Rivier College
For most "everyday" considerations an atom is composed of positive protons,neutral neutrons, and negative electrons. The protons and neutrons are aboutthe same mass and reside together in the nucleus of the atom. The electronsare much smaller (a proton is about 2000 times the mass of an electron) andreside outside of the nucleus. The position of the electron can only bedescribed in terms of probabilities. Using quantum mechanics the probabilityof finding an electron within a particular region of space can be calculated,but the exact position is not known. The electrons do NOT orbit aroundthe nucleus like planets around the sun.
All of the atoms of a particular element have the same number of protons, butcould differ in the number of neutrons or the number of electrons. If thenumber of electrons changes, the atom becomes a charged particle and is calledan ion.
Each atom has a set of energy levels associated with it. All of the atoms of aparticular element have the same set of energy levels, but every element has aunique set of energy levels associated with its atoms. Knowing the energylevels identifies the element.
Each electron in an atom is associated with a particular energy level of theatom. The electrons in the higher energy levels can often become excited bylight, heat, electricity, etc. and "jump" to an even higher energy level that isavailable in the atom. The excited electron will eventually "fall" back downto the lowest available energy. When it returns to the lower energy levelenergy is given off. If the energy given off is in the visible region, it canbe observed with the human eye.
An amazing thing about electrons is that they can only accept the exact amountof energy needed to get to a higher energy level. Less energy or more energydoes not cause the electron to "move" out of its present energy level. This isan example of selective absorption of energy. The atom only absorbs the energyassociated with a change in energy levels. In the same way the atom only givesoff energy associated with a change in energy levels. Since the energy levelsare characteristic of an element, the energies absorbed and given off are alsocharacteristic of an element. Each element has its own characteristic linespectrum.
Some molecules also selectively absorb light energy. When they absorb in thevisible energy range, we see the complement of thecolor that is absorbed. That is why leaves are green. The chlorophyll in theleaf absorbs the compliment of green and we see green. The blue color of wateris also due to selective absorption of light by the water molecules. This is much different from the scattering of light which is responsible for the blue sky.
Quantum mechanics predicts selective absorption of energy by atoms andmolecules. The discrete nature of energy levels is required in quantummechanics.
Light can be described in terms of energy (E), wavelength (lambda), or frequency(nu). These are related in the following way:
