Properties of Period 3 elements and their oxides (A-level only)

Here, we will look at how the elements in Period 3 (from sodium to sulfur) behave chemically. You will build on your knowledge of periodicity to understand how structure and bonding dictate macro-scale properties.

What you'll learn

How sodium and magnesium react with water.
The equations and observations for Period 3 elements reacting with oxygen to form their highest oxides.
Why the melting points of these oxides follow a distinctive trend across the period.
How to predict the pH of the solutions formed when these oxides dissolve in water, and how to write their acid-base equations.

1. Reactions of Sodium and Magnesium with Water

Sodium and magnesium are the two s-block metals in Period 3. Because sodium is in Group 1, it is more reactive than magnesium in Group 2.

Sodium Sodium reacts vigorously with cold water. It melts into a ball, dashes across the surface, and fizzes rapidly as hydrogen gas is produced. It forms a strongly alkaline solution of sodium hydroxide (pH 13–14):

2\text{Na (s)} + 2\text{H}_2\text{O (l)} \rightarrow 2\text{NaOH (aq)} + \text{H}_2\text{ (g)}

Magnesium Magnesium's reaction with cold water is very slow. It will eventually form a weakly alkaline solution of magnesium hydroxide (pH 9–10) and a few bubbles of hydrogen:

\text{Mg (s)} + 2\text{H}_2\text{O (l)} \rightarrow \text{Mg(OH)}_2\text{ (aq)} + \text{H}_2\text{ (g)}

However, magnesium reacts much more vigorously with steam. When heated in steam, it burns with a bright white flame to form solid magnesium oxide and hydrogen gas:

\text{Mg (s)} + \text{H}_2\text{O (g)} \rightarrow \text{MgO (s)} + \text{H}_2\text{ (g)}

Common Mistake

Watch the state symbols

Notice the difference! Magnesium forms magnesium hydroxide with liquid water, but it forms magnesium oxide when reacting with steam ( $(g)\text{H}_2\text{O (g)}$ ).

2. Reactions of Period 3 Elements with Oxygen

When heated, the Period 3 elements react with oxygen to form oxides. In most cases, they are oxidised to their highest oxidation states (which correspond to their group number).

Here are the key observations and equations you need to know:

Sodium: Burns with a yellow flame to produce a white solid.

4\text{Na (s)} + \text{O}_2\text{ (g)} \rightarrow 2\text{Na}_2\text{O (s)}

Magnesium: Burns with a brilliant white flame to produce a white solid.

2\text{Mg (s)} + \text{O}_2\text{ (g)} \rightarrow 2\text{MgO (s)}

Aluminium: Burns brightly (especially when powdered) to give a white solid powder.

4\text{Al (s)} + 3\text{O}_2\text{ (g)} \rightarrow 2\text{Al}_2\text{O}_3\text{ (s)}

Silicon: Requires strong heating to react, eventually forming a white solid.

\text{Si (s)} + \text{O}_2\text{ (g)} \rightarrow \text{SiO}_2\text{ (s)}

Phosphorus: Red phosphorus must be heated, but white phosphorus ignites spontaneously in air, burning with a brilliant white flame and producing a dense white smoke.

\text{P}_4\text{ (s)} + 5\text{O}_2\text{ (g)} \rightarrow \text{P}_4\text{O}_{10}\text{ (s)}

Sulfur: Burns in air with a distinctive blue flame to form a colourless, choking gas (sulfur dioxide).

\text{S (s)} + \text{O}_2\text{ (g)} \rightarrow \text{SO}_2\text{ (g)}

Common Mistake

Missing the 4 in phosphorus oxide

Students often write the formula for phosphorus(V) oxide as $P2O5\text{P}_2\text{O}_5$ . While that is the empirical formula, the molecular formula is $P4O10\text{P}_4\text{O}_{10}$ . Always use $P4O10\text{P}_4\text{O}_{10}$ in your equations to guarantee the marks.

Note on sulfur: Sulfur usually forms $SO2\text{SO}_2$ . It can form sulfur(VI) oxide ( $SO3\text{SO}_3$ ) but this requires a catalyst ( $V2O5\text{V}_2\text{O}_5$ ) and specific conditions (like in the Contact Process).

3. Melting Points of the Highest Oxides

The melting points of these oxides follow a distinct pattern that directly depends on their structure and bonding.

Melting points of Period 3 highest oxides

Key Idea

Structure dictates properties

To explain any melting point trend across a period, always state the type of structure (giant ionic, giant covalent, or simple molecular) and the forces that must be overcome.

Giant Ionic Lattices ( $Na2O\text{Na}_2\text{O}$ , $MgO\text{MgO}$ , $Al2O3\text{Al}_2\text{O}_3$ )

These metal oxides have high melting points because melting requires breaking strong electrostatic forces of attraction between oppositely charged ions.

$MgO\text{MgO}$ has a higher melting point than $Na2O\text{Na}_2\text{O}$ because the $Mg2+\text{Mg}^{2+}$ ion has a greater charge and is smaller than the $Na+\text{Na}^+$ ion. This creates a stronger electrostatic attraction to the $O2−\text{O}^{2-}$ ions.
$Al2O3\text{Al}_2\text{O}_3$ has a slightly lower melting point than $MgO\text{MgO}$ . Even though $Al3+\text{Al}^{3+}$ has a very high charge, it is so small and highly charged that it strongly distorts the electron cloud of the oxygen ions. This introduces some covalent character into the ionic lattice, lowering the melting point slightly.

Giant Covalent Lattice ( $SiO2\text{SiO}_2$ )

Silicon dioxide has a very high melting point. It forms a giant covalent structure (macro-molecular). Melting it requires breaking many strong covalent bonds between silicon and oxygen atoms, which requires a vast amount of energy.

Simple Molecular Lattices ( $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , $SO3\text{SO}_3$ )

These non-metal oxides have much lower melting points. They consist of separate molecules. Melting only requires breaking the weak intermolecular forces (van der Waals and dipole-dipole forces) between the molecules, not the strong covalent bonds within them.

$P4O10\text{P}_4\text{O}_{10}$ is a relatively large molecule with many electrons, so it has stronger van der Waals forces than $SO2\text{SO}_2$ or $SO3\text{SO}_3$ , giving it a higher melting point than the sulfur oxides.

4. Reactions of Oxides with Water and pH

The type of bonding (ionic vs covalent) also determines how the oxides behave when mixed with water.

The Ionic Basic Oxides: $Na2O\text{Na}_2\text{O}$ and $MgO\text{MgO}$

Ionic oxides contain the oxide ion, $O2−\text{O}^{2-}$ . This is a very strong base and reacts with water to form hydroxide ions ( $OH−\text{OH}^-$ ).

$Na2O\text{Na}_2\text{O}$ dissolves very readily to form sodium hydroxide.

\text{Na}_2\text{O (s)} + \text{H}_2\text{O (l)} \rightarrow 2\text{NaOH (aq)} \quad \text{(pH 13–14)}

$MgO\text{MgO}$ is only slightly soluble in water, forming magnesium hydroxide. Because fewer $OH−\text{OH}^-$ ions end up in solution, the pH is lower.

\text{MgO (s)} + \text{H}_2\text{O (l)} \rightarrow \text{Mg(OH)}_2\text{ (aq)} \quad \text{(pH 9–10)}

The Insoluble Oxides: $Al2O3\text{Al}_2\text{O}_3$ and $SiO2\text{SiO}_2$

Neither aluminium oxide nor silicon dioxide reacts with or dissolves in water. For $Al2O3\text{Al}_2\text{O}_3$ , the ionic lattice is too strong; for $SiO2\text{SiO}_2$ , the giant covalent lattice is too strong. Because they do not dissolve, the pH of the water remains 7.

The Covalent Acidic Oxides: $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , and $SO3\text{SO}_3$

The covalent oxides of phosphorus and sulfur react violently or readily with water to form acidic solutions.

$P4O10\text{P}_4\text{O}_{10}$ reacts vigorously to form phosphoric(V) acid.

\text{P}_4\text{O}_{10}\text{ (s)} + 6\text{H}_2\text{O (l)} \rightarrow 4\text{H}_3\text{PO}_4\text{ (aq)} \quad \text{(pH 0–1)}

$SO2\text{SO}_2$ dissolves to form sulfurous acid (sulfuric(IV) acid), which is a weak acid.

\text{SO}_2\text{ (g)} + \text{H}_2\text{O (l)} \rightleftharpoons \text{H}_2\text{SO}_3\text{ (aq)} \quad \text{(pH 2–3)}

$SO3\text{SO}_3$ reacts violently to form strong sulfuric(VI) acid.

\text{SO}_3\text{ (g)} + \text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{SO}_4\text{ (aq)} \quad \text{(pH 0–1)}

Below are the structures of the acids and their corresponding anions formed in these reactions:

Structures of the oxoacids and their anions

5. Acid-Base Reactions of the Oxides

You must be able to write equations for the reactions of these oxides with acids and bases.

Basic Oxides reacting with Acids

Basic oxides neutralise acids to form a salt and water.

Sodium oxide with hydrochloric acid:

\text{Na}_2\text{O (s)} + 2\text{HCl (aq)} \rightarrow 2\text{NaCl (aq)} + \text{H}_2\text{O (l)}

Magnesium oxide with sulfuric acid:

\text{MgO (s)} + \text{H}_2\text{SO}_4\text{ (aq)} \rightarrow \text{MgSO}_4\text{ (aq)} + \text{H}_2\text{O (l)}

The Amphoteric Oxide: Aluminium Oxide

Definition

Amphoteric

An amphoteric substance is one that can act as both an acid and a base. It will react with both bases and acids to form salts.

Aluminium oxide will neutralise an acid:

\text{Al}_2\text{O}_3\text{ (s)} + 6\text{HCl (aq)} \rightarrow 2\text{AlCl}_3\text{ (aq)} + 3\text{H}_2\text{O (l)}

It will also react with hot, concentrated alkalis to form an aluminate complex:

\text{Al}_2\text{O}_3\text{ (s)} + 2\text{NaOH (aq)} + 3\text{H}_2\text{O (l)} \rightarrow 2\text{NaAl(OH)}_4\text{ (aq)}

Tip

The Aluminate Ion

The formula of the aluminate ion formed is usually written as ${[Al(OH)}_4\text{]}^-$ . Knowing this structure makes balancing the water molecules in the equation much easier.

Acidic Oxides reacting with Bases

The covalent oxides on the right of the period are acidic, so they react with alkalis to form salts and water.

Silicon dioxide (even though it's insoluble in water) will react with hot, concentrated sodium hydroxide to form sodium silicate:

\text{SiO}_2\text{ (s)} + 2\text{NaOH (aq)} \rightarrow \text{Na}_2\text{SiO}_3\text{ (aq)} + \text{H}_2\text{O (l)}

Sulfur dioxide reacts with sodium hydroxide to form sodium sulfite (sodium sulfate(IV)):

\text{SO}_2\text{ (g)} + 2\text{NaOH (aq)} \rightarrow \text{Na}_2\text{SO}_3\text{ (aq)} + \text{H}_2\text{O (l)}

Sulfur trioxide reacts with sodium hydroxide to form sodium sulfate (sodium sulfate(VI)):

\text{SO}_3\text{ (g)} + 2\text{NaOH (aq)} \rightarrow \text{Na}_2\text{SO}_4\text{ (aq)} + \text{H}_2\text{O (l)}

Let's look closely at writing the most complex of these equations.

Example

Writing the equation for phosphorus(V) oxide and sodium hydroxide

Identify the salt produced. $P4O10\text{P}_4\text{O}_{10}$ is the anhydride of phosphoric acid ( $H3PO4\text{H}_3\text{PO}_4$ ). Reacting it with a base will yield a phosphate salt. The phosphate ion is $PO43−\text{PO}_4^{3-}$ . The sodium salt is therefore $Na3PO4\text{Na}_3\text{PO}_4$ .
Write the unbalanced framework.

\text{P}_4\text{O}_{10} + \text{NaOH} \rightarrow \text{Na}_3\text{PO}_4 + \text{H}_2\text{O}

Balance the primary non-metal (P). There are 4 phosphorus atoms on the left, so we need 4 phosphate ions on the right.

\text{P}_4\text{O}_{10} + \text{NaOH} \rightarrow 4\text{Na}_3\text{PO}_4 + \text{H}_2\text{O}

Balance the metal (Na). There are $\times 3 = 12$ sodium atoms on the right, so we need 12 $NaOH\text{NaOH}$ on the left.

\text{P}_4\text{O}_{10} + 12\text{NaOH} \rightarrow 4\text{Na}_3\text{PO}_4 + \text{H}_2\text{O}

Balance the hydrogen and oxygen. We have 12 hydrogen atoms on the left, so we need 6 $H2O\text{H}_2\text{O}$ on the right. Verify the oxygens: 10 + 12 = 22 on the left. $\times 4 + 6 = 22$ on the right.

\text{P}_4\text{O}_{10}\text{ (s)} + 12\text{NaOH (aq)} \rightarrow 4\text{Na}_3\text{PO}_4\text{ (aq)} + 6\text{H}_2\text{O (l)}

Exam technique

In the exam

Read the state symbols in the question. If asked for magnesium's reaction with steam, ensure you produce $MgO\text{MgO}$ and not $Mg(OH)2\text{Mg(OH)}_2$ .
When asked to explain the melting point of $SiO2\text{SiO}_2$ , always include the phrases "giant covalent structure" and "many strong covalent bonds require large amounts of energy to break".
Acid-base reactions of oxides often carry 2 marks. If you get stuck on balancing, write out the correct chemical formulas for the reactants and products first. One mark is almost always for getting the formulas correct, even if the stoichiometry is wrong.

Self review

Check yourself

Why does $MgO\text{MgO}$ have a higher melting point than $Na2O\text{Na}_2\text{O}$ ?
What are the observations when white phosphorus burns in oxygen?
Write the balanced equation for the reaction between aluminium oxide and sulfuric acid.
Why does $SiO2\text{SiO}_2$ have no effect on the pH of water?

What you'll learn

How sodium and magnesium react with water.
The equations and observations for Period 3 elements reacting with oxygen to form their highest oxides.
Why the melting points of these oxides follow a distinctive trend across the period.
How to predict the pH of the solutions formed when these oxides dissolve in water, and how to write their acid-base equations.

1. Reactions of Sodium and Magnesium with Water

Sodium and magnesium are the two s-block metals in Period 3. Because sodium is in Group 1, it is more reactive than magnesium in Group 2.

2\text{Na (s)} + 2\text{H}_2\text{O (l)} \rightarrow 2\text{NaOH (aq)} + \text{H}_2\text{ (g)}

Magnesium Magnesium's reaction with cold water is very slow. It will eventually form a weakly alkaline solution of magnesium hydroxide (pH 9–10) and a few bubbles of hydrogen:

\text{Mg (s)} + 2\text{H}_2\text{O (l)} \rightarrow \text{Mg(OH)}_2\text{ (aq)} + \text{H}_2\text{ (g)}

However, magnesium reacts much more vigorously with steam. When heated in steam, it burns with a bright white flame to form solid magnesium oxide and hydrogen gas:

\text{Mg (s)} + \text{H}_2\text{O (g)} \rightarrow \text{MgO (s)} + \text{H}_2\text{ (g)}

Common Mistake

Watch the state symbols

Notice the difference! Magnesium forms magnesium hydroxide with liquid water, but it forms magnesium oxide when reacting with steam ( $(g)\text{H}_2\text{O (g)}$ ).

2. Reactions of Period 3 Elements with Oxygen

When heated, the Period 3 elements react with oxygen to form oxides. In most cases, they are oxidised to their highest oxidation states (which correspond to their group number).

Here are the key observations and equations you need to know:

Sodium: Burns with a yellow flame to produce a white solid.

4\text{Na (s)} + \text{O}_2\text{ (g)} \rightarrow 2\text{Na}_2\text{O (s)}

Magnesium: Burns with a brilliant white flame to produce a white solid.

2\text{Mg (s)} + \text{O}_2\text{ (g)} \rightarrow 2\text{MgO (s)}

Aluminium: Burns brightly (especially when powdered) to give a white solid powder.

4\text{Al (s)} + 3\text{O}_2\text{ (g)} \rightarrow 2\text{Al}_2\text{O}_3\text{ (s)}

Silicon: Requires strong heating to react, eventually forming a white solid.

\text{Si (s)} + \text{O}_2\text{ (g)} \rightarrow \text{SiO}_2\text{ (s)}

Phosphorus: Red phosphorus must be heated, but white phosphorus ignites spontaneously in air, burning with a brilliant white flame and producing a dense white smoke.

\text{P}_4\text{ (s)} + 5\text{O}_2\text{ (g)} \rightarrow \text{P}_4\text{O}_{10}\text{ (s)}

Sulfur: Burns in air with a distinctive blue flame to form a colourless, choking gas (sulfur dioxide).

\text{S (s)} + \text{O}_2\text{ (g)} \rightarrow \text{SO}_2\text{ (g)}

Common Mistake

Missing the 4 in phosphorus oxide

3. Melting Points of the Highest Oxides

The melting points of these oxides follow a distinct pattern that directly depends on their structure and bonding.

Melting points of Period 3 highest oxides

Key Idea

Structure dictates properties

To explain any melting point trend across a period, always state the type of structure (giant ionic, giant covalent, or simple molecular) and the forces that must be overcome.

Giant Ionic Lattices ( $Na2O\text{Na}_2\text{O}$ , $MgO\text{MgO}$ , $Al2O3\text{Al}_2\text{O}_3$ )

These metal oxides have high melting points because melting requires breaking strong electrostatic forces of attraction between oppositely charged ions.

$MgO\text{MgO}$ has a higher melting point than $Na2O\text{Na}_2\text{O}$ because the $Mg2+\text{Mg}^{2+}$ ion has a greater charge and is smaller than the $Na+\text{Na}^+$ ion. This creates a stronger electrostatic attraction to the $O2−\text{O}^{2-}$ ions.
$Al2O3\text{Al}_2\text{O}_3$ has a slightly lower melting point than $MgO\text{MgO}$ . Even though $Al3+\text{Al}^{3+}$ has a very high charge, it is so small and highly charged that it strongly distorts the electron cloud of the oxygen ions. This introduces some covalent character into the ionic lattice, lowering the melting point slightly.

Giant Covalent Lattice ( $SiO2\text{SiO}_2$ )

Simple Molecular Lattices ( $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , $SO3\text{SO}_3$ )

$P4O10\text{P}_4\text{O}_{10}$ is a relatively large molecule with many electrons, so it has stronger van der Waals forces than $SO2\text{SO}_2$ or $SO3\text{SO}_3$ , giving it a higher melting point than the sulfur oxides.

4. Reactions of Oxides with Water and pH

The type of bonding (ionic vs covalent) also determines how the oxides behave when mixed with water.

The Ionic Basic Oxides: $Na2O\text{Na}_2\text{O}$ and $MgO\text{MgO}$

Ionic oxides contain the oxide ion, $O2−\text{O}^{2-}$ . This is a very strong base and reacts with water to form hydroxide ions ( $OH−\text{OH}^-$ ).

$Na2O\text{Na}_2\text{O}$ dissolves very readily to form sodium hydroxide.

\text{Na}_2\text{O (s)} + \text{H}_2\text{O (l)} \rightarrow 2\text{NaOH (aq)} \quad \text{(pH 13–14)}

$MgO\text{MgO}$ is only slightly soluble in water, forming magnesium hydroxide. Because fewer $OH−\text{OH}^-$ ions end up in solution, the pH is lower.

\text{MgO (s)} + \text{H}_2\text{O (l)} \rightarrow \text{Mg(OH)}_2\text{ (aq)} \quad \text{(pH 9–10)}

The Insoluble Oxides: $Al2O3\text{Al}_2\text{O}_3$ and $SiO2\text{SiO}_2$

The Covalent Acidic Oxides: $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , and $SO3\text{SO}_3$

The covalent oxides of phosphorus and sulfur react violently or readily with water to form acidic solutions.

$P4O10\text{P}_4\text{O}_{10}$ reacts vigorously to form phosphoric(V) acid.

\text{P}_4\text{O}_{10}\text{ (s)} + 6\text{H}_2\text{O (l)} \rightarrow 4\text{H}_3\text{PO}_4\text{ (aq)} \quad \text{(pH 0–1)}

$SO2\text{SO}_2$ dissolves to form sulfurous acid (sulfuric(IV) acid), which is a weak acid.

\text{SO}_2\text{ (g)} + \text{H}_2\text{O (l)} \rightleftharpoons \text{H}_2\text{SO}_3\text{ (aq)} \quad \text{(pH 2–3)}

$SO3\text{SO}_3$ reacts violently to form strong sulfuric(VI) acid.

\text{SO}_3\text{ (g)} + \text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{SO}_4\text{ (aq)} \quad \text{(pH 0–1)}

Below are the structures of the acids and their corresponding anions formed in these reactions:

Structures of the oxoacids and their anions

5. Acid-Base Reactions of the Oxides

You must be able to write equations for the reactions of these oxides with acids and bases.

Basic Oxides reacting with Acids

Basic oxides neutralise acids to form a salt and water.

Sodium oxide with hydrochloric acid:

\text{Na}_2\text{O (s)} + 2\text{HCl (aq)} \rightarrow 2\text{NaCl (aq)} + \text{H}_2\text{O (l)}

Magnesium oxide with sulfuric acid:

\text{MgO (s)} + \text{H}_2\text{SO}_4\text{ (aq)} \rightarrow \text{MgSO}_4\text{ (aq)} + \text{H}_2\text{O (l)}

The Amphoteric Oxide: Aluminium Oxide

Definition

Amphoteric

An amphoteric substance is one that can act as both an acid and a base. It will react with both bases and acids to form salts.

Aluminium oxide will neutralise an acid:

\text{Al}_2\text{O}_3\text{ (s)} + 6\text{HCl (aq)} \rightarrow 2\text{AlCl}_3\text{ (aq)} + 3\text{H}_2\text{O (l)}

It will also react with hot, concentrated alkalis to form an aluminate complex:

\text{Al}_2\text{O}_3\text{ (s)} + 2\text{NaOH (aq)} + 3\text{H}_2\text{O (l)} \rightarrow 2\text{NaAl(OH)}_4\text{ (aq)}

Tip

The Aluminate Ion

The formula of the aluminate ion formed is usually written as ${[Al(OH)}_4\text{]}^-$ . Knowing this structure makes balancing the water molecules in the equation much easier.

Acidic Oxides reacting with Bases

The covalent oxides on the right of the period are acidic, so they react with alkalis to form salts and water.

Silicon dioxide (even though it's insoluble in water) will react with hot, concentrated sodium hydroxide to form sodium silicate:

\text{SiO}_2\text{ (s)} + 2\text{NaOH (aq)} \rightarrow \text{Na}_2\text{SiO}_3\text{ (aq)} + \text{H}_2\text{O (l)}

Sulfur dioxide reacts with sodium hydroxide to form sodium sulfite (sodium sulfate(IV)):

\text{SO}_2\text{ (g)} + 2\text{NaOH (aq)} \rightarrow \text{Na}_2\text{SO}_3\text{ (aq)} + \text{H}_2\text{O (l)}

Sulfur trioxide reacts with sodium hydroxide to form sodium sulfate (sodium sulfate(VI)):

\text{SO}_3\text{ (g)} + 2\text{NaOH (aq)} \rightarrow \text{Na}_2\text{SO}_4\text{ (aq)} + \text{H}_2\text{O (l)}

Let's look closely at writing the most complex of these equations.

Example

Writing the equation for phosphorus(V) oxide and sodium hydroxide

Identify the salt produced. $P4O10\text{P}_4\text{O}_{10}$ is the anhydride of phosphoric acid ( $H3PO4\text{H}_3\text{PO}_4$ ). Reacting it with a base will yield a phosphate salt. The phosphate ion is $PO43−\text{PO}_4^{3-}$ . The sodium salt is therefore $Na3PO4\text{Na}_3\text{PO}_4$ .
Write the unbalanced framework.

\text{P}_4\text{O}_{10} + \text{NaOH} \rightarrow \text{Na}_3\text{PO}_4 + \text{H}_2\text{O}

Balance the primary non-metal (P). There are 4 phosphorus atoms on the left, so we need 4 phosphate ions on the right.

\text{P}_4\text{O}_{10} + \text{NaOH} \rightarrow 4\text{Na}_3\text{PO}_4 + \text{H}_2\text{O}

Balance the metal (Na). There are $\times 3 = 12$ sodium atoms on the right, so we need 12 $NaOH\text{NaOH}$ on the left.

\text{P}_4\text{O}_{10} + 12\text{NaOH} \rightarrow 4\text{Na}_3\text{PO}_4 + \text{H}_2\text{O}

Balance the hydrogen and oxygen. We have 12 hydrogen atoms on the left, so we need 6 $H2O\text{H}_2\text{O}$ on the right. Verify the oxygens: 10 + 12 = 22 on the left. $\times 4 + 6 = 22$ on the right.

\text{P}_4\text{O}_{10}\text{ (s)} + 12\text{NaOH (aq)} \rightarrow 4\text{Na}_3\text{PO}_4\text{ (aq)} + 6\text{H}_2\text{O (l)}

Exam technique

In the exam

Read the state symbols in the question. If asked for magnesium's reaction with steam, ensure you produce $MgO\text{MgO}$ and not $Mg(OH)2\text{Mg(OH)}_2$ .
When asked to explain the melting point of $SiO2\text{SiO}_2$ , always include the phrases "giant covalent structure" and "many strong covalent bonds require large amounts of energy to break".
Acid-base reactions of oxides often carry 2 marks. If you get stuck on balancing, write out the correct chemical formulas for the reactants and products first. One mark is almost always for getting the formulas correct, even if the stoichiometry is wrong.

Self review

Check yourself

Why does $MgO\text{MgO}$ have a higher melting point than $Na2O\text{Na}_2\text{O}$ ?
What are the observations when white phosphorus burns in oxygen?
Write the balanced equation for the reaction between aluminium oxide and sulfuric acid.
Why does $SiO2\text{SiO}_2$ have no effect on the pH of water?

What you'll learn

1. Reactions of Sodium and Magnesium with Water

2. Reactions of Period 3 Elements with Oxygen

3. Melting Points of the Highest Oxides

Giant Ionic Lattices ( $Na2O\text{Na}_2\text{O}$ , $MgO\text{MgO}$ , $Al2O3\text{Al}_2\text{O}_3$ )

Giant Covalent Lattice ( $SiO2\text{SiO}_2$ )

Simple Molecular Lattices ( $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , $SO3\text{SO}_3$ )

4. Reactions of Oxides with Water and pH

The Ionic Basic Oxides: $Na2O\text{Na}_2\text{O}$ and $MgO\text{MgO}$

The Insoluble Oxides: $Al2O3\text{Al}_2\text{O}_3$ and $SiO2\text{SiO}_2$

The Covalent Acidic Oxides: $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , and $SO3\text{SO}_3$

5. Acid-Base Reactions of the Oxides

Basic Oxides reacting with Acids

The Amphoteric Oxide: Aluminium Oxide

Acidic Oxides reacting with Bases

Recap questions

Review Properties of Period 3 elements and their oxides (A-level only) by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Properties of Period 3 elements and their oxides (A-level only)

What you'll learn

1. Reactions of Sodium and Magnesium with Water

2. Reactions of Period 3 Elements with Oxygen

3. Melting Points of the Highest Oxides

Giant Ionic Lattices ( $Na2O\text{Na}_2\text{O}$ , $MgO\text{MgO}$ , $Al2O3\text{Al}_2\text{O}_3$ )

Giant Covalent Lattice ( $SiO2\text{SiO}_2$ )

Simple Molecular Lattices ( $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , $SO3\text{SO}_3$ )

4. Reactions of Oxides with Water and pH

The Ionic Basic Oxides: $Na2O\text{Na}_2\text{O}$ and $MgO\text{MgO}$

The Insoluble Oxides: $Al2O3\text{Al}_2\text{O}_3$ and $SiO2\text{SiO}_2$

The Covalent Acidic Oxides: $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , and $SO3\text{SO}_3$

5. Acid-Base Reactions of the Oxides

Basic Oxides reacting with Acids

The Amphoteric Oxide: Aluminium Oxide

Acidic Oxides reacting with Bases

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Properties of Period 3 elements and their oxides (A-level only)

What you'll learn

1. Reactions of Sodium and Magnesium with Water

2. Reactions of Period 3 Elements with Oxygen

3. Melting Points of the Highest Oxides

Giant Ionic Lattices (Na2O\text{Na}_2\text{O}Na2​O, MgO\text{MgO}MgO, Al2O3\text{Al}_2\text{O}_3Al2​O3​)

Giant Covalent Lattice (SiO2\text{SiO}_2SiO2​)

Simple Molecular Lattices (P4O10\text{P}_4\text{O}_{10}P4​O10​, SO2\text{SO}_2SO2​, SO3\text{SO}_3SO3​)

4. Reactions of Oxides with Water and pH

The Ionic Basic Oxides: Na2O\text{Na}_2\text{O}Na2​O and MgO\text{MgO}MgO

The Insoluble Oxides: Al2O3\text{Al}_2\text{O}_3Al2​O3​ and SiO2\text{SiO}_2SiO2​

The Covalent Acidic Oxides: P4O10\text{P}_4\text{O}_{10}P4​O10​, SO2\text{SO}_2SO2​, and SO3\text{SO}_3SO3​

5. Acid-Base Reactions of the Oxides

Basic Oxides reacting with Acids

The Amphoteric Oxide: Aluminium Oxide

Acidic Oxides reacting with Bases

Recap questions

Review Properties of Period 3 elements and their oxides (A-level only) by teaching Genie

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Properties of Period 3 elements and their oxides (A-level only) Revision Guide

Properties of Period 3 elements and their oxides (A-level only)

What you'll learn

1. Reactions of Sodium and Magnesium with Water

2. Reactions of Period 3 Elements with Oxygen

3. Melting Points of the Highest Oxides

Giant Ionic Lattices (Na2O\text{Na}_2\text{O}Na2​O, MgO\text{MgO}MgO, Al2O3\text{Al}_2\text{O}_3Al2​O3​)

Giant Covalent Lattice (SiO2\text{SiO}_2SiO2​)

Simple Molecular Lattices (P4O10\text{P}_4\text{O}_{10}P4​O10​, SO2\text{SO}_2SO2​, SO3\text{SO}_3SO3​)

4. Reactions of Oxides with Water and pH

The Ionic Basic Oxides: Na2O\text{Na}_2\text{O}Na2​O and MgO\text{MgO}MgO

The Insoluble Oxides: Al2O3\text{Al}_2\text{O}_3Al2​O3​ and SiO2\text{SiO}_2SiO2​

The Covalent Acidic Oxides: P4O10\text{P}_4\text{O}_{10}P4​O10​, SO2\text{SO}_2SO2​, and SO3\text{SO}_3SO3​

5. Acid-Base Reactions of the Oxides

Basic Oxides reacting with Acids

The Amphoteric Oxide: Aluminium Oxide

Acidic Oxides reacting with Bases

Recap questions

Review Properties of Period 3 elements and their oxides (A-level only) by teaching Genie

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Remember key concepts with flashcards

Properties of Period 3 elements and their oxides (A-level only) Revision Guide

Giant Ionic Lattices ( $Na2O\text{Na}_2\text{O}$ , $MgO\text{MgO}$ , $Al2O3\text{Al}_2\text{O}_3$ )

Giant Covalent Lattice ( $SiO2\text{SiO}_2$ )

Simple Molecular Lattices ( $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , $SO3\text{SO}_3$ )

The Ionic Basic Oxides: $Na2O\text{Na}_2\text{O}$ and $MgO\text{MgO}$

The Insoluble Oxides: $Al2O3\text{Al}_2\text{O}_3$ and $SiO2\text{SiO}_2$

The Covalent Acidic Oxides: $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , and $SO3\text{SO}_3$

Giant Ionic Lattices ( $Na2O\text{Na}_2\text{O}$ , $MgO\text{MgO}$ , $Al2O3\text{Al}_2\text{O}_3$ )

Giant Covalent Lattice ( $SiO2\text{SiO}_2$ )

Simple Molecular Lattices ( $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , $SO3\text{SO}_3$ )

The Ionic Basic Oxides: $Na2O\text{Na}_2\text{O}$ and $MgO\text{MgO}$

The Insoluble Oxides: $Al2O3\text{Al}_2\text{O}_3$ and $SiO2\text{SiO}_2$

The Covalent Acidic Oxides: $P4O10\text{P}_4\text{O}_{10}$ , $SO2\text{SO}_2$ , and $SO3\text{SO}_3$