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Chemical Bonding and Structure - Ionic bonding and structure

Grade 12IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Ionic bonding is the electrostatic attraction between oppositely charged ions (cations and anions) arranged in a regular three-dimensional structure called a giant ionic lattice.

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Formation involves the transfer of electrons from a metal (low electronegativity) to a non-metal (high electronegativity). Generally, an electronegativity difference Δχ>1.8\Delta\chi > 1.8 indicates ionic character.

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The strength of the ionic bond is determined by lattice enthalpy, which is influenced by ionic charge and ionic radius. Higher charges (qq) and smaller radii (rr) lead to stronger attraction.

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Physical properties: High melting and boiling points due to strong electrostatic forces throughout the lattice; brittleness due to the repulsion of like-charges when layers shift.

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Electrical conductivity: Ionic compounds do not conduct electricity in the solid state as ions are in fixed positions. They conduct when molten or in aqueous solution because ions are free to move (mobile ionsmobile\ ions).

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Solubility: Most ionic compounds are soluble in polar solvents like H2OH_2O because ion-dipole attractions provide enough energy to overcome the lattice enthalpy.

📐Formulae

F=kq1q2r2F = k \frac{q_1 q_2}{r^2}

ΔHlat∝qcation⋅qanionrcation+ranion\Delta H_{lat} \propto \frac{q_{cation} \cdot q_{anion}}{r_{cation} + r_{anion}}

M(s)+12X2(g)→MX(s)M(s) + \frac{1}{2}X_2(g) \rightarrow MX(s)

💡Examples

Problem 1:

Predict and explain which compound has a higher melting point: NaClNaCl or MgOMgO.

Solution:

MgOMgO has a significantly higher melting point than NaClNaCl.

Explanation:

In MgOMgO, the ions are Mg2+Mg^{2+} and O2−O^{2-}, whereas in NaClNaCl, the ions are Na+Na^+ and Cl−Cl^-. According to Coulomb's Law, the force of attraction is proportional to the product of the charges (q1q2q_1 q_2). The product for MgOMgO is ∣2×−2∣=4|2 \times -2| = 4, while for NaClNaCl it is ∣1×−1∣=1|1 \times -1| = 1. Additionally, Mg2+Mg^{2+} has a smaller ionic radius than Na+Na^+. Higher charges and smaller radii result in a much stronger lattice enthalpy for MgOMgO, requiring more thermal energy to overcome the bonds.

Problem 2:

Explain why ionic crystals are brittle when a force is applied.

Solution:

Applying stress causes layers of ions to slide, leading to cleavage.

Explanation:

In a giant ionic lattice, ions are arranged in a regular alternating pattern of positive and negative charges. When an external force is applied, one layer of ions slides over another. This brings ions of the same charge (e.g., Na+Na^+ next to Na+Na^+) into alignment. The resulting electrostatic repulsion between like-charged ions causes the crystal lattice to shatter or split along a plane.

Ionic bonding and structure - Revision Notes & Key Formulas | IB Grade 12 Chemistry