Solids

12.1

Crystal Structure:

• Materials on Earth are crystals.

• Crystals are arrays of atoms.

• X-Rays confirm that crystals are three dimensional arrangements of atoms.

• The atoms are very close together.

  • They’re the same distance apart as X-ray wavelengths.

• A beam of X-rays directed on a crystal gets diffracted into a characteristic pattern.

• This pattern can be made visible on a photographic film.

  • The pattern is a diffraction pattern.
  • The film shows that crystals are mosaics of atoms located in regular lattices.

• Iron, copper, gold → simple crystal structures

• Tin, cobalt → slightly more complex structures

• All metals have many crystals.

  • Each crystal has the same regular lattice.
  • Each is slightly inclined to the crystal nearby.

• When metal surface is etched, the crystals can be seen.

Etched → cleaned with acid

• Crystal structures can be seen:
  • On the surface of galvanized iron
  • On brass doorknobs that get etched with perspiration from hands
• There’s a formula that shows how X-rays should scatter from the atomic layers in a crystal.
• This formula combines with the analysis of the pattern of spots in a diffraction pattern to find distances between atoms in a crystal.

X-ray diffraction is vital in biological and physical sciences.

• Non-crystalline solids are amorphous.
  • Amorphous → atoms and molecules are distributed randomly.
  • Particles can wander.
  • Eg: Rubber, glass, plastic
• In all atoms, atoms and ions vibrate in their positions.
• Atoms are tied together by electrical bonding forces.
  • Called atomic bonding.
  • Types of bonding in solids:
  • ionic
  • covalent
  • metallic
  • van der Waals (the weakest)
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