IB Chemistry SLTopic 3 — Classifying the ElementsPaper 1 & 2Structure & properties~11 min read
Metallic and Non-Metallic Oxides
Burn an element in oxygen, drop the product in water, and test the pH. That one experiment tells you whether you started with a metal or a non-metal — and repeating it across a period reveals one of the cleanest trends in the whole syllabus.
📚 What you need to know
Metal oxides are basic — they react with water to give alkaline solutions.
Non-metal oxides are acidic — they react with water to give acids.
Aluminium oxide is amphoteric — it reacts with both acids and bases.
Across Period 3 the oxides change from basic → amphoteric → acidic.
The cause is the electronegativity difference with oxygen shrinking across the period, so bonding shifts from ionic to covalent.
Acidic non-metal oxides cause acid rain; dissolved CO2 causes ocean acidification.
The trend across Period 3
Line up the oxides of the Period 3 elements in order and three things change together: the bonding, the structure, and the acid–base behaviour.
Bonding, structure and acid-base character all change together, because all three follow the electronegativity difference with oxygen.
Oxide
Bonding and structure
Melting point
Acid–base character
Na2O
Giant ionic
High
Basic
MgO
Giant ionic
Very high
Basic
Al2O3
Giant ionic with covalent character
Very high
Amphoteric
SiO2
Giant covalent
Very high
Weakly acidic
P4O10
Simple molecular covalent
Low
Acidic
SO2, SO3
Simple molecular covalent
Low
Acidic
Why the trend happens
Bonding type comes down to the difference in electronegativity between the two elements. Oxygen is highly electronegative (3.4). Across Period 3 the elements themselves become more electronegative, so the gap closes.
Sodium (0.9): difference of 2.5 — large, so electrons are transferred. Purely ionic oxide.
Aluminium (1.6): difference of about 1.8 — still ionic, but with noticeable covalent character because the small, highly charged Al3+ ion distorts the oxide ion.
Sulfur (2.6): difference of about 0.8 — small, so electrons are shared. Covalent molecular oxide.
And that maps directly onto acid–base behaviour: ionic oxides contain O2– ions, which grab protons from water and produce hydroxide ions. Covalent oxides have no O2– ions; instead they react with water to form molecules that release H+.
This is the payoff from the trends page. Electronegativity increases across a period, so the bond to oxygen slides from ionic to covalent, so the oxide slides from basic to acidic. Same chain, three consequences.
Basic oxides: the metals
Metal oxides contain the oxide ion, O2–, which is a strong base. In water it takes a proton from H2O and produces hydroxide ions:
Why metal oxides are alkaline
O2–(aq) + H2O(l) → 2OH–(aq)
Sodium oxide is very soluble and gives a strongly alkaline solution, around pH 13–14:
Na2O(s) + H2O(l) → 2NaOH(aq)
Magnesium oxide is much less soluble, so although magnesium hydroxide is a base, the solution only reaches about pH 10:
MgO(s) + H2O(l) → Mg(OH)2(aq)
Acidic oxides: the non-metals
Non-metal oxides are covalent molecules. They react with water to make oxoacids — acids in which the acidic hydrogen is attached to oxygen.
Silicon dioxide is the odd one out among the non-metal oxides. Its giant covalent structure is insoluble, so it does not acidify water — but it still counts as acidic because it reacts with hot alkali.
Amphoteric: aluminium oxide
Aluminium sits on the metal–non-metal boundary, and its oxide behaves like both. It is insoluble in water, but it reacts with acids (behaving as a base) and with bases (behaving as an acid). That double behaviour is what amphoteric means.
With an acid — acting as a base
Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)
With a base — acting as an acid
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
Why this matters outside the exam hall
Acid rain
Burning fossil fuels releases sulfur oxides, and high-temperature combustion in engines makes nitrogen oxides. Both are acidic non-metal oxides, so both dissolve in atmospheric water to make acids.
2NO2(g) + H2O(l) → HNO3(aq) + HNO2(aq)
The result is rainwater well below its natural pH, which damages forests, acidifies lakes and corrodes limestone buildings.
Ocean acidification
Carbon dioxide is also an acidic oxide. It dissolves in seawater to form the weak acid carbonic acid, which partly dissociates:
CO2(g) + H2O(l) ⇌ H2CO3(aq) ⇌ H+(aq) + HCO3–(aq)
More H+ means a lower ocean pH, which makes it harder for corals and shellfish to build calcium carbonate structures.
WORKED EXAMPLE
Each of these oxides is added to separate beakers of water: SO2, K2O, MgO. Place the resulting solutions in order of increasing pH.
Classify each oneSO₂ is a non-metal oxide → acidic. K₂O and MgO are metal oxides → basic.Separate the two basic onesK₂O is Group 1, very soluble, strongly alkaline. MgO is much less soluble, only weakly alkaline.SO₂ < MgO < K₂O
WORKED EXAMPLE
Explain why Na2O is ionic while SO3 is covalent, using electronegativity.
Find the differences with oxygen (3.4)Na: 3.4 − 0.9 = 2.5 S: 3.4 − 2.6 = 0.8Large difference → electrons transferredNa₂O is ionicSmall difference → electrons sharedSO₃ is covalentAnd that is why one gives an alkaline solution and the other an acidic one.
💡 Exam tip
These equations are named in the syllabus, so you can be asked to construct them from scratch. Practise writing them balanced with state symbols.
If asked to explain the trend, go through electronegativity difference → bonding type → acid–base character. All three steps.
Only one Period 3 oxide is amphoteric — Al2O3. Know both of its reactions.
Electronegativity values are in Section 9 of the data booklet. Use them rather than estimating.
⚠️ Common mix-up
Amphoteric is not the same as neutral. It means reacting with both acids and bases, not reacting with neither.
MgO is basic, just not very soluble. A pH of about 10 still means basic.
SiO2 does not dissolve to make an acidic solution — it is classed as acidic because it reacts with hot alkali.
Aluminium oxide has covalent character, but it is still described as a giant ionic solid.
Acid rain comes from sulfur and nitrogen oxides; CO2 is the ocean acidification one.
Up next: Oxidation States — the bookkeeping system that lets you track exactly where the electrons went in every reaction on these pages.
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