The Bohr Theory of the Hydrogen Atom

An atom consists of a nucleus with most of the mass and with a positive charge, surrounded by electrons which are negatively charged.  Classical physics requires that the electrons be in motion around the nucleus, in order to avoid being drawn into it because of the attractive force of opposite charges.  But classical physics would also require that the moving electron continually emit  energy in the form of light.  If this really happened the electron would spiral into the nucleus.  So classical physics fails to explain atomic structure, just as it failed to explain light and the photoelectric effect.

In 1913, Niels Bohr, who at the time was working with Rutherford, combined ideas from classical physics and the new quantum theory to explain the structure of the hydrogen atom.  Bohr made the assumption that certain properties of the electron in a hydrogen atom, in particular its energy can have only certain specified values, in other words the energy is quantized.  He derived an equation for the electron energy E_n.  Each value E₁, E₂,... is called an energy level of the atom, and is only allowed to have the values

where n is an integer, and B is a constant Plank's constant, and the mass and charge of the electron, B = 2.179 x 10^-18 J.  

The Bohr model of the hydrogen atom pictured the electron as orbiting about the nucleus as planets orbit the sun.  Different energy levels correspond to different orbits, and only a discrete set of energy levels or orbits is possible.

The emission spectrum of hydrogen consists of several series of lines.  One of these series is in the visible portion of the spectrum, another in the ultraviolet, and another in the infrared.  The electron in the hydrogen atom is usually in the lowest energy level.  At any given instant, most atoms will be in this state, which is called the ground state.  When hydrogen atoms are subjected to an electric discharge, a flame or some other source, some of the electrons absorb energy and are promoted to a higher energy level. Atoms with electrons in energy levels higher than the ground state are said to be in an excited state.  An atom in an excited state is very short lived, and the the electron drops to a lower energy level and ultimately reaches the ground state.  When the electron drops to a lower energy level from an excited state, it releases energy in the form of light and this is the origin of line spectra.