Atomic Spectra
From MyMCAT
Contents |
Introduction
Photons determine what colour we see, but how are specific colours given off from lights, neon lights, and the sun? Understanding the basic structure of the atom, and how energy can be absorbed or emitted or produce photons of uniquer colours will be addressed.
The Energy of an Atom
In general, the electrons of an atom determine the atom's energy. Electron's in higher orbits (further away from the nucleus, 4p, 3d, 5s, etc) have a higher energy than electrons closer to the center (1s, 2s, 2p, etc). If we refer to these electrons being at specific energy levels, we can begin to determine what happens when electrons drop to lower levels, or go to higher levels, and what these changes do to the atom. Orbits and energy levels are often used interchangeably, but it is important to realize that the classical view of an electron spinning around an atom is not true- electrons aren't really in a circular orbit around an atom, and thus energy levels are a less confusing concept.
Changes in Energy Level
If an atom loses energy because an electrons drops to a lower energy level, the energy doesn't simply dissapear, it has to go somewhere, and in the case of an atom, it turns into electromagnetic radiation. If the energy change is just right, it will result in a photon being given off (in a colour we can see!). Similarly, if an atom gains energy, the energy needs to have come from somewhere, and it is most often because a photon (with a specific energy) hit the atom.
When an electron drops orbit (goes to a lower energy level), the amount of energy released depends on how many levels the electron dropped. This energy released then turns into a photon which is emitted. The more energy that was released, the more energy the photon has and thus the higher the frequency. When this frequency occurs within the visible spectrum, we can actually see the photons being emitted.
The exact same principle holds of photons hitting an atom, the amount of energy in the photon is transfered into bumping electrons in the atom up to higher energy levels.
Atomic Line Spectra
If we keep in mind that atomic energy is quantized, that is, only exists at specific values and is not a continuous scale, then it is clear that atoms must only be able to give off and absorb specific values of energy. As a result of this, each atom (with its unique nucleus) has a specific set of frequencies of energy that it can absorb and emit. Hydrogen for example, has a specific set of frequencies we it can emit when energized, Lithium another, and copper yet another.
