Crystal Structures

Unit 6

Chapter 12

Madame Vorba says…

Crystals are highly structured arrangements of atoms in a lattice.

Crystals allow atoms/molecules to stick together with the least amount of energy.

The arrangement of atoms within a crystal repeats in 3-dimensions.

Particles arrange to pack in the most energetically favorable way.

They will pack to fill the most space.

I Just Wanna Get Close To You…

As Grocers have known for centuries… There are really only two ways to pack identical

spheres like oranges: Hexagonal Close Packing

AB (ABABABABAB) Cubic Close Packing

ABC (ABCABCABC)

There is always some amount of space that

remains empty (because atoms are spherical!)

Sneaky Little Buggers…

When crystals form between different sized atoms/molecules,

The smaller atoms can insert in between the larger atoms.

This is called an interstitial arrangement because the smaller atoms are in the interstices of the larger atoms.

Face It!

Ionic Crystals form from alternating Positive and Negative ions.

Usually, the anion makes up the bulk of the crystal

But didn’t you say…

The cation is always smaller than the parent atom.

The anion is always bigger than the parent atom.

Because of this, most common crystals have larger anions.

Who’s on First? Yes!

Since there are no physical bonds in ionic crystals, it is impossible to decide which atom is bonded to which.

There is no such thing as a molecule of salt! Instead, we say there is a formula unit of the

salt. The formula unit is the reduced, whole-

number ratio of elements in the crystal. (In practice, everybody says molecule.)

Growing up Geometric

Crystals tend to have very specific geometric patterns.

This arises because atoms attach in a specific manner.

Once a face is created; however, it is difficult for new atoms to attach.

Awww…SNAP!

Since ionic crystals are highly organized with alternating positive and negative,

If you try to bend an ionic crystal, the charges move out of alignment and similar charges line up with each other and…

SNAP! Crystals will shatter –

not bend.

It’s Like Facebook…

When covalent compounds form crystals, two types can occur:

Network Solid Atoms are covalently bonded in a 3-D crystal. Useful to think of them as one big molecule.

Molecular Crystal Individual molecules “sit” on top of each other. They arrange themselves into orderly patterns. Held together by intermolecular forces.

Compare the Two

Network Solid(Diamond)

Molecular Crystal(Ice)

Type Cast…or Not

Since atoms can arrange themselves into various packing configurations, different crystals can be made out of the same material:

QuartzSynthetic SiO2

Bendy-Bendy-SNAP!

Most simple covalent compounds form rigid crystals that will not bend much.

But, more complex molecules have some wiggle room.

If the molecule can twist, bend, or unwind, then the crystal can bend or stretch.

The more complex the molecule, the more likely the crystal can deform without shattering.

Up to a point.

Help! I’m Drowning!

Metals form crystals, too. Since their electrons are delocalized, the

crystal consists of positively charged metal atoms (not considered ions)

Surrounded by a sea of electrons.

It’s Almost Psychedelic…

Metals can form neat crystals.

Or, they can form polycrystalline structures.

Polycrystalline metals have competing crystals forming within the metal.

Usually most pronounced in alloys

I Love Shiny Things!

Metals are shiny because they have so many free-flowing electrons.

The energy levels are so close together that they can absorb (almost) all light.

They immediately fall back down and reemit the light with (almost) exactly the same wavelength.

The electron sea absorbs and reemits the light hitting it – making a mirror.

Bendy-Bendy-Bendy

Metals can bend (unlike ionic crystals) because the electron sea acts like a lubricant.

The metal atoms never come in close contact with each other,

So they never really repel each other like ionic crystals do.

There are exceptions (of course). Some metals are much more brittle than

others and will fracture.

I can see you…

A glass is not a crystal. Glass is an amorphous

arrangement of atoms. There is no apparent

order. Glass can be covalent

or metallic (not ionic) They can be pure

substances or even mixtures.