An ionic lattice in water
Predict firstIs NaCl a crystal made of many separate Na–Cl molecules packed together?
Understand hydration and dissolution in terms of the ions in the lattice.
Prerequisites: Ions carry a net charge; a water molecule is neutral overall, but its oxygen end is partly negative and its hydrogen ends partly positive.Look at the extended ionic structure
Predict firstIs NaCl a crystal made of many separate Na–Cl molecules packed together?
No. Na⁺ and Cl⁻ form an extended lattice, and the formula NaCl only gives the 1:1 ratio of numbers. Inside the bulk each ion has six nearest neighbors of opposite sign, while ions at the surface have fewer.
How to observe
- Cut the crystal open and count the six oppositely charged neighbours of an interior ion
- Show the polar orientation of the water molecules
- Follow an ion leaving the surface and forming a hydration shell along the illustrated path
Model notes
A finite fragment of the rock-salt NaCl structure. Na⁺ and Cl⁻ alternate on nearest-neighbour sites, with a nearest-neighbour distance of 0.282 nm; surface ions have lower coordination numbers. Hydration and ion separation are a teaching animation along a prescribed path, not a molecular dynamics, dissolution-rate or free-energy calculation; there is no NaCl molecule.