In simple terms
A friendly intro before the formal notes — no formulas yet.
Period 3: From Base to Acid
As we move across Period 3, the character of the elements' oxides changes dramatically from strongly basic to strongly acidic. This trend is mirrored by their chlorides, which hydrolyse to produce solutions of varying pH.
Imagine a line of chefs, each cooking with the same basic ingredient like oxygen. The first chef (sodium) makes a simple, strongly alkaline dish. As you go down the line, the chefs (elements) get more complex, and their dishes become increasingly acidic, with one chef (aluminium) able to make a dish that can be either acidic or basic depending on what it's served with.
- 1
Na₂O, MgO basic; Al₂O₃ amphoteric; SiO₂, P₄O₁₀, SO₃ acidic oxides.
- 2
Reaction with water: Na vigorous; Mg slow; Al/Si insoluble oxides.
- 3
Chlorides: NaCl neutral; AlCl₃, SiCl₄ hydrolyse to acidic solutions.
- 4
Max oxidation state rises to Group 15 then falls — inert pair effect later.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Full topic notes
Formal explanation with the rigour you need for the exam.
Reactions with Oxygen: The Period 3 Oxides
The elements from sodium to sulfur all react with oxygen when heated to form oxides. The nature and formula of the oxide formed depend on the element. Sodium burns with a yellow flame to form mainly sodium peroxide, Na₂O₂, but also some sodium oxide, Na₂O. For simplicity in this topic, we focus on Na₂O. Magnesium burns with a brilliant white light to form magnesium oxide, MgO. Aluminium forms a stable oxide layer, Al₂O₃, that passivates the metal, but powdered aluminium burns readily. Silicon, phosphorus, and sulfur also burn to form their highest oxides, SiO₂, P₄O₁₀, and SO₂/SO₃ respectively.
Sodium: 2Na(s) + ½O₂(g) → Na₂O(s)
Magnesium: 2Mg(s) + O₂(g) → 2MgO(s)
Aluminium: 4Al(s) + 3O₂(g) → 2Al₂O₃(s)
Silicon: Si(s) + O₂(g) → SiO₂(s)
Phosphorus: P₄(s) + 5O₂(g) → P₄O₁₀(s)
Sulfur: S(s) + O₂(g) → SO₂(g) (can be further oxidised to SO₃)
Acid-Base Behaviour of the Oxides
The most important trend across Period 3 is the change in the acid-base character of the oxides. This trend is directly linked to the change in bonding from ionic to covalent. The metallic elements on the left form ionic oxides which are basic. The non-metallic elements on the right form covalent oxides which are acidic. Aluminium oxide, in the middle, is amphoteric.
Basic oxides (Na₂O, MgO) are ionic lattices containing the O²⁻ ion. This ion is a strong base and reacts with water to produce hydroxide ions, forming alkaline solutions. Amphoteric aluminium oxide (Al₂O₃) has both ionic and covalent character and is insoluble in water, but it reacts with both strong acids and strong bases. Acidic oxides (SiO₂, P₄O₁₀, SO₃) are covalent. They react with water (or with bases if insoluble in water) to form acids.
MgO(s) + H₂O(l) \rightleftharpoons Mg(OH)₂(aq) \quad (pH \approx 9)
Period 3 Chlorides and Hydrolysis
The chlorides of the Period 3 elements also show a distinct trend in their reaction with water. This behaviour is determined by the bonding in the chloride and the polarising power (charge density) of the cation.
Hydrolysis of SiCl₄:
Hydrolysis of PCl₅:
Sodium & Magnesium Chloride (NaCl, MgCl₂): These are ionic compounds. They simply dissolve in water, with the ions becoming hydrated. The solutions are neutral (NaCl, pH 7) or very weakly acidic (MgCl₂, pH ~6.5) because the Na⁺ and Mg²⁺ ions have low charge densities and cause minimal hydrolysis.
Aluminium Chloride (AlCl₃): This has significant covalent character. In water, the Al³⁺ ion becomes hydrated to form [Al(H₂O)₆]³⁺. The high charge density of Al³⁺ polarises the coordinate bonds to the water ligands, weakening the O-H bonds. This leads to the release of H⁺ ions, making the solution strongly acidic (pH ~3).
Silicon & Phosphorus Chlorides (SiCl₄, PCl₅): These are simple molecular covalent compounds. They do not dissolve but react vigorously and irreversibly with water in a hydrolysis reaction. The reaction produces an acidic solution due to the formation of hydrogen chloride, along with an oxide or oxyacid of the Period 3 element.
When asked to compare the pH of solutions of sodium chloride and aluminium chloride, a complete answer must mention the difference in cation charge density (Al³⁺ > Na⁺), the resulting polarisation of water molecules by Al³⁺, and the subsequent release of H⁺ ions (hydrolysis). Simply stating 'AlCl₃ is acidic' is not enough for full marks.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Explain, with the aid of equations, why aluminium oxide is described as amphoteric.
- 1
Reaction as a base: Aluminium oxide contains oxide ions (O²⁻) which can accept protons from an acid, such as hydrochloric acid. It neutralises the acid to form a salt and water.
A student adds a small amount of silicon tetrachloride to a beaker of water. Describe two observations the student would make and write a balanced equation for the reaction. Explain why the resulting mixture is acidic.
- 1
A white precipitate/solid is formed (this is hydrated silicon dioxide).
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
What is the general trend in the acid-base nature of Period 3 oxides?
They trend from basic (Na₂O, MgO) to amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂, SO₃). This reflects the change from metallic to non-metallic character across the period.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Sodium: 2Na(s) + ½O₂(g) → Na₂O(s)
- ✓
Magnesium: 2Mg(s) + O₂(g) → 2MgO(s)
- ✓
Aluminium: 4Al(s) + 3O₂(g) → 2Al₂O₃(s)
- ✓
Silicon: Si(s) + O₂(g) → SiO₂(s)
- ✓
Phosphorus: P₄(s) + 5O₂(g) → P₄O₁₀(s)
- ✓
Sulfur: S(s) + O₂(g) → SO₂(g) (can be further oxidised to SO₃)
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Periodicity of Chemical Properties
Periodicity of Chemical Properties
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Periodicity of Chemical Properties on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.