2.3 Structure of Ionic Solids

1. AX is bonded only to the O²⁻ ion that took the two electrons from it
   A student who thinks an ion is bonded only to the ion it exchanged electrons with picks this. Once the ions exist, X attracts every nearby O²⁻ ion according to charge and distance, whichever ion gained its electrons.
2. BX and one O²⁻ ion form an MgO molecule that is apart from the other ions
   A student who thinks ionic solids are made of molecules picks this. X is equally close to several O²⁻ ions, and none of them is set apart with X as a separate unit; MgO is the ratio of ions in the array.
3. CX is attracted to each of the O²⁻ ions that surround it in the solid Correct
   The diagram shows X with O²⁻ ions on every side and no single partner. Each of those oppositely charged neighbors attracts X by Coulombic attraction; the crystal is a continuous array, not a collection of pairs.
4. DX is bonded to two O²⁻ ions, one for each unit of its 2+ charge
   A student who thinks an ion forms one bond per unit of charge picks this. The 2+ charge does not limit X to two neighbors; it makes X's attraction to each surrounding O²⁻ ion stronger.

CED 2.3.A.1 · Read this in Fix

1. ANa⁺ and Cl⁻ ions alternate in a regular pattern that repeats in three dimensions Correct
   A regular, repeating three-dimensional array of ions produces flat planes of ions throughout the crystal, so the crystal grows and breaks along flat faces. The alternation of Na⁺ and Cl⁻ maximizes attractions and keeps like charges apart.
2. BEach Na⁺ ion and each Cl⁻ ion is itself shaped like a tiny crystal with flat faces
   A student who gives particles the properties of the bulk substance picks this. Ions are not flat-faced; the shape of the crystal comes from the regular arrangement of the ions.
3. CNaCl molecules, each one Na⁺ and one Cl⁻, are stacked side by side in neat rows
   A student who thinks ionic solids are made of molecules picks this. Solid NaCl has no NaCl molecules: each ion is surrounded by ions of the opposite charge in a continuous array.
4. DEach Na⁺ shares an electron pair with one Cl⁻, which holds the ions at fixed angles
   A student who pictures ionic bonds as shared electron pairs picks this. The ions in NaCl are held by Coulombic attractions between whole ions, which act in all directions; there are no shared pairs.

CED 2.3.A.1 · Read this in Fix

1. ADiagram 1
   A student who thinks ions are the same size as their atoms picks this, because a Li atom is larger than an F atom. The arrangement alternates correctly, but Li⁺ has lost its only valence shell and is much smaller than F⁻, which has gained an electron.
2. BDiagram 2
   A student who thinks ionic solids are made of molecules picks this. Solid LiF is a continuous array in which each ion is surrounded by ions of the opposite charge, not separate Li⁺F⁻ pairs.
3. CDiagram 3
   A student who considers only attractions picks this, because each Li⁺ still touches F⁻ ions on both sides. But each Li⁺ also has Li⁺ ions directly above and below it, so the like-charge repulsions are not minimized.
4. DDiagram 4 Correct
   Diagram 4 places every Li⁺ next to F⁻ ions and keeps like charges apart, which maximizes attractions and minimizes repulsions. It also shows Li⁺ (about 76 pm) as smaller than F⁻ (about 133 pm).

CED 2.3.A.1 · Read this in Fix

1. A0.00
   A student who thinks only the attractions between oppositely charged ions matter in an ionic solid picks this, treating the force between two M⁺ ions as negligible. Every pair of ions interacts by Coulomb's law; two M⁺ ions 0.42 nm apart repel with about half the force with which M⁺ and Z⁻ attract at 0.30 nm.
2. B0.51 Correct
   Both pairs involve charges of magnitude 1, so the ratio F(M⁺–M⁺)/F(M⁺–Z⁻) depends only on the distances: (0.30/0.42)² = 0.51. Keeping like-charged ions farther apart than oppositely charged neighbors makes each repulsion about half as strong as each attraction.
3. C0.71
   A student who treats the force as inversely proportional to the distance, not its square, picks this: 0.30/0.42 = 0.71. The distance enters Coulomb's law as r², so the ratio is 0.71² = 0.51.
4. D1.00
   A student who thinks the force depends only on the sizes of the charges picks this. The charges have equal magnitudes, but the M⁺ ions are farther apart, so their repulsion is weaker than the attraction between neighboring M⁺ and Z⁻ ions.

Working F ∝ q₁q₂/r². Both interactions involve charges of magnitude 1, so F(M⁺–M⁺)/F(M⁺–Z⁻) = (0.30 nm/0.42 nm)² = (0.714)² = 0.51.

CED 2.3.A.1 · Read this in Fix

1. ANa⁺ and Cl⁻ pair off into molecules, as each Na atom gave its electron to one Cl atom
   A student who thinks an ion is bonded only to the ion it exchanged electrons with picks this. In the solid each Na⁺ attracts all the Cl⁻ ions around it, whichever Cl atom received its electron.
2. BEach Na⁺ is surrounded by Cl⁻ ions, as only attractions decide where the ions sit
   A student who considers only the attractions between opposite charges picks this. The arrangement is right, but the reason is not: repulsions between like-charged ions also act, and the arrangement keeps them as small as possible.
3. CNa⁺ and Cl⁻ pair off into molecules, as the formula NaCl shows one of each ion
   A student who reads the formula of an ionic compound as a molecule picks this. NaCl gives the 1:1 ratio of ions in a continuous array; no Cl⁻ ion is paired with one Na⁺ ion.
4. DEach Na⁺ is surrounded by Cl⁻ ions, as this maximizes attraction and minimizes repulsion Correct
   Coulomb's law applies to every pair of ions. Surrounding each Na⁺ with Cl⁻ ions (and each Cl⁻ with Na⁺ ions) places opposite charges close together and keeps like charges farther apart, which maximizes the attractions and minimizes the repulsions.

CED 2.3.A.1 · Read this in Fix