Chemistry · Ch 6 — Solid State
Crystal Lattice and Unit Cell
Crystal Lattice and Unit Cell
A crystalline solid is characterised by a definite orientation of its atoms, ions or molecules relative to one another, extending as a regular three-dimensional pattern throughout the whole solid. This regular, repeating arrangement of particle positions is called the crystal lattice, and the smallest structural block that repeats to build up the entire lattice is called the unit cell.
A crystal can be thought of as a huge number of unit cells, each in direct contact with its nearest neighbours, all identically oriented in space and stacked without gaps. The number of nearest-neighbour particles surrounding any one particle in the crystal is called that particle's coordination number -- a quantity that recurs throughout the rest of this unit as a way of characterising how tightly a given arrangement is packed. …
What this figure shows. A 3D grid of pink spheres (lattice points) joined by grey construction lines into a large cube built from many identical smaller cubes; one small cube near the front-left is shaded pale blue and outlined to mark it out as 'the unit cell' among the full array of 'lattice points', with a legend at the foot of the figure keying the blue-shaded cube as Unit cell and the pink spheres as L …
What this figure shows. A single 3D cube (pink spheres at each of its 8 corners, joined by construction lines) with its three edges explicitly labelled a (front-bottom), b (right-bottom) and c (vertical), and the three inter-edge angles marked at the front-bottom-left vertex: alpha (between b and c), beta (between a and c) and gamma (between a and b) -- captioned 'Unit cell'. This is the general-case picture behind the seven crystal systems of Section 6.5: a cubic cell is simply the special case where all three edges a, b, c come out equal and all three angles alpha, beta, gamma come out equal to 90 degrees, while every other crystal …