450 B.C.

DEMOCRITUS, a Greek philosopher, proposed that all matter is made up of particles called atoms, meaning indivisible.



CHRISTIAN HUYGENS, postulated that light is a wave which moves and acts like waves in water.



SIR ISAAC NEWTON stated that "matter is formed of solid, massy impenetrable particles", of some definite size which combine in various ways to produce substance.



SIR ISAAC NEWTON developed the "corpuscular theory of light." Light is thought to be the result of "luminous corpuscles" or particles which produce the waves we see as light.



CLERK MAXWELL developed a series of equations expressing the relationship between electric and magnetic forces.



CLERK MAXWELL stated "we have strong reason to conclude that light itself is an electromagnetic disturbance."



HEINRICH HERTZ discovered the photoelectric effect. If a beam of light falls on a clean metal plate in a vacuum, the plate becomes positively charged.



SIR JOSEPH THOMPSON proved the existence of a negatively charged particle, termed the electron, which existed as part of the atom.



MAX PLANCK developed the basis of modern Quantum Theory by finding that light is emitted or absorbed by an atom in discrete amounts called quanta.



ALBERT EINSTEIN in his explanation of the photoelectric effect proposed that light must have both the properties of particles as well as those of waves.



LORD ERNEST RUTHERFORD discovered that the atom's nucleus is very small in relation to the entire atom. He proposed that the negatively charged electrons were revolving around a heavier, charged nucleus.



NIELS BOHR synthesized Rutherford's discovery into a reasonable model of an actual atom, using hydrogen as his example. Bohr proposed a positively charged central nucleus with electrons moving about it in circular orbits. The important feature in Bohr's theory was that electron orbits could occur only in specific, predetermined paths. If an electron absorbs energy, it is moved to an orbit further from the nucleus. Conversely, when it drops to an orbit nearer the nucleus, it gives off energy in the form of light. Different colors of light are produced depending on which orbit the electron starts from and to which orbit it drops.



ARNOLD SOMMERFELD proposed elliptical orbits in addition to Bohr's circular ones. Sommerfeld's ellipses altered Bohr's model by showing electrons moving inwardly and outwardly without radiating or absorbing energy.



LOUIS DE BROGLIE proposed that all objects have properties of waves. The lighter the object, the more pronounced the wave effect. An object as small as the electron would act very much like a wave, forming stationary waves around the nucleus.



WOLFGANG PAULI developed the Pauli Exclusion Principle which states that no two electrons within the same atom can have the same set of quantum numbers.



UHLENBECK & GOUDSMIT showed that the electron possesses a spin in either direction upon its axis.



ERWIN SCHROEDINGER developed an equation, based on de Broglie's wave idea, expressing the probable location of an electron. These probable regions of occupancy were conceived as clouds of charge around the nucleus. Different shapes occurred for different types of orbitals.



WERNER HEISENBERG derived his "Uncertainty Principle" which states that it is impossible to determine simultaneously the momentum and position of an electron.



LINUS PAULING showed how 2 electrons could form a more stable wave arrangement if their spins were antiparallel.



At the center of the atom is a small, dense positively charged nucleus consisting primarily of protons and neutrons.


Moving around the nucleus are negatively charged electrons which account for only 1/5000 of the atom's mass -- the rest of the mass being in the nucleus. Most of the atom is empty space. The motion of the electrons is not described.


The electrons in an atom are allowed to have only certain energies. The allowed states are described by a set of "quantum numbers", which indicate their average distance from the nucleus, their angular momentum and its direction, and the electrons' spin direction.


Light of a specific color is emitted or absorbed when electrons change from one state to another.


The "Heisenberg Uncertainty Principle" states that the position and momentum of an electron cannot be simultaneously determined. The interpretation of the Heisenberg principle is that the atom's structure and the interactions of its electrons are random and can be discussed only statistically.


Even though the electron's exact position cannot be determined, if its energy is known, the theory predicts the probability that an electron could be at a particular place.


If the probability location of an electron of known energy is plotted in space, the plot looks like a fuzzy cloud of varying density, the shape varying with differences in angular momentum. It always has a definite symmetry about the nucleus. Some of the clouds or orbitals are spherical, others are like dumbbells, while others are more complex.


In describing an atom with many electrons, the charge clouds of one shell are superimposed in space with those of other shells.

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