Early Atomic Theory

1. Early Atomic Theory How the atom came to be

2. 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.

3. 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.

4. 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.

5. 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.

6. Right and Not So Right of Daltons Theory • Right • 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

7. 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.

8. 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.

9. 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.

10. 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.

11. 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

12. Niels Bohr 1885-1962 • Danishphysicist 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

13. 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.