For the present purpose, the best model we have of the atom is that of a miniature solar system, with a nucleus or heavy part at the center and a cloud of electrons dashing around the nucleus in fixed orbits. The term “fixed orbits” is used advisedly. Our planet moves in a certain orbit around the sun. If we attached a large enough rocket to the earth we theoretically _could_ move it closer to or farther away from the sun. In the atom, we have learned, this cannot be done. An electron can only exist in one of a certain number of fixed orbits; different kinds of atoms have different numbers of orbits. We might think in terms of an elevator that can only stop at the various floors of an apartment building. Each upper floor is like an orbit of the electron. But you get nothing for nothing in the world of physics, and just as it takes energy to raise an elevator to a higher floor, it takes energy to move an electron to an outer orbit. Hence the atom is said to be raised to higher _energy_ levels when an electron is nudged to an outer orbit. The energy input can be of many different kinds. Examples are heat, pressure, electrical current, chemical energy, and various forms of electromagnetic radiation. If too much energy is put into the elevator it goes flying out the roof. If too much energy is put into the atom, one or more of its electrons will go flying out of the atom. This is called _ionization_, and the atom, now minus one of its negative electrons and therefore positively charged, is called a positive _ion_. But if the _right_ amount of energy is put into the atom, one of its electrons will merely be raised to a higher energy level. Shown in Figure 9, for instance, are the ground state (Circle No. 1) and two possible higher energy levels. As you can see there are three possible transitions. [Illustration: Figure 9 _Schematic representation of the electron orbits and energy levels of an atom. Each circle represents a separate possible orbit and each arrow a possible energy level difference._] The higher energy levels are abnormal, or excited, states, however, and the electron will shortly fall back to its normal (ground state) orbit (assuming some other electron has not fallen into it first). In order for the electron to do this (go back to its normal orbit), it must give off the energy it has acquired. This it does in the form of electromagnetic radiation.