The History of Atomic Theory
Atomic Models This model of the atom may look familiar to you. This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels. A model uses familiar ideas to explain unfamiliar facts observed in nature. A model can be changed as new information is collected.
Who are these men? In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.
Democritus 400 BC Greek philosopher Asked if matter could be divided into smaller and smaller particles. Proposed that the atom was the smallest divisible part of matter
Democritus In 400 BC, thought an atom looked like a billiard ball →
Atomos He named the smallest piece of matter “atomos,” meaning “not to be cut.”
Crazy but true! This theory was ignored and forgotten for more than 2000 years!
Why? Aristotle and Plato favored a model of matter being earth, fire, air and water. They were wrong but well known and therefore their theory was accepted
Dalton’s Model It wasn’t until the Early 1800’s that Dalton added to Democritus’ model of the atom
Dalton’s Theory 1. all elements are composed of atoms. Atoms are indivisible and indestructible particles. 2. Atoms of the same element are exactly alike. 3. Atoms of different elements are different. 4. Compounds are formed by the joining of atoms of two or more elements.
Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson came up with a different model for the atom
Thomson Model Plum Pudding Model Electrons are stuck in the atom like raisins stuck in a cake
Thomson Model Cathode ray tube (CRT) experiments showed the existence of charged particles in atoms, which were later identified as electrons 4th
Thomson Model Where did they come from? This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?
Rutherford’s Gold Foil Experiment In 1908, the English physicist Ernest Rutherford
Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)
http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html
Rutherford All positive charges in nucleus All negative charges surround nucleus
Disproved plum pudding model. Atoms must have empty spaces He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole.
Bohr Model In 1913, the Danish scientist Niels Bohr Discovered energy levels for electrons. Called them orbits
Bohr Model
Wave Model
The Wave Model Electrons have wave-like properties They do not fill orbits, but are within an electron cloud. Schrodinger contributed to this concept.
The Wave Model It is impossible to determine exact location of an electron within an electron cloud
Electron Cloud: A space in which electrons are likely to be found. Electrons are locked in clouds depending on their energy Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.
Democritus X Dalton Thomson Rutherford Bohr Schrodinger Indivisible Electron Nucleus Orbit Electron Cloud Democritus X Dalton Thomson Rutherford Bohr Schrodinger
The Wave-Particle Model Modern theory accepts both Bohr and Heisenberg-Schrodinger models. In the case of the electron, it acts both as a particle and a wave.