-
340 BCE
Aristotle
Aristotle believed that there were 5 elements. Air was light, earth was cool and heavy, water was wet, fire was hot and Aether, which he viewed as a divine substance,was made up of stars and planets. He believed that all matter was made up one or more of the elements with the exception of Aether. -
500
Heraclitus & Democritus
Prior to Democritus, Heraclitus believed that fire was the driving force behind atomic structure. Democritus believed that everything, including people, were made up of tiny invisible particles. -
John Dalton
Dalton's Atomic Theory has 5 principles.
1. All matter is made up of atoms.
2. All atoms of a given element element are identical in size, shape and mass. Other elements may differ in size, mass and other properties,
3. Atoms can not be created or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated or rearranged. -
J.J Thomson
Thomson began working with cathode ray tubes. Cathode ray tubes are sealed glass tubes where most of the air has been expelled. High voltage is applied across 2 electrodes at one end of the tube which causes a beam of particles to flow from the cathode to the anode. To test his theory Thomson placed 2 oppositely charged electrodes around the cathode. The cathode ray was deflected away from the negatively charged electrode towards the positive, indicating the cathode ray was negatively charged. -
Plum Pudding Model
Thomson believed that atoms are uniform spheres of which negative electrons embedded within a positive "soup". -
Gold Foil Experiment
Rutherford performed the Gold Foil Experiment which was a series of experiments where a radioactive source able to emit alpha particles was enclosed in a lead shield. Radiation was focused into a narrow beam. There was a slit in the lead screen and a piece of gold foil was placed in front of the slit. The screen was coated with Zinc Sulfide so it was fluorescent and served as a way to detect alpha particles. There was also a microscope attached to the back of the screen for viewing the reaction. -
Gold Foil Experiment Findings
As each alpha particle struck the screen it would produce a burst of light whcih was visible through the microscope. Most alpha particles passed though the foil which implied that atoms have large amounts of open space. Some particles were deflected suggesting they were reacting to other positive particles. -
Rutherford Model
In 1911 Ernest Rutherford proposed his model of the atom, also called the planetary model. He proposed the atom was tiny and dense with a positively charged core, called a nucleus. He also stated that light negative constituents, electrons, circulate at the same distance. Like planets around the sun. -
Bohr Model
The Bohr model shows that the electrons in atoms are in orbits of differing energy around the nucleus (think of planets orbiting around the sun). ... The energy level an electron normally occupies is called its ground state. -
Bohr
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. -
Schrodinger
Schrodinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. -
Chadwick
Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. -
