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Crystal Structures

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.

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Crystal Structures

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  1. Crystal Structures Unit 6 Chapter 12

  2. 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.

  3. 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!)

  4. 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.

  5. Face It! • Ionic Crystals form from alternating Positive and Negative ions. • Usually, the anion makes up the bulk of the crystal

  6. 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.

  7. 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.)

  8. 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.

  9. 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.

  10. 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.

  11. Compare the Two Network Solid (Diamond) Molecular Crystal (Ice)

  12. Type Cast…or Not • Since atoms can arrange themselves into various packing configurations, different crystals can be made out of the same material: Synthetic SiO2 Quartz

  13. 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.

  14. 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.

  15. 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

  16. 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.

  17. 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.

  18. 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.

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