How does the bonding in the oxides of Period 3 elements explain the change from basic to acidic behaviour across the period?
The reactions of the Period 3 elements sodium and magnesium with water. The reactions of the Period 3 elements with oxygen to form oxides. The structure and bonding of the Period 3 oxides and the trends in their melting points. The reactions of the oxides with water and the acid-base nature of the resulting solutions. The behaviour of the oxides as acids or bases in their reactions with acids and bases.
A focused answer to the AQA A-Level Chemistry 3.2.4 specification points on Period 3 elements and their oxides. Covers the reactions of sodium and magnesium with water, the reactions with oxygen, the structure and bonding of the oxides, their melting point trend, their reactions with water, and the change from basic to amphoteric to acidic across the period.
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What this dot point is asking
AQA wants you to describe the reactions of sodium and magnesium with water, the reactions of the Period 3 elements with oxygen, the structure, bonding and melting points of the oxides, and how the oxides behave with water, acids and bases as you move from metal oxides (basic) through aluminium oxide (amphoteric) to non-metal oxides (acidic).
Reactions with water
Sodium reacts vigorously with cold water, fizzing and melting into a ball, giving a strongly alkaline solution:
Magnesium reacts very slowly with cold water to give the hydroxide, but reacts well with steam to give the oxide and hydrogen:
Reactions with oxygen
The elements burn in oxygen to form their oxides, for example:
The oxides to know are , , , , and /.
Structure, bonding and melting points
Reactions of the oxides with water
The ionic oxides of the metals dissolve to give alkaline solutions:
and are insoluble in water (giant structures). The molecular oxides dissolve to give acidic solutions:
So the pH of the resulting solutions falls from strongly alkaline (, around pH 13 to 14) through weakly alkaline (, around pH 9), to insoluble in the middle ( and , no pH change), and then to acidic ( giving phosphoric acid around pH 1 to 2, and giving sulfuric acid, also strongly acidic) across the period. The underlying reason is the change in bonding: the metal oxides are ionic, so the oxide ion reacts with water to give hydroxide ions and an alkaline solution, whereas the non-metal oxides are covalent and react with water to form oxoacids that release .
Acid-base nature of the oxides
Try this
Q1. Write an equation for sodium reacting with cold water and state the pH of the product. [2 marks]
- Cue. ; strongly alkaline (about pH 13 to 14).
Q2. Explain why silicon dioxide has a very high melting point. [2 marks]
- Cue. Giant covalent structure; many strong covalent bonds must be broken.
Q3. Give an equation showing acting as a base. [1 mark]
- Cue. .
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20183 marksExplain why magnesium oxide has a much higher melting point than phosphorus(V) oxide.Show worked answer →
A 3-mark answer compares the structure and bonding.
is ionic with a giant ionic lattice; melting it requires breaking many strong electrostatic forces between and ions, so it has a very high melting point. is simple molecular with only weak van der Waals forces between molecules, which need little energy to overcome, so it melts at a much lower temperature.
Markers reward giant ionic versus simple molecular, the named forces, and the link to energy needed.
AQA 20202 marksExplain why aluminium oxide is described as amphoteric.Show worked answer →
A 2-mark answer needs both behaviours.
reacts with acids (acting as a base) and with bases/alkalis (acting as an acid), so it is amphoteric. With acid: ; with alkali it forms an aluminate.
Related dot points
- The classification of an element as s, p, d or f block according to its outer electron configuration. Trends in atomic radius and first ionisation energy across Period 3 and down a group, explained by nuclear charge, shielding and atomic radius. The trend in melting point across Period 2 and Period 3, explained by the structure and bonding of the elements.
A focused answer to the AQA A-Level Chemistry 3.2.1 specification points on periodicity. Covers s, p, d and f block classification, the trends in atomic radius and first ionisation energy across Period 3 and down a group, and the melting point trend across Periods 2 and 3 explained by structure and bonding.
- The trend in atomic radius, first ionisation energy and melting point down Group 2. The reactions of Group 2 elements with water. The trend in solubility of the hydroxides and sulfates of Group 2 elements. Uses of magnesium in the extraction of titanium, of calcium hydroxide in agriculture, of barium sulfate in medicine and of Group 2 compounds in neutralising acidity.
A focused answer to the AQA A-Level Chemistry 3.2.2 specification points on Group 2. Covers the trends in atomic radius, ionisation energy and reactivity down the group, reactions with water, the opposite solubility trends of the hydroxides and sulfates, and the key uses of Group 2 compounds.
- The trends in electronegativity and boiling point of the halogens. The trend in oxidising ability of the halogens down the group, including displacement reactions of halide ions in aqueous solution. The trend in reducing ability of the halide ions, including the reactions of solid sodium halides with concentrated sulfuric acid. The use of acidified silver nitrate to identify and distinguish halide ions, and the use of chlorine in water treatment.
A focused answer to the AQA A-Level Chemistry 3.2.3 specification points on Group 7. Covers the boiling point and electronegativity trends, the decrease in oxidising power down the group with displacement reactions, the increase in reducing power of the halide ions with concentrated sulfuric acid, the silver nitrate test and the use of chlorine in water treatment.
- The definition of a transition metal in terms of an incomplete d sub-shell. The characteristic properties of transition metals: complex formation, coloured ions, variable oxidation states and catalytic activity. The shapes of complex ions and the meaning of coordination number and ligand. Stereoisomerism in complexes. Ligand substitution reactions and the chelate effect. The origin of colour in transition metal ions and its use in colorimetry. The role of transition metals as homogeneous and heterogeneous catalysts.
A focused answer to the AQA A-Level Chemistry 3.2.5 specification points on transition metals. Covers the d-sub-shell definition, complex ions, ligands and coordination number, the shapes and stereoisomerism of complexes, ligand substitution and the chelate effect, the origin of colour and colorimetry, variable oxidation states and homogeneous and heterogeneous catalysis.
- The acidity of metal-aqua ions in terms of the charge density of the metal ion and the polarisation of coordinated water. The reactions of metal-aqua ions with bases such as sodium hydroxide and ammonia, and with carbonate ions. The amphoteric character of the aluminium hydroxide complex. The use of these reactions to identify metal ions in solution by the colours and behaviour of the precipitates formed.
A focused answer to the AQA A-Level Chemistry 3.2.6 specification points on reactions of ions in aqueous solution. Covers the acidity of metal-aqua ions and the link to charge density, the reactions of 2+ and 3+ aqua ions with sodium hydroxide, ammonia and carbonate, the amphoteric behaviour of aluminium hydroxide, and how the precipitate colours identify metal ions.
Sources & how we know this
- AQA A-level Chemistry (7405) specification — AQA (2015)