Mark Planck, German physicist. Energy carried by radiation could only be whole number of energy units.

1885 Johann Balmer found a straightforward formula that contained all the known freq of the spectral line of H2: each frequency could be written as a fundamental frequency mutiplied by the difference between the inverse squares of two whole numbers.

Ernest Rutherfold. Most of alpha particles sailed through the gold foils, but a small proportion bounced back toward the source.. The alpha particle must have run into something more soild and massive than itself. The explanation was not hard to find. The light, negatively charged particlesknown as electrons had been recently discovered and were known to be a componentof atoms. Rutherfold concluded that atoms must contain a dense, hard core-the atomic nucleus-about which the electrons orbited like the planet around the Sun.

Niel Bohr, Dane, becamed the first great innovator and interpreter of what soon be called __ quantum mechanics__. He addedthe condition that the electronorbits were quantized-that is, the electrons could circle around the nucleus not at any old distance but only at certain fixed radii. He then proposed that the emmission and absorption of energy by the atom was due to electrons jumping from one orbit to another. If a unit of electromagnetic radiation of just the right frequency struck the atom, it would knock an electron from one orbit to the next; this is why atoms absorb radiation only at certain energies. Similarly, if an electron moved down the ladder from one orbit to the next, it would send out a unit of electromagenetic radiation of a spewcific freq, this is why atoms emit radiation only at certain energies.

1923, Prince Louis de Broglie: The energy of eachorbit was known, but since the electron was not a wave but a little tiny planet barreling around the the atomic nucleus, there was no obvious meaning to be attached to an electron frequency. Undaunted, de Broglie figured out what the freq and wavelength would have to be, and in so doing came across a remarkable result: for the first Bohr orbitthe wavelenght of the electron was exactly equal to the circumference of the orbit.

American physicists C.J. Davisson and L.H. Germer conducted careful experiments in which a beam of electron was bounced off a crystal. They interferenced by varying the angle of bombardment. Electron could behave as if they were waves, not particle. At about hte same time , de Broglie’s hypothesis was set on surer theoretical gorund with the publication by the nAustrian physicist Erwin Schrodinger of the equation that bears his name. It is a wave equation, or the equation that describe the vibration of a drumskin, it gives a mathematical basis for calculating undulation in a field according to the force applied to it. When a drumskin is struck, it vibrates in a complex way, but mathematical analysis of the relevant wave equation shows that any vibration, no matter how complex, can be broken down into a mixture od simpler solutsion, which are called the fundamental modes. The simplest mode is one in which the centre of the drum rises and falls.