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QLDChemistryQuick questions
Unit 1: Chemical fundamentals (structure, properties and reactions)
Quick questions on Ionic bonding and the properties of ionic compounds (QCE Chemistry Unit 1)
14short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is forming an ionic compound?Show answer
Metals lose electrons to form cations (positive ions); non-metals gain electrons to form anions (negative ions). Both species typically reach a noble-gas electron configuration (closed-shell stability).
What is predicting formulae?Show answer
The compound must be electrically neutral overall. Use the smallest whole-number ratio that balances charge.
What is the ionic lattice?Show answer
In the solid state, ions are arranged in a regular repeating 3D pattern (a crystal lattice). The geometry depends on the relative sizes of the cation and anion (the radius ratio). Common QCE-relevant lattices:
What is physical properties from lattice structure?Show answer
High melting and boiling points. Strong electrostatic attractions throughout the lattice; large lattice energy. Comparing within ionic compounds, higher charges and smaller ions give higher melting points. MgO (charges 2+/2-) melts above 2800 degrees C; NaCl (1+/1-) melts at 801 degrees C.
What is worked example?Show answer
Question. Predict whether sodium oxide or magnesium oxide has the higher melting point, and explain your reasoning.
What is high melting and boiling points?Show answer
Strong electrostatic attractions throughout the lattice; large lattice energy. Comparing within ionic compounds, higher charges and smaller ions give higher melting points. MgO (charges 2+/2-) melts above 2800 degrees C; NaCl (1+/1-) melts at 801 degrees C.
What is hard but brittle?Show answer
The lattice resists deformation, so the solid is hard. But under stress, one layer of ions can shift over another so that like charges align; the resulting repulsion shatters the crystal along that plane. This is why ionic crystals cleave cleanly under impact.
What is electrical conductivity only when molten or dissolved?Show answer
Conduction requires mobile charge carriers. In the solid, ions are fixed in the lattice. In the molten state, the lattice has broken down and ions can migrate.
What is variable solubility in water?Show answer
Water is a polar solvent that solvates ions effectively. Solubility depends on the balance between lattice energy (must be overcome to dissociate the solid) and hydration energy (released when ions are solvated). Compounds with comparable lattice and hydration energies tend to dissolve; compounds with very high lattice energy relative to hydration energy do not.
What is writing formulae with non-simplified subscripts?Show answer
Mg^2+ with O^2- gives 1:1, so MgO. Never Mg_2O_2.
What is calling ionic compounds "molecules"?Show answer
They are formula units, not molecules. NaCl in the solid is an extended lattice, not discrete pairs.
What is claiming solid ionic compounds conduct because they contain ions?Show answer
The ions must be free to move. In the solid lattice they are not.
What is forgetting the brittleness mechanism?Show answer
Brittleness is not just "ionic compounds are weak"; it is specifically the like-charge repulsion when layers shift.
What is using "ionic bond strength" loosely?Show answer
The relevant property is lattice energy, set by charge magnitudes and ionic radii. Quote both factors.