Atomic Theory “History of. . . ”

The Ancient Greeks Democritus and other Ancient Greeks were the first to describe the atom around 400 B.C. The atom was nature’s basic particle that makes up all matter. Since there was no scientific evidence, many others believed Aristotle’s ideas instead, since he was more popular.

The Ancient Greeks The Greek philosopher Aristotle did not believe in atoms, and instead believed all matter was continuous. He thought the elements were Earth, Fire, Wind and Water – this is not correct. Aristotle’s belief was accepted for ~2000 years, as there was no evidence to support either theory.

What changed? In the 1700’s scientists' began to seriously question whether Aristotle was correct. Bernoulli (1738), Dalton (1808) and Faraday (1834) began studies which proved him wrong

John Dalton’s Atomic Theory (1808) He proposed a simple model of an atom that was small but indivisible and uniform throughout. His atomic theory was . . . All matter is composed of extremely small particles called atoms. Atoms of an element are identical to each other and different from atoms of other elements.

Dalton’s Theory continued 3) Atoms cannot be created or destroyed. Atoms are combined, separated or rearranged when chemical reactions occur. 4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Dalton’s Model Dalton’s model of the atom is a small, solid, and indivisible particle. (Ball-Bearing Model – similar to a marble)

But Dalton wasn’t entirely correct… Today we know that…. Atoms are divisible into even smaller particles Atoms of the same element can have different masses. (called isotopes)

JJ Thompson (1897) Conducted the Cathode-Ray Tube Experiment Thomson concludes that all cathode rays are composed of identical negatively charged sub-atomic particles, which he calls electrons. Proposed the “Plum-Pudding Model” of an atom.

Plum-Pudding Model Thomson’s experiments show that (1) atoms are divisible and (2) they include negatively charged electrons. Thomson’s Plum-Pudding Model of the Atom has negative particles spread evenly throughout a solid, positively charged sphere.

Thompson’s Experiment Passed electric current through a tube. The rays were deflected away from magnetic fields and negatively charged objects . Demo

Ernest Rutherford (1911) Conducted the Gold-Foil Experiment Positively charged alpha particles were fired at a thin piece of gold foil. About 1 out of every 20,000 particles were actually deflected back toward the source. Video Clip

Ernest Rutherford Concludes that there must be a very small and densely packed region of positive charge in the atom that would repel these alpha particles. Discovered nucleus and credited with discovery of proton

Rutherford’s model: He proposed a model of the atom where the negatively charged electrons orbit a small, dense, positively charged nucleus.

Niels Bohr (1913) Bohr added the idea of quantized energy levels to his Rutherford’s model. This explained why different elements give off different colors of light when heated in a flame.

Weakness of Bohr’s model His model was only accurate for predicting the behavior of Hydrogen atoms

The Nuclear Model of the Atom Nucleus = very small and dense center of the atom, where protons and neutrons are found. Protons have a positive charge. Neutrons are electrically neutral. Negatively charged electrons orbit the nucleus like the planets orbit the sun. Most of the atom is empty space.

Subatomic Particles Proton Neutron Electron Charge + — Mass* 1 amu ~ 0 Location Nucleus Electron Cloud *Mass of protons and neutrons = 1.67 x10-27 kg compared to the electron’s mass of 9.109 x10-31 kg