A solid is made of a combination of atoms. Each atom is composed of a positively charged nucleus and several electrons, the wavefunctions of which are around the nucleus. In most cases, the atoms in solids are well organized, like oranges in a crate or a Rubik’s cube. Their periodic arrangement is the subject of crystallography. In order to imagine the sizes and numbers involved, a one cubic centimetre copper cube contains 10 to the power of 23 atoms (i.e. a hundred thousand billion billions atoms), and each atom is separated from its neighbours by 2 ten billionths of a metre.
The weight of a solid is linked to its nuclei, much heavier than the electrons. However, most other properties concerning matter are linked to electrons: their nature, their number and the way they move or do not move in a solid. Because the electron is a quantum object, the properties of the solid are generally linked to the quantum physics of the electrons composing it as in the case of resistance in a metal. Understanding these properties has been one of the successes of quantum physics in the 20th century.
Solid-state physicists have indeed managed to apply quantum laws to the billions and billions of atoms and electrons in order to deduce the physical laws ruling matter. This intimate understanding of what a solid is has aided in the discovery of, among other things, the transistor effect that paved the way for modern electronics and computer science. This solid-state and condensed matter physics is today one of the most studied subjects in laboratories, because some questions have not yet been answered and because there are many technological applications.
