Physics · Chapter 27
Study notes aligned to the official NEB syllabus.
Solids are held together by strong, fixed bonds between neighbouring atoms, which is why they keep both a definite shape and a definite volume (unlike liquids and gases). This chapter looks first at how atoms are arranged inside a solid, and then at how that arrangement decides whether the solid conducts electricity, resists it partially (semiconductor), or blocks it almost entirely (insulator).
Solids are broadly classified by how regularly their atoms are arranged.
A crystalline solid has its particles arranged in an ordered, repeating pattern that extends throughout the solid, whereas an amorphous solid has its particles arranged randomly with no long-range order, only short-range order at best.
The smallest repeating block of a crystal lattice, which when stacked in three dimensions in every direction reproduces the entire crystal, is called the unit cell. A unit cell is described by its edge lengths and the angles between them. Common lattice types encountered in introductory physics include simple cubic, body-centred cubic (BCC) and face-centred cubic (FCC) arrangements, which differ in how many atoms touch (coordination number) and how densely the atoms are packed.
In the simple cubic arrangement atoms sit only at the corners of the cube, in the body-centred cubic (BCC) arrangement there is an additional atom at the centre of the cube, and in the face-centred cubic (FCC) arrangement there is an additional atom at the centre of each face, so the three structures differ in coordination number and packing density.
The regular spacing of atoms in a crystal lattice is also what makes X-ray diffraction possible, since the lattice planes act like a three-dimensional diffraction grating for X-rays of comparable wavelength to the atomic spacing.