Ionic bonding & formulae: OCR A A-level Chemistry

NO₃^- nitrate [1]; CO₃^2- carbonate [1]; SO₄^2- sulfate [1]; OH^- hydroxide [1]; NH₄⁺ ammonium [1]; Zn²⁺ zinc [1]; Ag⁺ silver [1]

Aluminium oxide Al₂O₃ [1]; calcium nitrate Ca(NO₃)₂ [1]; ammonium sulfate (NH₄)₂SO₄ [1].

Magnesium hydroxide Mg(OH)₂ [1]; zinc carbonate ZnCO₃ [1]; silver nitrate AgNO₃ [1].

Use brackets only when more than one of a compound ion is needed, so the subscript applies to the whole ion [1]

(a) Ionic bonding is the electrostatic attraction [1] between positive and negative ions [1]. [2]

(b) Electron transfer is the process that forms the ions [1]; the bond is the attraction between the ions after transfer [1]. [2]

One Mg²⁺ ion and two Cl^- ions, all bracketed and correctly charged [2].

Each chloride ion has eight outer-shell electrons [1], with one electron on each chloride shown as transferred from Mg [1]. Magnesium has lost two electrons in total [1]. [5]

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(a) Two Na⁺ ions are needed for one O^2- ion, so the formula is Na₂O [1]. [1]

(b) Two bracketed Na⁺ ions and one bracketed O^2- ion [2]; oxide has eight outer-shell electrons [1], including two electrons shown as transferred from sodium [2]; electron totals and charges are consistent with Na₂O [1]. [6]

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(a) Charge balance requires 2 × (+3) and 3 × (−2), so the formula is X₂Y₃ [2]. [2]

(b) Two X atoms each lose three electrons, so six electrons are transferred in total [2]. [2]

(c) Two bracketed X³⁺ ions and three bracketed Y^2- ions [2]; each Y^2- has eight outer-shell electrons [1], with two transferred electrons shown on each [2]; all charges and the 2:3 ratio are correct [1]. [6]

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(a) X has 12 electrons, so X is magnesium, Mg [1]. Y has 9 electrons, so Y is fluorine, F [1]. [2]

(b) Mg loses two electrons to form Mg²⁺ [1]; F gains one electron to form F^- [1]. [2]

(c) One Mg²⁺ requires two F^- ions, so the formula is MgF₂ [1]. [1]

(d) One bracketed Mg²⁺ and two bracketed F^- ions [2]; each fluoride has eight outer-shell electrons [1], including one electron shown as transferred from Mg [1]; final electron totals and charges are correct [1]. [5]

(e) In solid MgF₂, ions are fixed and cannot carry charge [1]. When molten, the ions are mobile and carry charge [1]. [2]

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CaO has the higher melting point. Ca²⁺ and O²⁻ have larger charges than K⁺ and Cl⁻ [1], giving stronger electrostatic attractions between oppositely charged ions [1]. More energy is required to overcome these attractions when CaO melts [1].

The smaller F⁻ ion allows the centres of opposite ions to be closer together [1]. With the same ion charges, electrostatic attractions are stronger in NaF [1], so more energy is needed to overcome them on melting [1].

(a) NaCl has many strong electrostatic attractions between oppositely charged ions throughout a giant lattice [1]; much energy is required to overcome them [1]. [2]

(b) In solid NaCl, ions are fixed in position, so there are no mobile charge carriers [1]. When molten, the ions are mobile [1] and carry charge through the liquid [1]. [3]

(c) In aqueous NaCl, dissolved Na⁺ and Cl^- ions are mobile and carry charge [1]. [1]

(d) Ionic compounds are often, but not always, soluble in water; solubility depends on whether new water-ion attractions compensate for separating the ions in the lattice [1]. [1]

(a) Transfer forms the ions [1]; ionic bonding is the electrostatic attraction between oppositely charged ions [1]. [2]

(b) The charge carriers are mobile Na⁺ and Cl^- ions [1], not free electrons [1]. [2]

(c) Melting iodine overcomes intermolecular forces between I₂ molecules [1]; the I-I covalent bonds remain intact [1]. [2]

(d) Ionic compounds are often but not always water-soluble [1]; solubility depends on the balance between attractions broken and new solvent-ion attractions formed [1]. [2]