STPM Chemistry Form 6 Notes – Terminology and Concepts: Liquid and Solid States (Part 3)

by BerryBerryTeacher

in Berry Reference (Notes)

This is Part 3 of the series of notes on “Liquid and Solid State” from Berry Berry Easy. In the previous part, the crystal systems were discussed in tabulated form. With that knowledge in hand, it is time to look at the packing of it. It is absolutely important that you draw the following structure at least once and try to have a 3D mental image of it. This action of visualising will help you to understand the information more easily. And when you understand, you don’t have to memorise it anymore, it will come natural to you. (Nonetheless, at least for the beginning, try to memorise them. Subsequent topics will require understanding from this topic, so do not skip this topic)

STPM Form 6 – Terminology and Concepts: Liquid and Solid States (Part 3)

Four types of lattice points:

  1. Lattice point at the corner of the unit cell (1/8)
  2. Lattice point on the edge of the unit cell (1/4)
  3. Lattice point on the face of the unit cell (1/2)
  4. Lattice point in the centre of the unit cell (1)

Coordination number – the number of atoms, molecules or ions (called the nearest neighbours) that surrounds a given atom, molecule or ion in a crystal lattice.

A) Simple cubic cell

Example: Caesium chloride & Polonium

  • Sphere touches six other spheres.
  • Four sphere in its own layer, one sphere above the layer and one sphere below the layer.
  • Coordination number = 6
  • Unit cell contains in total one atom (8 corners x 1/8 = 1)

B) Body-centre cubic lattice

Example: Sodium, Barium, Potassium, Iron, Manganese, Chromium & Vanadium

  • Sphere touches eight other spheres.
  • Second layer are placed in the hollows between the spheres in the first layer.
  • Each sphere atom is in contact with four atoms in the layer above and four atoms in the layer below.
  • Coordination number = 8
  • Unit cell contains in total of two lattice points per unit cell (8 corners x 1/8 + 1 = 2)

C) Close-packed structures

Example: Sodium chloride

  • Unit cell contains in total of four atoms per unit cell (8 corners x 1/8) + (6 faces x 1/2)

i) Cubic close packing (ABCABCABC) / Face-centered cubic / Simple cubic close packing

Thomas Harriot (1585) first pondered the mathematics of the cannonball arrangement or cannonball stack, which has a face-centered cubic lattice.

  • Sphere touches twelve other spheres.
  • First layer of spheres is packed as closely and each sphere atom is in contact with six other atoms.
  • Second layer of spheres is placed on top of the first layer, so that each sphere in the second layer rests on the hollows between the spheres in the first layer.
  • Each sphere atom is in contact with six atoms in its own layer, three spheres (atoms) in the layer above and three spheres (atoms) in the layer below.
  • Coordination number = 12

ii) Hexagonal close packing (ABABABABA)

  • Sphere touches twelve other spheres.
  • First layer and the second layer of spheres are packed in the same way as cubic close packing.
  • (Difference = the third layer of spheres is placed on top of the first layer)
  • Coordination number = 12

Stay tune to Berry Berry Easy for the next part (Part 4) in this series covering allotropy and enantiotropy.

Previous post:

Next post: