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Questions on ionic bonding and ionic lattices

Electron transfer and the charges on simple and compound ions, dot-and-cross diagrams for NaCl, MgO and CaCl₂, formula units and counting ions in a mass, the sodium chloride lattice and 6:6 coordination, ion migration and electron density maps as evidence for ions, the strength of ionic bonding and how it depends on ionic charge and radius, ion sizes down a group and across an isoelectronic series, and the melting points, conductivity, brittleness and solubility of ionic compounds.

18 original questions · 59 marks · the ionic bonding and ionic lattices notes · Bonding and structure · Flashcards for this unit

To print: the questions (PDF, 77 KB) · the mark schemes (PDF, 84 KB)

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• 6 marks · Explain · IBI-Q18

  Explain, in terms of structure and bonding, the high melting point of magnesium oxide, its electrical conductivity as a solid and when molten, and its brittleness.

  Mark scheme

  Indicative content

  1. magnesium oxide is a giant ionic lattice of Mg²⁺ and O²⁻ ions
  2. there are strong electrostatic attractions between oppositely charged ions in all directions, made stronger by the 2+ and 2− charges
  3. a great deal of energy is needed to overcome these many strong attractions, so the melting point is high
  4. as a solid it does not conduct because the ions are fixed in position and there are no delocalised electrons
  5. when molten the ions are free to move and carry charge, so it conducts
  6. it is brittle because a displaced layer brings ions of like charge together, they repel and the lattice splits

  Levels

  • Level 3 (5–6 marks) all three properties are explained from the giant ionic lattice: the strong attraction between 2+ and 2− ions giving the high melting point, the ions fixed in the solid and free when molten for conductivity, and the like charges brought together on displacement for brittleness; ions, not electrons, carry the current in the melt
  • Level 2 (3–4 marks) the lattice is identified and two of the three properties are explained, or all three are tackled and one is thin, such as conductivity with nothing said about what carries the charge
  • Level 1 (1–2 marks) magnesium oxide is said to be ionic with strong bonds; the molten oxide is typically said to conduct through free electrons, and brittleness is left unexplained

• 5 marks · Explain · IBI-Q9 Edexcel · CAIE

  Explain why the ionic radius decreases along the isoelectronic series N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺, and why it increases down Group 1 from Li⁺ to Na⁺ to K⁺.

  Mark scheme

  1. B1 all six ions have the same number of electrons, ten, in the same arrangement 1s² 2s² 2p⁶
  2. B1 the number of protons, and so the nuclear charge, increases from 7 to 13 along the series
  3. B1 with the same shielding, the larger nuclear charge attracts the electrons more strongly and pulls them closer to the nucleus
  4. B1 down Group 1 each ion has one more occupied shell of electrons
  5. B1 the outer electrons are further from the nucleus, which outweighs the greater nuclear charge because of the extra shielding; ignore 'more electrons' alone

• 4 marks · Give · IBI-Q2

  Give the formulae of the ions formed by aluminium and by sulfur, and the formulae of aluminium sulfide, ammonium sulfate and calcium nitrate.

  Mark scheme

  1. B1 aluminium forms Al³⁺ and sulfur forms S²⁻; both needed for the mark
  2. B1 aluminium sulfide is Al₂S₃, since two Al³⁺ (+6) balance three S²⁻ (−6)
  3. B1 ammonium sulfate is (NH₄)₂SO₄; reject NH₄₂SO₄ without the brackets
  4. B1 calcium nitrate is Ca(NO₃)₂, two NO₃⁻ ions for each Ca²⁺ ion; reject CaNO₃₂

• 4 marks · Explain · IBI-Q7

  Explain why magnesium oxide has a much higher melting point than sodium chloride, although both have the same lattice structure.

  Mark scheme

  1. B1 magnesium oxide contains Mg²⁺ and O²⁻ ions, with larger charges than Na⁺ and Cl⁻
  2. B1 its ions are smaller, so their centres are closer together
  3. B1 so the electrostatic attraction between oppositely charged ions is stronger in magnesium oxide
  4. B1 more energy is needed to overcome the stronger attraction and free the ions, so the melting point is higher

• 4 marks · Compare · IBI-Q16 Edexcel · CAIE

  Compare the radius of a sodium atom with that of a sodium ion, and the radius of a chlorine atom with that of a chloride ion, explaining each difference.

  Mark scheme

  1. B1 the Na⁺ ion is smaller than the Na atom
  2. B1 because Na⁺ has lost its outer shell, the 3s electron, so its outermost electrons are in a shell closer to the nucleus
  3. B1 the Cl⁻ ion is larger than the Cl atom
  4. B1 because the extra electron increases repulsion between the electrons while the nuclear charge is unchanged, so the electron cloud expands

• 4 marks · Explain · IBI-Q19 Edexcel

  Explain how an electron density map of sodium chloride provides evidence that the compound is made of ions, and how the map of a crystal of iodine, I₂, would differ.

  Mark scheme

  1. B1 the contours form a separate, nearly spherical region of electron density round each nucleus
  2. B1 between neighbouring sodium and chlorine centres the electron density falls almost to zero, so no electrons are shared between them
  3. B1 about 18 electrons surround each chlorine and about 10 each sodium, showing that one electron has transferred from sodium to chlorine
  4. B1 in iodine the contours of the two atoms in each I₂ molecule merge, with high density between the nuclei where the shared pair is, and fall to almost zero between molecules

• 3 marks · Describe · IBI-Q4

  Describe the dot-and-cross diagram for magnesium oxide, showing outer electrons only.

  Mark scheme

  1. B1 the magnesium ion is drawn in square brackets with a 2+ charge and an empty outer shell, or no outer electrons shown
  2. B1 the oxide ion is drawn in square brackets with a 2− charge and eight electrons in its outer shell
  3. B1 of the oxide ion's eight electrons, six are one symbol and two are the other symbol, the two transferred from magnesium

• 3 marks · Explain · IBI-Q6

  Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.

  Mark scheme

  1. B1 in the solid the ions are held in fixed positions in the lattice and cannot move
  2. B1 when molten the ions are free to move
  3. B1 the moving ions carry the charge, Na⁺ towards the negative electrode and Cl⁻ towards the positive electrode; there are no free electrons in either state

• 3 marks · Explain · IBI-Q8

  Explain why ionic compounds such as sodium chloride are brittle.

  Mark scheme

  1. B1 a force can move one layer of ions relative to the next by one ion
  2. B1 ions of the same charge are then next to each other across the layers
  3. B1 the like charges repel, so the layers push apart and the crystal splits

• 3 marks · Calculate · IBI-Q11

  Calculate the total number of ions in 1.00 g of sodium chloride. Mr of NaCl = 58.5; L = 6.022 × 10²³ mol⁻¹.

  Mark scheme

  1. M1 amount of NaCl = 1.00 ÷ 58.5 = 0.01709 mol
  2. M1 each formula unit contains two ions, so ions = 2 × 0.01709 × 6.022 × 10²³
  3. A1 2.06 × 10²² ions, to three significant figures

• 3 marks · Calculate · IBI-Q12

  Calculate the number of chloride ions in 4.44 g of calcium chloride, CaCl₂. Ar: Ca 40.1, Cl 35.5; L = 6.022 × 10²³ mol⁻¹.

  Mark scheme

  1. M1 Mr of CaCl₂ = 40.1 + 2 × 35.5 = 111.1, so amount = 4.44 ÷ 111.1 = 0.03996 mol
  2. M1 two chloride ions per formula unit, so chloride ions = 2 × 0.03996 × 6.022 × 10²³
  3. A1 4.81 × 10²² chloride ions, to three significant figures; allow error carried forward from a wrong Mr

• 3 marks · Suggest · IBI-Q15 Edexcel

  Suggest what is seen when a crystal of purple potassium manganate(VII), KMnO₄, is placed in the middle of a strip of filter paper soaked in salt solution, and a potential difference is applied across the strip for twenty minutes. Explain the observation.

  Mark scheme

  1. B1 a purple colour spreads from the crystal towards the positive electrode
  2. B1 the purple colour is due to the MnO₄⁻ ions, which are negative and are attracted to the positive electrode
  3. B1 the K⁺ ions move towards the negative electrode but are colourless, so the movement shows that the compound is made of ions that are free to move in solution

• 3 marks · Evaluate · IBI-Q17

  Evaluate the claim that solid sodium chloride is made of NaCl molecules because its formula shows one sodium and one chlorine.

  Mark scheme

  1. B1 the claim is wrong: solid sodium chloride is a giant ionic lattice of Na⁺ and Cl⁻ ions
  2. B1 each Na⁺ is attracted equally to six Cl⁻ ions around it, so no Na⁺ is paired with any particular Cl⁻
  3. B1 the formula NaCl shows only the ratio of ions in the lattice, one Na⁺ for each Cl⁻, not a molecule

• 3 marks · Calculate · IBI-Q20

  Calculate the number of oxide ions in 2.04 g of aluminium oxide, Al₂O₃. Ar: Al 27.0, O 16.0; L = 6.022 × 10²³ mol⁻¹.

  Mark scheme

  1. M1 Mr of Al₂O₃ = 2 × 27.0 + 3 × 16.0 = 102.0, so amount = 2.04 ÷ 102.0 = 0.0200 mol
  2. M1 three oxide ions per formula unit, so oxide ions = 3 × 0.0200 × 6.022 × 10²³
  3. A1 3.61 × 10²² oxide ions, to three significant figures

• 2 marks · State · IBI-Q1

  State what is meant by the term ionic bonding.

  Mark scheme

  1. B1 a strong electrostatic attraction
  2. B1 between oppositely charged ions, positive and negative, acting in all directions in a lattice

• 2 marks · Name · IBI-Q5

  Name the type of structure in solid sodium chloride, and give the coordination number of each of its ions.

  Mark scheme

  1. B1 a giant ionic lattice
  2. B1 each Na⁺ has six Cl⁻ nearest neighbours and each Cl⁻ has six Na⁺, so 6:6 coordination

• 2 marks · Calculate · IBI-Q13

  Calculate the distance between the centres of two chloride ions on the same line of a sodium chloride crystal, with one sodium ion between them and all three touching. Radii: Na⁺ 0.102 nm, Cl⁻ 0.181 nm.

  Mark scheme

  1. M1 the distance is one Cl⁻ radius, one Na⁺ diameter and one Cl⁻ radius: 2 × 0.181 + 2 × 0.102
  2. A1 0.566 nm, to three significant figures

• 2 marks · Suggest · IBI-Q14

  Suggest why barium oxide melts at 1923 °C, lower than magnesium oxide at 2852 °C.

  Mark scheme

  1. B1 Ba²⁺ has more electron shells than Mg²⁺, so it is a larger ion with the same charge
  2. B1 the ion centres in barium oxide are further apart, so the attraction between the ions is weaker and less energy is needed to melt the lattice

Practise ionic bonding and ionic lattices one question at a time