Atomic Models

Even with the discovery of the proton, Rutherford's atomic model still did not explain how electrons could have stable orbits around the nucleus. The development of a mathematical constant by the German physicist Max Planck (1858–1947) served as the basis for the next atomic model. Planck developed his constant in 1900 when explaining how light was emitted from hot objects. He hypothesized that electromagnetic radiation could only be associated with specific amounts of energy, which he called quanta. The energy lost or gained by an atom must occur in a quantum, which can be thought of as a "packet" containing a minimum amount of energy. He described the relationship between a quantum of energy and the frequency of the radiation emitted mathematically by the equation E=λ (where E is the energy, in joules, of one quantum of radiation, and λ is the frequency of the radiation). The letter h symbolizes Planck's constant.

In 1905 Albert Einstein (1879-1955) developed a theory stating that light has a dual nature. Light acts not only as a wave, but also as a particle. Each particle of light has a quantum of energy associated with it and is called a photon. The energy of a photon can be expressed using Planck's equation. Einstein's hypothesis helped explain the light emitted when current is passed through a gas in a cathode ray tube. An atom that has the lowest potential energy possible is said to be in the ground state. When a current passes through a gas at low pressure, the potential energy of the atoms increase. An atom having a higher potential energy than its ground state it is said to be in an excited state. The excited state atom is unstable and will return to the ground state. When it does, it gives off the lost energy as electromagnetic radiation (a photon). When an electric current is passed through an elemental gas, a characteristic color of light is emitted. This light can be passed through a prism where it splits into various bands of light at specific wavelengths. These bands are known as the line-emission spectrum for that element. The line-emission spectrum for hydrogen was the first to be described mathematically. Scientists now faced the task of developing a model of the atom that could account for this mathematical relationship.