Wave Interference

When two pulses traveling on a string meet each other, the amplitudes of the pulses are added together to produce the shape of the resulting pulse. If the pulses are identical but travel on opposite sides of the string, then the sum of the amplitudes is zero and the string will appear flat for one instant (A). This is called destructive interference. When the two identical pulses travel on the same side of the string, then the sum of the amplitudes is double the amplitude of a single pulse when the pulses are together (B). This is called constructive interference.

 

Models of the Atom

Experimental data has been the impetus behind the creation and dismissal of physical models of the atom. Rutherford's model, in which electrons move around a tightly packed, positively charged nucleus, successfully explained the results of scattering experiments, but was unable to explain discrete atomic emission—that is, why atoms emit only certain wavelengths of light. Bohr began with Rutherford's model, but then postulated further that electrons can only move in certain quantized orbits; this model was able to explain certain qualities of discrete emission for hydrogen, but failed completely for other elements. Schrödinger's model, in which electrons are described not by the paths they take but by the regions where they are most likely to be found, can explain certain qualities of emission spectra for all elements; however, further refinements of the model, made throughout the 20th century, have been needed to explain all observable spectral phenomenon.

 

Electron Density and Orbital Shapes

Atomic orbitals are mathematical descriptions of where the electrons in an atom (or molecule) are most likely to be found. These descriptions are obtained by solving an equation known as the Schrödinger equation, which expresses our knowledge of the atomic world. As the angular momentum and energy of an electron increases, it tends to reside in differently shaped orbitals. The orbitals corresponding to the three lowest energy states are s, p, and d, respectively. The illustration shows the spatial distribution of electrons within these orbitals. The fundamental nature of electrons prevents more than two from ever being in the same orbital. The overall distribution of electrons in an atom is the sum of many such pictures. This description has been confirmed by many experiments in chemistry and physics, including an actual picture of a p-orbital made by a Scanning Tunneling Microscope.

 

Light Absorption and Emission

When a photon, or packet of light energy, is absorbed by an atom, the atom gains the energy of the photon, and one of the atom's electrons may jump to a higher energy level. The atom is then said to be excited. When an electron of an excited atom falls to a lower energy level, the atom may emit the electron's excess energy in the form of a photon. The energy levels, or orbitals, of the atoms shown here have been greatly simplified to illustrate these absorption and emission processes.

 

Spectral Lines of Atomic Hydrogen

When an electron makes a transition from one energy level to another, the electron emits a photon with a particular energy. These photons are then observed as emission lines using a spectroscope. The Lyman series involves transitions to the lowest or ground state energy level. Transitions to the second energy level are called the Balmer series. These transitions involve frequencies in the visible part of the spectrum. In this frequency range each transition is characterized by a different color.

 

Characteristic Spectra

Every chemical element has a characteristic spectrum, or particular distribution of electromagnetic radiation. Because of these “signature” wavelength patterns, it is possible to identify the constituents of an unknown substance by analyzing its spectrum; this technique is called spectroscopy. Emission spectrums, such as the representative examples shown here, appear as several lines of specific wavelength separated by absolute darkness. The lines are indicative of molecular structure, occurring where atoms make transitions between states of definite energy.