Structure 3.1.5 – Trends in Metallic and Non-Metallic Oxides

Oxides Across a Period

• As we move across Period 3 of the periodic table (from sodium to chlorine), the nature of the oxides of these elements changes significantly. This reflects the transition from metallic to non-metallic character and results in a progression from basic oxides to acidic oxides.
• General trend: Basic → Amphoteric → Acidic

Acidic and Basic Nature of Period 3 Oxides

• Basic oxides: Sodium oxide (Na₂O), Magnesium oxide (MgO)
• Amphoteric oxide: Aluminium oxide (Al₂O₃)
• Acidic oxides: Silicon dioxide (SiO₂), Phosphorus pentoxide (P₄O₁₀), Sulfur oxides (SO₂, SO₃)

Basic oxides react with acids and form hydroxide ions in water. Acidic oxides react with bases and form acids in water. Amphoteric oxides can react with both acids and bases.

Structure, Bonding and Electronegativity of Period 3 Oxides

Electronegativity increases across the period, which influences bonding:

Electronegativity trend: Na (0.9) < Mg (1.2) < Al (1.5) < Si (1.8) < P (2.1) < S (2.5) < Cl (3.0) | O = 3.4

Large ΔEN (e.g., Na–O) leads to ionic bonding; smaller ΔEN (e.g., P–O) leads to covalent bonding.

Amphoteric Oxides

Aluminium oxide (Al₂O₃) is amphoteric. It reacts with both acids and bases:

• With acid: \( \text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O} \)
• With base: \( \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4 \)

Predicting Oxide Properties

You can predict the properties of an oxide based on its element’s:

• Metallic vs. non-metallic character
• Electronegativity and ionization energy

Across a period:

• • Ionic → Covalent
  • Basic → Acidic

Down a group:

• • More ionic (due to decreased electronegativity).

Summary:

• Metals form basic, ionic oxides (e.g., Na, Mg)
• Aluminium forms amphoteric oxide (Al₂O₃)
• Non-metals form acidic, covalent oxides (e.g., P, S, Cl)

Reactions of Oxides with Water

The chemical behavior of metal and non-metal oxides with water varies depending on their acidity or basicity. Oxides of Group 1 and 2 metals form alkaline solutions (basic oxides), while non-metal oxides like carbon and sulfur form acids (acidic oxides).

Group 1 Metal Oxides

Oxides of Group 1 metals (e.g., Na, K) react readily with water to form strong alkaline solutions:

• Sodium oxide (Na₂O):

\( \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \)

• Potassium oxide (K₂O):

\( \text{K}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{KOH} \)

These hydroxides are highly soluble and produce solutions with pH ≈ 14.

Group 2 Metal Oxides

Group 2 metal oxides also form basic hydroxides, but their solubility decreases down the group.

• Magnesium oxide (MgO):

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

Partially soluble → weakly alkaline (pH ≈ 9–10)

• Calcium oxide (CaO):

\( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \)

More soluble than Mg(OH)₂ → pH ≈ 12

Non-Metal Oxides

Non-metal oxides such as carbon dioxide and sulfur dioxide form acidic solutions when dissolved in water. These oxides contribute to acid rain.

• Carbon dioxide (CO₂):

\( \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \) (carbonic acid)

Weak acid → pH ≈ 4–5

• Sulfur dioxide (SO₂):

\( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \) (sulfurous acid)

Weak-to-moderate acid → pH ≈ 3–4

• Sulfur trioxide (SO₃):

\( \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \) (sulfuric acid)

Strong acid → pH ≈ 1

Note:

Al₂O₃ and SiO₂ do not dissolve in water.

Comparison Table: Oxides Reacting with Water

Environmental Links

Acid Rain: Formed when non-metal oxides dissolve in atmospheric water.

• \( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \)
• \( \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \)
• \( 2\text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_3 + \text{HNO}_2 \)

Ocean Acidification: Carbon dioxide dissolves in water and forms carbonic acid:

\( \text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^- \)