio that makes total charge zero.
Use parentheses when a polyatomic ion appears more than once.

3. Molecular compounds: read the prefixes

Molecular compounds contain non-metal elements. Their names often use prefixes to show how many atoms of each element are in one molecule. Common prefixes include mono- for one, di- for two, tri- for three, and tetra- for four.

Write the element symbols in the order named. Use each prefix to decide the subscript for that element. For example, dinitrogen tetroxide names nitrogen first and oxygen second. Di- means two nitrogen atoms, and tetra- means four oxygen atoms, so its formula is

\mathrm{N_2O_4}

.

A prefix mono- is often left off the first element in a molecular compound name. For example, carbon dioxide has one carbon atom and two oxygen atoms. The formula is

\mathrm{CO_2}

. Do not write a subscript 1. Molecular formulas use the name's prefixes to count atoms; ionic charges do not set their subscripts.

\mathrm{di}=2,\quad \mathrm{tri}=3,\quad \mathrm{tetra}=4

Use prefixes in the name to count atoms.
Keep the elements in the order named.
Leave off a subscript when the count is one.

4. Acids: connect the name to hydrogen and an ion

Common acid formulas begin with hydrogen. The rest of the formula is often based on a negative ion. Choose enough hydrogen to balance that ion's negative charge, so the complete formula has no overall charge.

A binary acid contains hydrogen and one other non-metal element. Its name includes hydro- and ends in -ic acid. Hydrochloric acid is based on chloride. One hydrogen balances one chloride, so the formula is

\mathrm{HCl}

. The word hydro- is part of the name pattern, not part of the formula.

Other common acids are connected to polyatomic ions. Sulfuric acid is associated with sulfate, written as

\mathrm{SO_4^{2-}}

. Each hydrogen ion has a

1+

charge, so two hydrogen ions balance one sulfate ion. The formula is

\mathrm{H_2SO_4}

. Nitric acid is associated with nitrate, written as

\mathrm{NO_3^-}

. One hydrogen balances one nitrate, giving

\mathrm{HNO_3}

.

Names and an approved ion list or classroom reference provide evidence for which ions to use. Appearance alone does not identify a compound. Do not taste or handle unknown substances to identify them. In class, follow the teacher's safety directions and use only approved materials.

\mathrm{2H^+ + SO_4^{2-} \rightarrow H_2SO_4}

Common acid formulas begin with hydrogen.
Balance the hydrogen charge with the negative ion's charge.
Use compound names and approved references, not appearance, to identify formulas.

Compare the formula clues

Compound type	Useful clue	How to choose subscripts
Ionic	Positive and negative ions	Balance charges to make the total zero
Molecular	Non-metals; prefixes may appear	Use prefixes to count atoms
Acid	Name includes acid; formula begins with hydrogen	Balance hydrogen with the negative ion

Worked example

Ionic compound: calcium chloride

Write the formula for calcium chloride. Calcium ions have a

2+

charge and chloride ions have a

1-

charge.

Identify the ions
Calcium is the positive ion, and chloride is the negative ion. Write calcium first and chloride second.
$\mathrm{Ca^{2+},\ Cl^-}$
Balance the charges
One calcium ion has a total charge of $2+$ . Two chloride ions have a total charge of $2-$ . Together, they make a neutral compound.
$(+2)+2(-1)=0$
Write the formula
Use one calcium symbol and two chlorine atoms. Leave the charge signs out of the formula for the neutral compound.
$\mathrm{CaCl_2}$

Answer:

\mathrm{CaCl_2}

Check: One calcium ion and two chloride ions have a combined charge of zero.

Worked example

Molecular compound: dinitrogen tetroxide

Write the formula for dinitrogen tetroxide.

Read the element names
The name gives nitrogen first and oxygen second. Both are non-metals, so use the molecular-compound prefix pattern.
$\mathrm{N,\ O}$
Use the prefixes
Di- means two nitrogen atoms, and tetra- means four oxygen atoms. Put those counts after the matching symbols.
$\mathrm{N_2O_4}$

Answer:

\mathrm{N_2O_4}

Check: The formula shows two nitrogen atoms and four oxygen atoms, as named.

Worked example

Acid: sulfuric acid

Write the formula for sulfuric acid using the sulfate ion,

\mathrm{SO_4^{2-}}

.

Identify the related ion
Sulfuric acid is associated with the sulfate ion. Its charge is $2-$ .
$\mathrm{SO_4^{2-}}$
Balance the charge
Each hydrogen ion has a $1+$ charge. Two hydrogen ions balance one sulfate ion.
$2(+1)+(-2)=0$
Write the formula
Write hydrogen first, followed by the sulfate group. Since one sulfate group is needed, parentheses are not required.
$\mathrm{H_2SO_4}$

Answer:

\mathrm{H_2SO_4}

Check: Two hydrogen ions balance the charge of one sulfate ion.

Common mistakes and how to avoid them

Writing ion charges as subscripts in the final ionic formula.

Correction: Use charges to work out the ratio, then leave charge signs out of the neutral compound formula.

Using ionic charge balancing for a molecular compound.

Correction: Use the prefixes in the molecular compound's name to count atoms.

Writing a subscript of 1 for one atom.

Correction: Leave the subscript off when the count is one.

Treating a polyatomic ion's atoms as separate ions when writing the formula.

Correction: Keep the group together. Use parentheses if more than one whole group is needed.

Lesson summary

Identify the compound type before choosing a formula rule.
For ionic compounds, balance positive and negative charges.
For molecular compounds, use the name prefixes to count atoms.
For common acids, use hydrogen and the related negative ion so the total charge is zero.
Check that symbol order and subscripts match the name.

Check your understanding

Question 1

What is the formula for magnesium oxide if magnesium ions have a

2+

charge and oxide ions have a

2-

charge?

$\mathrm{MgO}$
$\mathrm{Mg_2O}$
$\mathrm{MgO_2}$
$\mathrm{Mg_2O_2}$

Show answer and explanation

\mathrm{MgO}

One magnesium ion balances one oxide ion because their charges are equal in size and opposite.

Question 2

What is the formula for carbon dioxide?

$\mathrm{CO}$
$\mathrm{CO_2}$
$\mathrm{C_2O}$
$\mathrm{C_2O_2}$

Show answer and explanation

\mathrm{CO_2}

Di- means two oxygen atoms. No prefix before carbon means one carbon atom.

Question 3

How many hydrogen ions are needed to balance one sulfate ion with a

2-

charge?

One
Two
Three
Four

Show answer and explanation

Two

Each hydrogen ion has a

1+

charge, so two are needed to balance a

2-

sulfate ion.

Key terms

Compound: A substance containing two or more different elements in a fixed combination.
Formula: Element symbols and subscripts used to represent a compound.
Subscript: A small number after a symbol that shows how many atoms or groups are present.
Ion: An atom or group of atoms with an electric charge.
Polyatomic ion: A group of atoms that acts as one charged unit.
Molecular compound: A compound made from non-metal elements joined in molecules.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C3.8. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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C3.8 · Identify and write formulas for common ionic, molecular, and acid compounds

Ontario Grade 10 Science

Ionic, molecular, and acid compounds | Ontario Grade 10 Science C3.8

What you will learn

1. From element symbols to compound type

2. Ionic compounds: make the total charge zero

3. Molecular compounds: read the prefixes

4. Acids: connect the name to hydrogen and an ion

Compare the formula clues

Ionic compound: calcium chloride

Molecular compound: dinitrogen tetroxide

Acid: sulfuric acid

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SNC2D

About this lesson and its review