Early Atomic Theory How the atom came to be

Early Atomic Theory The history of atomic theory explains how scientists came to understand the structure of the atom.   This includes the existence, mass, behavior, and location of protons, neutrons, and electrons.

Democritus 460 – 370 BC All matter is made up of various imperishable, indivisible elements which he called atomos or "indivisible units", from which we get the English word atom. The idea was lost for centuries until scientific interest was rekindled during the Renaissance Period.

John Dalton 1766-1844 used the concept of atoms to explain why elements always reacted in simple proportions He proposed that each element consists of atoms of a single, unique type, and that these atoms could join to each other, to form compound chemicals.

Dalton’s Atomic Theory All elements are composed of atoms. Atoms are indivisible and indestructible. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by the joining of atoms of two or more elements.

Right and Not So Right of Daltons Theory All elements are composed of atoms. Atoms of different elements are different. Compounds are formed by the joining of atoms of two or more elements. Not So Right Atoms are indivisible and indestructible. Atomic Decay Atomic Particles Atoms of the same element are exactly alike. Isotopes Ions

J.J. Thomson 1856-1940 Thomson is credited for the discovery of the electron, of isotopes, and the invention of the mass spectrometer. Was the first to say that the atom is made of even smaller particles referred to as sub-atomic particles. Discovery of electrons made by the cathode tube ray experiments.

CATHODE RAY EXPERIMENT A cathode ray is a sealed glass cylindrical tube in which two electrodes are separated by a vacuum Thomson discovered that the negative charge could not be separated from the rays (by the application of magnetism), and that the rays could be deflected by an electric field. He concluded that these rays, rather than being waves, were composed of negatively charged particles he called "corpuscles“. Later these ‘corpuscles’ would be named electrons. Was unable to find positively charged particles but figured they were there.

Plum Pudding Model Thomson proposed atomic model which had both negatively and positively charged particles randomly scattered around the atom. Referred to as the ‘Plum Pudding’ model because the way the particles were scattered like plums in pudding. Thomson was correct about positively and negatively charged particles, but his arrangements of these particles were not.

Ernest Rutherford 1871-1937 Performed a gold foil experiment to show that positive charges are located in the center of the atom. Discovered the nucleus and positively charged particles. Neutrons were later discovered by James Chadwick in 1932.

Gold Foil Experiment Plum Pudding Model Alpha Particle Nucleus Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge. Plum Pudding Model Alpha Particle Gold Foil Experiment Nucleus Rutherford’s Atomic Model

Niels Bohr 1885-1962 Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in 1922. Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus

Bohr Model In the Bohr Model the neutrons and protons (symbolized by red and green balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). Electrons in clouds or rings.