AS Chem Memorisation

State the trend in atomic radius across Period 3 (Na → Ar).

Atomic radius decreases across Period 3.

Why does atomic radius decrease across Period 3?

Nuclear charge increases while shielding remains almost constant, increasing attraction between the nucleus and outer electrons.

Why does shielding remain approximately constant across Period 3?

Electrons are added to the same principal energy level, so no additional electron shell is introduced.

What is the general trend in ionic radius across the cations Na⁺, Mg²⁺ and Al³⁺?

Ionic radius decreases.

Why do the ionic radii of Na⁺, Mg²⁺ and Al³⁺ decrease across the period?

Nuclear charge increases while the ions remain isoelectronic.

What is the trend in ionic radius from P³⁻ to Cl⁻?

Ionic radius decreases.

Why is there a large jump in ionic radius between Al³⁺ and P³⁻?

The change is from small positive ions to much larger negative ions with an extra electron shell.

State the trend in melting point across Period 3.

It increases from Na to Si, then decreases sharply at P₄, rises slightly at S₈, then decreases through Cl₂ to Ar.

Why does the melting point increase from Na to Al?

Metallic bonding becomes stronger due to increasing positive charge and more delocalised electrons.

Which Period 3 element has the highest melting point?

Silicon.

Why does silicon have the highest melting point in Period 3?

It has a giant covalent structure with many strong covalent bonds.

Why does phosphorus have a low melting point?

It consists of simple molecular P₄ molecules held together by weak intermolecular forces.

Why is sulfur's melting point higher than phosphorus'?

S₈ molecules are larger and have stronger intermolecular forces than P₄ molecules.

Why does chlorine have a lower melting point than sulfur?

Cl₂ molecules are much smaller and have weaker intermolecular forces.

Why does argon have the lowest melting point?

It is monatomic with only weak instantaneous dipole-induced dipole attractions.

State the trend in electrical conductivity across Period 3.

It increases from Na to Al, drops to almost zero at Si, remains very low through P, S, Cl and Ar.

Why does electrical conductivity increase from Na to Al?

The number of delocalised electrons increases.

Why is aluminium the best electrical conductor among Na, Mg and Al?

Each atom contributes three delocalised electrons.

Why does silicon conduct electricity poorly compared with metals?

It has no delocalised electrons but is a semiconductor.

Why do phosphorus, sulfur, chlorine and argon not conduct electricity?

They contain no mobile charged particles.

What oxide is formed when sodium burns in oxygen?

Na₂O.

Write the equation for sodium reacting with oxygen.

4Na + O₂ → 2Na₂O

What oxide is formed when magnesium burns in oxygen?

MgO.

Write the equation for magnesium reacting with oxygen.

2Mg + O₂ → 2MgO

What oxide is formed when aluminium burns in oxygen?

Al₂O₃.

Write the equation for aluminium reacting with oxygen.

4Al + 3O₂ → 2Al₂O₃

What oxide is formed when phosphorus burns in excess oxygen?

P₄O₁₀.

Write the equation for phosphorus reacting with oxygen.

P₄ + 5O₂ → P₄O₁₀

What oxide is formed when sulfur burns in oxygen?

SO₂.

Write the equation for sulfur reacting with oxygen.

S + O₂ → SO₂

What chloride is formed when sodium reacts with chlorine?

NaCl.

Write the equation for sodium reacting with chlorine.

2Na + Cl₂ → 2NaCl

What chloride is formed when magnesium reacts with chlorine?

MgCl₂.

Write the equation for magnesium reacting with chlorine.

Mg + Cl₂ → MgCl₂

What chloride is formed when aluminium reacts with chlorine?

AlCl₃.

Write the equation for aluminium reacting with chlorine.

2Al + 3Cl₂ → 2AlCl₃

What chloride is formed when silicon reacts with chlorine?

SiCl₄.

Write the equation for silicon reacting with chlorine.

Si + 2Cl₂ → SiCl₄

What chloride is formed when phosphorus reacts with chlorine?

PCl₅.

Write the equation for phosphorus reacting with chlorine.

P₄ + 10Cl₂ → 4PCl₅

Write the equation for sodium reacting with water.

2Na + 2H₂O → 2NaOH + H₂

Describe the reaction of sodium with water.

Very vigorous; sodium floats, melts into a ball, moves rapidly, releases hydrogen and forms sodium hydroxide.

Write the equation for magnesium reacting with cold water.

Mg + 2H₂O → Mg(OH)₂ + H₂

Describe the reaction of magnesium with cold water.

Very slow reaction producing magnesium hydroxide and hydrogen.

State the oxidation number of sodium in Na₂O.

+1

State the oxidation number of magnesium in MgO.

+2

State the oxidation number of aluminium in Al₂O₃.

+3

State the oxidation number of phosphorus in P₄O₁₀.

+5

State the oxidation number of sulfur in SO₂.

+4

State the oxidation number of sulfur in SO₃.

+6

Why does oxidation number increase across Period 3?

The number of valence electrons increases.

What happens when Na₂O reacts with water?

It forms sodium hydroxide.

Write the equation for Na₂O reacting with water.

Na₂O + H₂O → 2NaOH

What is the approximate pH of sodium oxide solution?

About 14.

What happens when MgO reacts with water?

It forms magnesium hydroxide slowly.

Write the equation for MgO reacting with water.

MgO + H₂O → Mg(OH)₂

What is the approximate pH of magnesium oxide solution?

About 9–10.

Does Al₂O₃ react with water?

No.

Does SiO₂ react with water?

No.

What happens when P₄O₁₀ reacts with water?

It forms phosphoric acid.

Write the equation for P₄O₁₀ reacting with water.

P₄O₁₀ + 6H₂O → 4H₃PO₄

What is the approximate pH after dissolving P₄O₁₀ in water?

About 1–2.

What happens when SO₂ reacts with water?

It forms sulfurous acid.

Write the equation for SO₂ reacting with water.

SO₂ + H₂O ⇌ H₂SO₃

What is the approximate pH of sulfur dioxide solution?

About 2–3.

What happens when SO₃ reacts with water?

It forms sulfuric acid.

Write the equation for SO₃ reacting with water.

SO₃ + H₂O → H₂SO₄

What is the approximate pH of sulfur trioxide solution?

About 1.

Which Period 3 oxide is amphoteric?

Al₂O₃.

Which Period 3 oxides are basic?

Na₂O and MgO.

Which Period 3 oxides are acidic?

P₄O₁₀, SO₂ and SO₃.

Which oxide is a neutral covalent giant oxide that does not react with water?

SiO₂.

Write the equation for Na₂O reacting with hydrochloric acid.

Na₂O + 2HCl → 2NaCl + H₂O

Write the equation for MgO reacting with hydrochloric acid.

MgO + 2HCl → MgCl₂ + H₂O

Write the equation for Al₂O₃ reacting with hydrochloric acid.

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Write the equation for Al₂O₃ reacting with sodium hydroxide.

Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄

Why is aluminium oxide amphoteric?

It reacts with both acids and bases.

State the acid-base behaviour of NaOH.

Strong alkali.

State the acid-base behaviour of Mg(OH)₂.

Weak alkali due to low solubility.

State the acid-base behaviour of Al(OH)₃.

Amphoteric.

Does NaCl react with water?

No.