The Periodic Table: chemical periodicity

Question 1

Describe and explain the trends in atomic and ionic radius across a period

Answer 1

Atomic radius decreases

• no. of protons ↑ so nuclear +ve charge ↑
• more attraction so outer shell is closer to nucleus

Same principle for ionic radius but anions are larger than cations, as cation lose outer shell

Question 2

Describe and explain the trend in melting point across period 3

Answer 2

• Metallic lattice (Na, Mg, Al): high, increases
  
  • charge density increases as more e^-s donated to electron cloud, smaller radius
• Giant molecular structure (Si): very high
• Simple molecular structure: low
  
  • Ar → Cl₂ → P₄ → S₈
  • increasing van der Waals’

Question 3

Describe and explain the trend in electrical conductivity across period 3

Answer 3

• Metallic lattice (Na, Mg, Al): high, increases
  
  • more e-s donated to electron cloud
• Giant molecular structure (Si): lower than metals, higher than non-metals, i.e. metalloid
  
  • ​conducts under certain circumstances e.g. high temperature
  • ‘doping’ with similar-size elements e.g. Al
• Simple molecular structure: very low
  
  • covalent bonding
  • no mobile e^-s or ions

Question 4

Describe and explain the trend in 1^st ionisation energy across a period

Answer 4

Ionisation energy increases across a period

• no. of protons / nuclear +ve charge ↑
• shielding almost constant - slight increase when adding a sub-shell / spin-pair repulsion
• distance of valence e^- from nucleus ↑

Down a group, 1^st I.E. decreases as shielding and radius increase

Question 5

Explain the strength, high melting point and electrical insulating properties of ceramics in terms of their giant structure; to include magnesium oxide, aluminium oxide and silicon dioxide

Answer 5

• MgO
  
  • ​giant ionic lattice structure, so high melting point
• Al₂O₃
  
  • ​ionic bonding with some covalent character
• SiO₂
  
  • ​giant covalent structure - tetrahedral shape is very hard

Question 6

List the reactions of period 3 elements with oxygen

Answer 6

Na₂O

MgO

Al₂O₃

P₄O₁₀

SO₂, SO₃

Question 7

List the reactions of period 3 elements with chlorine

Answer 7

NaCl , MgCl2, Al2Cl6, SiCl4, PCl5

Question 8

List the reactions of sodium and magenesium with water

Answer 8

2Na + 2H2O → 2NaOH + H2

Mg + 2H2O(l) → Mg(OH)2 (very slow reaction)

Mg + H2O(g) → MgO+ H2 (much faster)

Question 9

State and explain the variation in oxidation number of the oxides (sodium to sulfur only) and chlorides (sodium to phosphorus only) in terms of their valence shell electrons

Answer 9

• maximum oxidation number increases across period 3 as no. of valence e^-s increases

Question 10

Describe the properties of period 3 oxides in terms of:

• structure & m.p.
• bonding & acid/base nature

Answer 10

• Structure & m.p.
  
  • Na₂O, MgO & Al₂O₃: giant ionic lattice, high m.p.
  • SiO₂: giant covalent lattice, very high m.p.
  • P₄O₁₀, SO₂ & SO₃: simple molecular lattice, low m.p.
• Bonding & acid/base nature
  
  • Na₂O & MgO: ionic bonding, basic
  • Al₂O₃: ionic with some covalent character (high charge density polarises O^2- ion), amphoteric
  • SiO_2,P₄O₁₀, SO₂ & SO₃: covalent bonding, acidic

Electronegativity increases across period 3, so the difference in electronegativity between oxygen and the element decreases. Larger difference → ionic; smaller difference → covalent

Question 11

Describe the reactions of period 3 oxides with water

Answer 11

• Na₂O(s) + H₂O(l) → 2NaOH(aq)
  
  • dissolves readily, pH 13
• MgO(s) + H₂O(l) → Mg(OH)₂(aq)
  
  • dissolves partially, pH 9
• Al₂O₃ & SiO₂ - no reaction
  
  • insoluble due to high lattice energy
• P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)
  
  • reacts vigorously, pH2
• SO₂(g) + H₂O(l) → H₂SO₃(aq)
  
  • sulphuric (IV) acid, pH1
• SO₃(g) + H₂O(l) → H₂SO₄(aq)
  
  • sulphuric (VI) acid, pH1

Question 12

List the reactions of Al₂O₃ with acid & concentrated alkali, and of SiO₂ with concentrated alkali

Answer 12

Al₂O₃(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂O(l)

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq)

SiO₂(s) + 2NaOH(aq) → NaSiO₃(aq) + H₂O(l)

salt is close to neutral

Question 13

Describe the reactions of period 3 chlorides with water

Answer 13

• NaCl(s) + aq → Na⁺(aq) + Cl^-(aq)
  
  • pH 7
• MgCl₂(s) + aq → Mg²⁺(aq) + 2Cl^-(aq)
  
  • pH 6.5
  • white solid dissolves readily due to dipole-dipole attraction → colourless solution
• AlCl₃ + 6H₂O(l) → [Al(H₂O)₆]³⁺(aq) + 3Cl^-(aq)
  
  • HCl fumes given off
• → [Al(H₂O)₅(OH)]²⁺(aq) + H⁺(aq)
  
  • pH 3
• SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(g)
  
  • off-white ppt, HCl fumes given off, vigorous & exothermic hydrolysis
• PCl₅(s) + 4H₂O(l) → H₃PO₄(aq) + 5HCl(g)
  
  • colourless solution pH 2, HCl fumes given off, vigorous & exothermic hydrolysis