The ionic model: notes and practice questions
- This topic extends the understanding of the ionic model by exploring its connections to other higher level concepts.
- Relate the ionic model to the variable oxidation states of transition elements, explained by successive ionization energies.
- Apply formal charge to predict the preferred structures of polyatomic ions.
- Connect the stability of polyatomic anions to their role as conjugate bases and acid dissociation constants.
How it is examined
Formula deduction is a 1-mark Paper 1A staple. The explanation parts want the mechanism spelled out: "conducts when molten or in solution because ions are free to move" earns the mark, "conducts electricity" alone does not. Lattice enthalpy comparisons want both variables named. May 2025 HL Paper 2 TZ1 4(c)(ii) asked why MgCl₂ and CaCl₂ differ in lattice enthalpy, [1], and the Notes required a clear reference to ionic radius: "larger radius" without the word ionic was not enough.
Ionic radii. Lattice enthalpy values. The seven polyatomic ions above are recall.
- 2.1.1 When metal atoms lose electrons they form positive ions called cations. When non-metal atoms gain electrons they form negative ions called anions. Students predict the charge of an ion from the electron configuration of the atom.
- 2.1.2 The ionic bond is formed by electrostatic attractions between oppositely charged ions. Students deduce the formula and name of an ionic compound from its component ions, including polyatomic ions. Binary ionic compounds are named with the cation first, followed by the anion, and the anion adopts the suffix "ide". Students interconvert names and formulas of binary ionic compounds.
- 2.1.3 Ionic compounds exist as three-dimensional lattice structures, represented by empirical formulas. Students explain the physical properties of ionic compounds, to include volatility, electrical conductivity and solubility.
Guiding questions
- What determines the ionic nature and properties of a compound?
Linking questions
- Structure 3.1 How does the position of an element in the periodic table relate to the charge of its ion(s)? How can lattice enthalpies and the bonding continuum explain the trend in melting points of metal chlorides across period 3?
- Structure 1.3 (HL) How does the trend in successive ionization energies of transition elements explain their variable oxidation states?
- Reactivity 3.2 Why is the formation of an ionic compound from its elements a redox reaction?
- Structure 2.2 (HL) How is formal charge used to predict the preferred structure of sulfate?
- Reactivity 3.1 (HL) Polyatomic anions are conjugate bases of common acids. What is the relationship between their stability and the conjugate acid's dissociation constant, Ka?
- Tool 1, Inquiry 2 What experimental data demonstrate the physical properties of ionic compounds?
Practice questions
4 questions · 1 easy · 1 medium · 2 hardQuestion 1
EasyPaper 1A · calculator1 markWhich compound has the most exothermic lattice enthalpy?
A. Magnesium fluoride
B. Magnesium chloride
C. Calcium fluoride
D. Calcium chloride
Consider the factors that affect the magnitude of lattice enthalpy. How do ionic charge and ionic radius influence the electrostatic attraction between ions in a crystal lattice?
Question 2
MediumPaper 1A · calculator1 markWhich ionic compound has the most exothermic lattice enthalpy?
A. LiF
B. LiI
C. CsF
D. CsI
Lattice enthalpy is a measure of the strength of the ionic bond. Consider how ionic size and charge affect the electrostatic attraction between ions.
Question 3
HardPaper 2 · calculator24 marksAntimony (Sb) and Bismuth (Bi) are elements in group 15 of the periodic table.
(a) Antimony has two stable isotopes. 57.21% of antimony atoms contain 70 neutrons and the remainder contain 72 neutrons.
(i) Deduce the nuclear symbol of the isotope of antimony containing 72 neutrons. Use section 6 of the data booklet.
(ii) Calculate, to two decimal places, the relative atomic mass of antimony.
Bismuth(III) nitrate, , is a common salt of bismuth.
(b) (i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.
(ii) Distinguish between covalent and ionic bonding in terms of electron distribution.
(iii) State the enthalpy term that characterizes the strength of the bonding between the ions in an ionic solid.
(iv) Write an equation for the formation of aqueous bismuth(III) nitrate from solid bismuth(III) oxide and nitric acid.
(v) Calculate the volume, in , of nitric acid required to react completely with of solid bismuth(III) oxide.
(vi) Predict, with a reason, whether bismuth(III) oxide is expected to be primarily acidic, basic or amphoteric.
(vii) Discuss how the relative reactivity of zinc and bismuth could be established using the metals and aqueous solutions of their nitrates.
(viii) Discuss the products formed at the electrodes during the electrolysis of aqueous bismuth(III) nitrate. Use the standard electrode potential and section 24 of the data booklet.
Bismuth compounds are sometimes used in fireworks to produce special effects.
(c) (i) State the feature of the atomic emission spectrum of an element that corresponds to its first ionization energy.
(ii) Calculate the wavelength, in nm, that corresponds to the first ionization energy of bismuth. Use sections 1, 2 and 8 of the data booklet.
(iii) Explain why the first ionization energy of bismuth is lower than that of polonium (Po), in terms of nuclear charge and electron shielding.
First, find the atomic number (number of protons) for Antimony from the periodic table. The mass number is the sum of protons and neutrons. The nuclear symbol is written with the mass number as a superscript and the atomic number as a subscript to the left of the element symbol.
The relative atomic mass is the weighted average of the masses of its isotopes. You'll need the mass number and abundance of both isotopes. The abundance of the second isotope is 100% minus the abundance of the first.
Consider the structure of the polyatomic nitrate ion and how it interacts with the bismuth cation. Covalent bonds typically form between non-metal atoms, while ionic bonds form between metal cations and non-metal anions.
Think about what happens to the valence electrons in each type of bond. Are they shared or transferred?
This term refers to the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.
Bismuth(III) oxide is a basic oxide. It will react with an acid in a neutralization reaction to form a salt and water. Remember to balance the equation and include state symbols.
First, calculate the moles of bismuth(III) oxide using its mass and molar mass. Then, use the stoichiometry from your balanced equation in (b)(iv) to find the moles of nitric acid required. Finally, use the concentration of the nitric acid to find the volume.
Consider the position of bismuth in the periodic table and the trend in metallic character down group 15. How does the acid-base character of oxides change with metallic character?
A more reactive metal can displace a less reactive metal from a solution of its salt. Describe a simple experiment to test this.
At the cathode (negative electrode), reduction occurs. Compare the standard electrode potentials for the reduction of and water. At the anode (positive electrode), oxidation occurs. Compare the oxidation of water and the nitrate ion.
Ionization corresponds to the removal of an electron, which means the electron transitions to the n=∞ energy level. What happens to the spectral lines as they approach this limit?
First, find the first ionization energy of Bismuth from the data booklet (in kJ mol⁻¹). Convert this to energy per atom (in J) using Avogadro's constant. Then use the Planck-Einstein relation () and the wave equation () to find the wavelength.
Bismuth and Polonium are in the same period. Consider how the number of protons and the location of the valence electrons change as you move from Bi to Po.
Question 4
HardPaper 2 · calculator23 marksA sample of chlorine consists of two isotopes, and .
(a) Contrast the sub-atomic structure of these two isotopes.
(b) (i) The sample of chlorine is analysed in a mass spectrometer, producing a spectrum for the ion. The spectrum shows three peaks at m/z values of 70, 72 and 74. Explain the origin and relative heights of these three peaks, given that the abundance of is approximately three times that of .
(ii) A more precise measurement finds the composition by mass to be: : 75.76%, : 24.24%. Calculate the relative atomic mass of chlorine from this sample, giving your answer to two decimal places. (Use isotopic masses of 35.0 and 37.0 for this calculation).
Magnesium chloride, , and manganese(II) chloride, , are two ionic compounds.
(c) (i) Deduce the type of bonding in magnesium chloride, , using electronegativity values from section 9 of the data booklet.
(ii) Determine the lattice enthalpy of magnesium chloride, assuming the bonding is purely ionic. Use sections 9, 10 and 12 of the data booklet and the following data:
Enthalpy of formation of magnesium chloride =
(iii) Explain, with reference to electron configurations, why the ionic radii of , and are different. Use section 10 of the data booklet.
(iv) Predict, with a reason, which has the stronger ionic bonding, manganese(II) chloride, , or magnesium chloride.
Magnesium chloride is white, but manganese(II) chloride is pale pink.
(d) (i) State the condensed electron configuration of a manganese atom.
(ii) State the reason, in terms of electron configuration, why manganese(II) chloride is coloured.
(iii) Manganese(II) chloride absorbs light with a wavelength of approximately 530 nm. Describe why this is consistent with the observed colour of the compound. Use sections 2 and 15 of the data booklet.
A copper key is to be electroplated with manganese using an aqueous solution of manganese(II) chloride as the electrolyte.
(e) (i) Deduce the half-equations for the reactions occurring at the anode (made of pure manganese) and the cathode (the copper key).
(ii) Deduce a balanced chemical equation for the reaction of fluorine gas with the aqueous chloride ions in the electrolyte.
Isotopes of an element have the same number of protons but a different number of another sub-atomic particle. What is this particle and how does its number differ between and ?
The peaks correspond to different combinations of the two chlorine isotopes in a diatomic molecule. The height of each peak is related to the probability of that specific combination occurring. Consider the relative abundances of the isotopes.
The relative atomic mass is the weighted average of the isotopic masses. Multiply each isotopic mass by its fractional abundance and sum the results.
Find the electronegativity values for magnesium and chlorine. The difference in their electronegativity values will indicate the type of bonding.
Construct a Born-Haber cycle for the formation of from and . Use Hess's Law to find the unknown lattice enthalpy. Remember to account for the stoichiometry, especially for chlorine.
Compare the number of electron shells and the nuclear charge (number of protons) for each ion.
The strength of ionic bonding depends on the charge of the ions and the distance between them (ionic radii). Compare these factors for and .
Manganese is in the first row of the d-block. Remember the filling order of the 4s and 3d sub-levels.
The colour of transition metal compounds is related to the electronic structure of the transition metal ion. What is special about the d-sublevel in coloured ions?
The colour we see is the complementary colour to the one that is absorbed. Use the colour wheel in the data booklet to find the complementary colour of the absorbed light.
In electroplating, the object to be plated is the cathode, and the metal used for plating is the anode. Oxidation occurs at the anode and reduction occurs at the cathode.
Consider the relative oxidizing strengths of the halogens. A more reactive halogen will displace a less reactive halide from its salt solution.
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Where marks are lost
- Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
- Joining the dots instead of drawing a curve.
- Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.
