Ch 2 · Periodicity of chemical properties of the elements in Period 3 · AS & A Level Chemistry 9701

Topic 2 of 3 in The Periodic Table: chemical periodicity — AS & A Level Chemistry 9701 — 57 lessons.

Introduction

The physical trends across Period 3 come from changes in structure and bonding; the chemical trends come from the same cause, seen in how the elements and their oxides and chlorides react with oxygen, chlorine, water and acids or bases. The key idea running through this lesson is that bonding shifts from ionic (Na, Mg) to covalent (Si, P, S, Cl) as electronegativity and ionisation energy increase across the period.

Key ideas — Reactions of the elements with oxygen and chlorine

Every Period 3 element (except Ar) burns in oxygen or reacts with chlorine to form an oxide or chloride in which the element shows its maximum oxidation number (equal to its group number):

🧑‍🏫 Teacher's explanation

Think of the Period 3 elements as a group of people trying to reach a state of 'maximum stability' by filling their outer electron shells. As you move from Sodium to Chlorine, the atoms become increasingly 'greedy' for electrons because their nuclear charge increases, pulling the electrons closer and making them harder to lose.

📖 Definitions

🔍 Worked example — Easy

Calculate the oxidation number of Sulfur in SO_2.

🔍 Worked example — Standard

Predict the bonding type in MgCl_2 and SiCl_4 based on electronegativity differences.

🔍 Worked example — Exam-style

A sample of Al_2O_3 reacts with excess HCl and excess NaOH. Write the balanced equation for the reaction with HCl and state the nature of the oxide.

⚠️ Common errors

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🎯 Exam question

Key ideas — Reactions of the elements with water

Na reacts vigorously with cold water: 2Na(s) + 2H_2O(l) arrow 2NaOH(aq) + H_2(g). Mg reacts very slowly with cold water but readily with steam: Mg(s) + H_2O(g) arrow MgO(s) + H_2(g). Al does not react with water under normal conditions, because a thin, unreactive, adherent layer of aluminium oxide protects the metal surface. Si, P, S and Cl₂ do not react directly with cold water.

🧑‍🏫 Teacher's explanation

Think of the elements in Period 3 as a group of people moving from being highly reactive 'extroverts' to very stable 'introverts'. On the far left, Sodium is like a hyperactive toddler; it wants to get rid of its one outer electron so badly that it reacts violently with water. As you move right, the atoms get smaller and hold onto their electrons much more tightly because the nuclear charge increases.

📖 Definitions

🔍 Worked example — Easy

Write a balanced chemical equation for the reaction of Magnesium with steam.

🔍 Worked example — Standard

Predict the pH of a solution formed when Na_2O reacts with water and identify the type of oxide.

🔍 Worked example — Exam-style

Explain why Aluminum does not react readily with water under normal conditions.

⚠️ Common errors

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🎯 Exam question

Key ideas — Oxidation number and bonding across the period

The maximum oxidation number in the oxides and chlorides rises steadily from +1 (Na) to +5 (P) as all of the outer-shell electrons become available for bonding: Na has one outer electron to lose, Mg two, Al three, Si four, and P can use all five outer electrons in mathrmP_4O_10. Alongside this, bonding changes from ionic (Na₂O, MgO, NaCl, MgCl₂), through polar/partly covalent (Al_2O_3, AlCl_3), to giant covalent (SiO_2) and simple molecular covalent (mathrmP_4O_10, SO_2, SiCl_4, PCl_5). This shift happens because electronegativity increases across the period, so the difference in electronegativity between the Period 3 element and oxygen or chlorine decreases, reducing the ionic character of the bond. The small, highly charged mathrmAl^3+ ion also strongly polarises neighbouring anions, pulling electron density back towards itself and giving the bonding significant covalent character.

🧑‍🏫 Teacher's explanation

Imagine a group of people playing a game of tug-of-war with a rope. In the beginning (Sodium), one person is much stronger than the others, so they simply snatch the rope away entirely—this is ionic bonding. As we move across the period, the players become more evenly matched in strength (electronegativity). Instead of one person winning the rope, they end up sharing it. If they share it very unevenly, it is polar covalent; if they share it almost equally, it is non-polar covalent.

📖 Definitions

🔍 Worked example — Easy

Predict the maximum oxidation state for the elements in Period 3 and explain why it increases from Na to P.

🔍 Worked example — Standard

Explain why AlCl_3 exhibits significant covalent character despite being formed from a metal and a non-metal.

🔍 Worked example — Exam-style

Compare the structures and bonding of SiO_2 and P_4O_10.

⚠️ Common errors

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🎯 Exam question

(a) Write the equation for the reaction of Na_2O with water and state the pH of the resulting solution. [2] (b) Explain why Al_2O_3 is described as amphoteric in terms of its bonding. [2] (c) Predict the state of matter and bonding type for SO_2 at room temperature. [2] [6 marks]

Key ideas — Reactions of the oxides with water and acid–base behaviour

Na_2O and MgO are basic oxides. Na_2O dissolves readily to give a strongly alkaline solution (pH ≈ 13–14): Na_2O(s) + H_2O(l) arrow 2NaOH(aq). MgO is only sparingly soluble, giving a weakly alkaline solution (pH ≈ 9–10): MgO(s) + H_2O(l) arrow Mg(OH)_2(aq). Al_2O_3 is insoluble and amphoteric: it reacts with both acids and bases.

🧑‍🏫 Teacher's explanation

Think of the Period 3 oxides as a sliding scale of 'chemical personality'. On one end, you have the metals (Na, Mg) which are very generous with their electrons, making them strongly basic. As you move right across the period, the atoms become much more 'greedy' for electrons due to increasing nuclear charge. This shift in electronegativity changes the nature of the bond from purely ionic to covalent.

📖 Definitions

🔍 Worked example — Easy

Write a balanced equation for the reaction between magnesium oxide and water.

🔍 Worked example — Exam-style

Explain why Al_2O_3 is amphoteric in terms of its bonding and electronegativity.

⚠️ Common errors

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🎯 Exam question

Key ideas — Reactions of the chlorides with water

Ionic chlorides simply dissolve, dissociating fully into hydrated ions with little or no reaction with water. NaCl gives a neutral solution (pH ≈ 7); MgCl_2 gives a weakly acidic solution (pH ≈ 6.5) because the small, highly charged hydrated mathrmMg^2+ ion polarises surrounding water molecules, weakening an O–H bond enough for a proton to be released. Covalent chlorides undergo vigorous hydrolysis, releasing steamy fumes of hydrogen chloride and leaving a strongly acidic solution:

🧑‍🏫 Teacher's explanation

Think of the reaction of a chloride with water as a competition for attention. In an ionic chloride like NaCl, the ions are 'happy' just floating around in water, separated from their partners. They don't care about the water molecules; they just dissociate. However, in covalent chlorides, the central atom is 'hungry' for electrons because it is bonded to highly electronegative chlorine atoms.

Ch 2 · Periodicity of chemical properties of the elements in Period 3 · AS & A Level Chemistry 9701

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