atom

The smallest part of an element of matter that retains the basic characteristics of the element. An atom consists of a tightly packed nucleus composed of protons and neutrons surrounded by a "cloud" of electrons that arrange themselves in an orderly pattern according to the rules of quantum mechanics. Atoms with equal numbers of protons and electrons are electrically neutral; ions are simply atoms that contain more or fewer electrons than protons. Atoms are very small -- much smaller than the wavelength of visible light. The simplest and most abundant atom in the universe is hydrogen, which contains one proton and one electron.