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The History of Atomic Structure AKA: The old, dead guy parade! CA State Standards: 1h and 1i Democritus (460 - 370 BC) Was the first person to come up with the idea of atom Believed that all matter was composed of indivisible particles he called “ATOMS” Was he right? Are atoms indivisible?? He also believed that different atoms: Are different sizes Have different properties Objectives To identify the major scientists and what they contributed to our knowledge of atomic structure To describe the evolution of our understanding of atomic structure To describe Rutherford’s Gold Foil experiment and what it concluded John Dalton (1766-1844) • Dalton is the “Father of Atomic Theory” • Dalton’s ideas were so brilliant that they have remained essentially intact up to the present time and has only been slightly corrected. Dalton’s Atomic Theory (1803) Dalton’s Atomic Theory (1803) aka: 5 Postulates aka: 5 Postulates 1. All matter is composed of extremely small particles called atoms. (I agree with Democritus!) 2. All atoms of a given element are identical, having the same: - size - mass - chemical properties 3. All atoms of different elements are different. (a Sodium atom is different than chlorine) 4. Atoms cannot be created, divided into smaller particles, or destroyed. **In a chemical reaction, atoms of different elements are separated, joined or rearranged. They are never changed into the atoms of another element. We will learn more later** 5. Law of Definite Proportions: Different atoms combine in simple whole number ratios to form compounds (you can’t have a ½ of a Carbon bonding with Oxygen; it’s a whole atom or no atom) 1 JJ Thomson (1856-1940) Solid Ball Model Dalton’s TinyAtomic •Based on Dalton’s Atomic Theory (5 postulates), most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. • Used cathode rays to prove that Dalton’s Solid-ball model could be broken into smaller particles •(Sorry Dalton! No more tiny SOLID balls!) •Thomson is credited with discovering electrons •Dalton was credited for the three Atomic Laws that were proven after his time. •Milikan confirmed Thomson’s work with the oil droplet experiment. Cathode Ray Tubes • Cathode rays had been used for Cathode + some time before Thompson’s experiments. Rutherford (1871-1937) Anode • A cathode ray is a tube that has a piece of metal, called an electrode, at each end. Each electrode is connected to a power source (battery). • When the power is turned on, the electrodes become charged and produce a stream of negatively charged particles. They travel from cathode, across the tube to the anode. Gold Foil Experiment Rutherford directed a narrow beam of alpha particles (+ charges) at a thin piece of gold foil Based on the Plum Pudding Model, he predicted that the + charges would go through the foil Because the atom overall was neutral, he thought the alpha particles would travel straight through Took Thomson’s Plum Pudding Model and added to it “Gold Foil Experiment” in 1913 was used to discover the existence: Protons Nucleus You must be able to explain the Gold Foil Experiment…it will be on the CST Bohr (1885-1962) Worked in Rutherford’s lab Wondered why – electrons are not attracted to the + nucleus and cluster around it Disproved Thomson’s Plum Pudding Model Used Rutherford’s Planetary Model and Einstein’s photon discovery to propose a new model Bohr said that electrons travel at the speed of light. 2 Bohr’s Energy Level Model Energy Level Model: Electrons are arranged in circles around the nucleus. Each circle has a different energy. •Electrons are in constant motion, traveling around the nucleus at the speed of light. •Electrons can “jump” from one circle (orbit) to the next •But they are not attracted to the nucleus they traveling too fast to be fully attracted. Bohr’s Energy Level Model He proposed the following: 1. Protons and neutrons are in the nucleus 2. Electrons can only be found at certain distances from the nucleus. 3. The electrons orbit the nucleus at fixed energy levels. 4. The electrons must absorb or emit a fixed amount of energy to travel between these energy levels Bohr’s & Energy Level Model Video Electron Cloud Model • Bohr thought that electrons traveled in a perfect circle every revolution. (circumference) • We know that the orbits of electrons are not that perfect. • Electrons have a bit of wobble in their travel, similar to the path of a runner on a track. Electrons can be anywhere in their lane. The lanes are called electron clouds. • The Electron Cloud Model improved upon Bohr’s model by using mathematics to determine the position of an electron in an orbit (lane). •An electron will be found in it’s cloud about 90% of the time. The picture above shows all of the places an electron can be found in an atom. 3