Major upsetting discoveries: Today s Objectives/Agenda. Notice: New Unit: with Ms. V. after school Before Friday 9/22.

Transcription

1 Bellwork Monday Tuesday *This should be the last box on bellwork: 1. What are some major points in history where common knowledge was upset by a new discovery? 2. Draw what you think an atom should look like. 3. You get a present, completely wrapped so you can t see what s inside. How could you figure out what s inside without looking? Major upsetting discoveries: Today s Objectives/Agenda I will know: how models are developed how the idea of the atom was developed I will be able to: make a model based on observations Compare and contrast different models of the atom Agenda: Think Tube activity Atomic Theory!!! (How did they figure it out?) Story time Notice: IF you were GONE LAST CLASS, You need to make up the TEST with Ms. V. after school Before Friday 9/22. Please check the availability calendar. New Unit: Coming Soon: Atomic Theory But first how did scientists find out what is inside something SOOOO small?? Atomic Structure For example, 1 carbon atom weighs grams. = How did they figure out what was inside the atom? 1

2 Think Tube Think Tube Summary Scientists had to make observations about matter to figure out the atom. Those observations began long ago Let s take a trip back in time Atomic Theory Objectives: 1. Describe the particle theory of matter as it evolved throughout history 2. Compare the early atomic models to the electron cloud model with respect to their ability to accurately represent the structure of the atom 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. Hypothesis: possible explanation for your observations Theory: broad, extensively tested explanation of why experiments give certain results, can never be proved. Law: statement that summarizes results of experiments but does not attempt to explain it. 2

3 Who are these men? In this lesson, we ll learn about the scientists whose quests for knowledge about the fundamental nature of the universe helped define our views. 1) Democritus 400 BC This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. He asked.could matter be divided into smaller and smaller pieces forever, or was there a LIMIT to the number of times a piece of matter could be divided? Atomos His theory: Matter cannot be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. I shall call it atomos meaning not to be cut. Atomos To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. Examples of his theory: Atomos- Not to be cut Problem is- Nobody listened. Everybody accepted a different man s theory for more than 2000 years. Why? 3

4 The rock star philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately WRONG) theory. Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea from Democritus was buried for approximately 2000 years. The Slow Years For a long time, science in terms of Chemistry and Physics did not advance. Dark Ages, war, death, famine, etc. What people picture: Alchemists What it actually was: Alchemists in the middle ages tried to get rich quick by searching for a formula to turn lead into gold, or create the philosopher s stone. However, we got some neat-o lab equipment out of it, discovered some new elements, AND started using symbols for elements! 2) Dalton s Model In the early 1800s, the English chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. By George, I think Democritus knew what he was talkin bout Atoms = POPULAR with the public again. Dalton s Atomic Theory Dalton s Main Ideas: 1. All elements are composed of atoms. 2. Atoms of the same element are exactly alike; Atoms of different elements are different. 3. Compounds are formed by the joining of atoms of two or more elements, in whole number ratios. 4. Atoms are indivisible and indestructible particles. 4

5 Law of Conservation of Mass Atoms are indivisible and indestructible particles. This means ATOMS are NOT created or destroyed in chemical reactions; a chemical reaction simply rearranges the way the atoms are grouped together. A substance undergoes a change in identity, but the atoms that make up the old and new substance are not changed, just rearranged. Thomson s Plum Pudding Model 1897 English scientist (3) J.J. Thomson hypothesized that maybe an atom is made of even smaller particles. Thomson Model Thomson studied the passage of an electric current through a gas in a Cathode Ray Tube. As the current passed through the gas, it produced rays of negatively charged particles. Cathode Ray Tube used by J.J. Thomson See how it works! Thomson Model This surprised him, because the atoms of the gas were uncharged. Where did they come from?? The atom can be broken down into smaller particles! We call these particles electrons. He won a Nobel Prize for this discovery! Since the gas was known to be neutral, having no charge, he reasoned that there must also be positively charged particles in the atom. rats.. But he could never find them. Thomson concluded that the negative charges came from within the atom. BIG DEAL:A particle smaller than an atom had to exist! Whoopee! A New Model It explained: Atoms were made from a positively charged substance (pudding) with negatively charged electrons scattered about (like raisins in a pudding). To balance out the negative charge found within a neutral atom, J.J. proposed a model of the atom called the Plum Pudding Model. 5

6 Examples from today: We might relate to these items better than plum pudding. Remember, a model uses familiar ideas to explain unfamiliar facts observed in nature. A Decade Later English physicist 4) Ernest Rutherford was studying how positive charges interacted with matter. Unexpectedly, he made the next breakthrough in atomic understanding! This was called the Gold Foil Experiment Cheerio this gold is expensive! Rutherford s experiment fired a stream of alpha particles (tiny, positively charged) at a thin sheet of gold foil ( only 2000 atoms thick!) Rutherford s experiment involved firing a stream of alpha particles (tiny, positively charged) at a thin sheet of gold foil ( only 2000 atoms thick!) Gold Foil Experiment Results Most of the positively charged bullets passed right through the gold atoms in the sheet of gold foil without changing course at all. HOWEVER, Some of the positively charged bullets bounced away from the gold sheet as if they had hit something solid. He knew that like charges repel each other. Gold Foil Experiment Most of the positively charged bullets passed right through the gold atoms in the sheet of gold foil without changing course at all. Some of the positively charged bullets, however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges. 6

7 Experiment Conclusions 1. The gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. 2. The atom must have a small, dense, positively charged center (because it repelled his positively charged bullets. I I will call the center of the atom the nucleus! I will call the center of the atom the nucleus! 3. This center is tiny compared to the whole atom. X He reasoned the negatively charged particles were scattered outside the nucleus around the atom s edge. Rutherford s Model Rutherford reasoned that all of an atom s positively charged particles were contained in the nucleus. The Nucleus Refined 1920 Rutherford proposed the proton, the positively charged particle that gives the nucleus its charge James Chadwick wins the Nobel Prize for discovering a neutral particle also in the nucleus, called a neutron. Bohr Model 1913 Danish scientist 5) Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level. Bohr Model Electrons move in circular orbits around the nucleus, much like planets circle the sun. Problems again Bohr s model only work These orbits are specific energy levels, located at certain distances from the nucleus. When excited, electrons move to a higher energy level, like moving up a ladder. 7

8 Bohr s Energy levels Think of these energy levels like rungs on a ladder it takes energy to move to a higher rung. In an atom, the energy levels are NOT evenly spaced. Quantum Mechanical Model 1926: Austrian physicist 6) Erwin Schrodinger developed our current model based on research that suggested electrons sometimes behave as WAVES, not particles Instead of electron orbits, there is an electron cloud surrounding the nucleus. Electrons whirl about the nucleus billions of times in one second!! They are NOT moving around in random patterns. It is impossible to determine the EXACT location of an electron, but the PROBABLE location of an electron can be determined. Electrons with the lowest energy are found in the energy level closest to the nucleus Electron Cloud: Location of an electron depends upon how much ENERGY the electron has. Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. Homework, D1 READ Ch. 4.1 & 4.2 Take NOTES over anything not covered here who else helped with Atomic Theory? What was discovered? And so ends the history for now until something new is observed, and the model is changed again 8