STATES OF MATTER
First consider the behavior of collections of atoms Matter exists in three states: In a solid atomic particles are bound to permanent positions relative to each other; in a liquid the particle bonds are weak and temporary; by contrast in a gas there is no significant bonding between atomic particles, and the particles have no permanent positions relative to each other. Most of the matter in the universe is in the form of gas. Frequently matter is in the form of plasma, a gas composed of free electrons and positive ions, which are atoms from which one or more electrons have been stripped. Such ionization is generally the result of very high temperatures, as we shall discuss in the next section.
The particles of a gas can be molecules (which consist of two or more atoms), atoms themselves, or ions and electrons. In the molecule the atoms may be of the same element, as the two atoms of the oxygen molecule we breathe, or of different elements, such as the two hydrogen atoms and one oxygen atom in the water molecule. In the plasma there is a variety of . possibilities. Each atom can be stripped of one electron-so that there are two independent particles per atom-or stripped of two electrons-three particles per atom-and so on. Only some of the atoms may lose an electron, or in the extreme case all atoms lose all their electrons.
In the atomic world (as in a gas) gravity is not the cause for changes in motion as it is in the macroscopic world. The atomic world is dominated by electromagnetic forces. As with the force of gravity the intensity of electric and magnetic fields weakens as the inverse square of the distance from their source. At first glance this suggests that Newtonian mechanics ought to describe motion in the atomic domain, gravity as a cause simply being replaced by electromagnetic forces. Such is not the case in general, and the mechanics of the atom is called quantum mechanics. Its details go beyond our needs in this book, so we only point out that motion in the atomic world has a discrete nature rather than the continuous characteristics of our everyday experience.
