IB Chemistry SLTopic 3 — Classifying the ElementsPaper 1 & 2Trends~12 min read
Metallic and Non-Metallic Oxides
Burn sodium and drop the product in water: you get a solution that will strip your skin, pH around 13. Burn sulfur and do the same: you get an acid, pH around 1. Both are just “an element plus oxygen”, and the difference between them is the single clearest chemical trend across a period.
📘 What you need to know
Across period 3 the oxides change from basic, through amphoteric, to acidic.
Metal oxides are basic. The soluble ones react with water to give alkaline hydroxide solutions.
Non-metal oxides are acidic. They react with water to give acids.
Aluminium oxide is amphoteric — it reacts with both acids and bases.
The pattern follows the bonding: ionic on the left, giant covalent in the middle, simple molecular covalent on the right.
The underlying cause is the electronegativity difference with oxygen, which shrinks as you move right, making the bonding progressively more covalent.
Non-metal oxides in the atmosphere cause acid rain and ocean acidification. You should be able to write those equations.
The spectrum across period 3
Silicon dioxide is the quiet one. It is a giant covalent network, so it does not dissolve in water at all — but it will react with hot concentrated sodium hydroxide, which is what makes it acidic in character.
Why the change happens
Oxygen has an electronegativity of 3.4, which is very high. On the left of period 3 the other element has a very low value, so the difference is large and the bonding is essentially ionic. Sodium oxide contains genuine O2− ions, and the oxide ion is a powerful base:
Why metal oxides are basic
O2−(aq) + H2O(l) → 2OH−(aq)
Move right and the electronegativity difference shrinks. By the time you reach phosphorus and sulfur, the difference is small and the oxides are covalent molecules. There are no oxide ions to release — instead the molecule reacts with water to form a species that donates H+, which is what an acid does.
If a question asks you to explain the trend, the two words that earn marks are electronegativity difference and bonding. Saying “metals give bases and non-metals give acids” describes the trend without explaining it.
The equations to know
Oxide
Reaction with water
Approximate pH
Sodium oxide
Na2O(s) + H2O(l) → 2NaOH(aq)
13–14
Magnesium oxide
MgO(s) + H2O(l) → Mg(OH)2(aq)
about 10
Aluminium oxide
No reaction with water — insoluble
7
Silicon dioxide
No reaction with water — insoluble
7
Phosphorus(V) oxide
P4O10(s) + 6H2O(l) → 4H3PO4(aq)
about 2
Sulfur dioxide
SO2(g) + H2O(l) → H2SO3(aq)
about 1
Sulfur trioxide
SO3(g) + H2O(l) → H2SO4(aq)
about 1
Magnesium hydroxide is only slightly soluble, which is why its pH is around 10 rather than 14 even though it is a strong base in principle. That is the chemistry behind indigestion remedies: alkaline enough to neutralise stomach acid, not alkaline enough to do damage.
Magnesium hydroxide only reaches about pH 10 because it barely dissolves. Being a strong base and producing a high pH are not the same claim.
Amphoteric aluminium oxide
Aluminium sits on the boundary between metals and non-metals, and its oxide behaves accordingly — it reacts with acids like a base, and with bases like an acid.
Acting as a base: Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)
Acting as an acid: Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
The reason is exactly the covalent character discussed earlier in these notes: Al3+ is small and highly charged, so the bonding in Al2O3 is not purely ionic. It sits between the two behaviours because its bonding does too.
Environmental consequences
Non-metal oxides released into the atmosphere dissolve in rainwater and acidify it.
Acid rain: SO2(g) + H2O(l) → H2SO3(aq), and nitrogen oxides do something similar, giving nitric and nitrous acids.
Ocean acidification: CO2(g) + H2O(l) ⇌ H2CO3(aq), which then releases H+ and lowers the pH of seawater.
These are the same chemistry, not a separate topic. Acid rain and ocean acidification are simply “non-metal oxide plus water gives an acid” happening on a planetary scale.
Worked examples
WORKED EXAMPLE
Predict whether the oxide of barium is acidic or basic, write its reaction with water, and state the approximate pH.
Step 1: classify the element
Barium is in group 2, so it is a metal, and its oxide will be basic.
Step 2: work out the formula
Ba forms Ba2+ and oxygen forms O2−, so the oxide is BaO.
Step 3: react it with waterBaO(s) + H₂O(l) → Ba(OH)₂(aq)Basic oxide, giving an alkaline solution of pH around 13group 2 hydroxides get more soluble down the group, so barium hydroxide is more alkaline than magnesium hydroxide
WORKED EXAMPLE
Explain, in terms of bonding, why Na2O is basic but SO3 is acidic. [3]
Take χ(Na) = 0.9, χ(S) = 2.6 and χ(O) = 3.4.
Mark 1: compare the electronegativity differencesNa—O: 3.4 − 0.9 = 2.5 S—O: 3.4 − 2.6 = 0.8Mark 2: state the bonding that follows
The large difference makes Na2O ionic, containing O2− ions. The small difference makes SO3 a covalent molecule.
Mark 3: link to acid-base behaviour
Oxide ions react with water to give OH−, so the solution is alkaline. SO3 reacts with water to give H2SO4, which releases H+.
Ionic oxide gives a base, covalent oxide gives an acidquote the actual numbers — a stated electronegativity difference is usually worth a mark on its own
WORKED EXAMPLE
Aluminium oxide dissolves in both hydrochloric acid and sodium hydroxide solution. What does this tell you, and what is the term for it?
Step 1: interpret the acid reaction
Reacting with an acid means it is behaving as a base.
Step 2: interpret the alkali reaction
Reacting with a base means it is behaving as an acid.
Step 3: name it and explain
A substance that does both is amphoteric. This happens because the Al—O bonding lies between ionic and covalent.
Aluminium oxide is amphoteric — it sits on the boundary in both position and bondingamphoteric and amphiprotic are different words; amphoteric is the one used for oxides
💡 Exam tip
The syllabus expects you to construct these equations, not just recognise them. Practise writing them from the formula of the oxide alone.
Balance P4O10 carefully — it needs six waters and gives four H3PO4.
For trend questions, always link back to electronegativity difference and the bonding it produces.
Remember that insoluble is not the same as neutral. Silicon dioxide is an acidic oxide that happens not to dissolve.
Give aluminium oxide two equations when asked to show it is amphoteric. One is only half the answer.
Quote approximate pH values where you can — around 13 for sodium hydroxide, around 1 for sulfuric acid.
⚠ Common mix-up
Saying all metal oxides give alkaline solutions. Only the soluble ones do; many metal oxides are insoluble but still basic.
Writing NaOH2 or Mg(OH). Get the charges right: Na+ gives NaOH, Mg2+ gives Mg(OH)2.
Calling SiO2 neutral. It is acidic in character, just insoluble in water.
Confusing amphoteric with amphiprotic. Amphoteric means it reacts with both acids and bases; amphiprotic specifically means it can donate or accept a proton.
Forgetting that P4O10 has four phosphorus atoms. The balancing follows from that.
Explaining the trend by “metals are basic”. That restates the observation instead of explaining it.
Up next: Oxidation States — a bookkeeping system for electrons that lets you spot at a glance whether a reaction is redox, and which species did what.
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