Electromagnetic radiationEnergy in the form of oscillating, mutually perpendicular electric and magnetic fields. The energy is quantized in units called photons. of sufficient energyA system's capacity to do work. to remove one or more electrons from a moleculeA set of atoms joined by covalent bonds and having no net charge. or atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. to form an ion.