Topic 9: The Periodic Table: Chemical Periodicity

Question 1

Definitions I:
(a) Atomic radius
(b) Ionic radius
(c) Periodicity

Answer 1

Definitions I:
(a) Distance from nucleus to outer electron shell of an atom.
(b) Distance from nucleus to outer electron shell of an ion.
(c) Repeating trends in properties across periods in the Periodic Table.

Question 2

Definitions II:
(a) Oxidation number
(b) Amphoteric
(c) Electronegativity

Answer 2

Definitions II:
(a) Charge an atom would have if bonds were fully ionic.
(b) Substance reacting with both acids and bases, e.g., Al₂O₃.
(c) Power of an atom to attract electrons in a covalent bond.

Question 3

How do melting points vary across Period 3 from Na to Ar, and why? [3 Marks]

Answer 3

Na to Si: increases, metallic bonding strengthens (1 mark)
P₄ to Cl₂: decreases, weak van der Waals’ (1 mark)
Ar: lowest, monatomic gas (1 mark)

Question 4

Write equations for the reactions of:
(a) Mg with O₂ [1 Mark]
(b) P₄ with Cl₂ (to PCl₅) [1 Mark]

Answer 4

(a) 2Mg + O₂ → 2MgO (1 mark)
(b) P₄ + 10Cl₂ → 4PCl₅ (1 mark)

Question 5

What are the oxidation numbers of:
(a) Na in Na₂O? [1 Mark]
(b) P in PCl₅? [1 Mark]

Answer 5

(a) Na: +1 (O = -2, 2Na = +2) (1 mark)
(b) P: +5 (Cl = -5, P = +5) (1 mark)

Question 6

Describe the reaction of SiO₂ and P₄O₁₀ with water, including pH. [3 Marks]

Answer 6

SiO₂: no reaction, neutral pH (1 mark)
P₄O₁₀ + 6H₂O → 4H₃PO₄, acidic pH ~2 (1 mark)
Covalent bonding, electronegativity drives acidity (1 mark)

Question 7

Why does Al₂O₃ react with both HCl and NaOH, unlike MgO? [2 Marks]

Answer 7

Al₂O₃ amphoteric, reacts with H⁺ and OH⁻ (1 mark)
MgO basic only, ionic, no acid reaction (1 mark)

Question 8

Write equations for NaCl and AlCl₃ reacting with water, noting pH. [2 Marks]

Answer 8

NaCl + H₂O: no reaction, pH 7 (1 mark)
AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl, pH < 7 (1 mark)

Question 9

In Period 3, why does SO₃ form an acidic solution while Na₂O forms a basic one? [2 Marks]

Answer 9

SO₃ covalent, high electronegativity, acidic H₂SO₄ (1 mark)
Na₂O ionic, low electronegativity, basic NaOH (1 mark)

Question 10

Predict the bonding in MgCl₂ and SiCl₄ based on their properties. [2 Marks]

Answer 10

MgCl₂: ionic, high melting point (1 mark)
SiCl₄: covalent, liquid, hydrolyses (1 mark)

Question 11

An element reacts with O₂ to form XO₂ and has low conductivity. Suggest its identity in Period 3. [2 Marks]

Answer 11

Likely Si: forms SiO₂ (covalent), poor conductor (1 mark)
Period 3, Group 14 (1 mark)

Question 12

Predict the reaction of a Group 1 element with water based on Na’s behavior. [2 Marks]

Answer 12

Vigorous reaction, forms XOH + H₂ (1 mark)
Like Na, increases down Group 1 (1 mark)