9.11 Atomic Theory. My Future Matters. Objectives: - SWBAT describe and explain the four historical models of the atom and how they evolved over time.

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1 9.11 Atomic Theory Objectives: - SWBAT describe and explain the four historical models of the atom and how they evolved over time. Read & Analyze: Directions: Read the following text. While you are reading annotate according to the strategies below. At the end of the reading complete the reading comprehension and analysis questions. Highlight definitions Underline examples Star interesting facts & your questions Bracket & Number answers to questions Theory: Atom Are Building Blocks Scientist: John Dalton In the 1800 s, an English scientist named John Dalton came up with his own idea of what atoms were like. Even though he couldn t see atoms he could make observations based on his experiences with matter in life. Based on these observations he develop the Atomic Theory of Matter that included the following ideas: Everything is made of atoms (EVERYTHING!!!) All atoms of an element are identical in every way (Ex: A block of gold is only made of gold atoms) Atoms of different elements are different (Ex: gold atoms are different from oxygen atoms) Atoms can t be broken without changing their identity Atoms combine in whole number ratios to form compounds (i.e. you can t have half an atom in a compound). In chemical reactions, atoms are rearranged. They don t get created or destroyed. Although incredibly important observations and theories, Dalton did not create a model for what is inside an atom. Check for Understanding 1. What happens to atoms in chemical reactions? Explain. 2. You have an atom of Gold and an atom of Silver. Are they the same atom? Why or why not? 1

2 Theory: Plum Pudding Model Scientist: J.J. Thomson The plum pudding model of the atom was developed by J. J. Thomson in In this model, the Thomson hypothesized that the atom is composed of small particles called electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "plums" surrounded by positively- charged "pudding". Essentially, his idea was that electrons and protons were mixed evenly throughout the atom. Think of chocolate cookie dough were the chocolate chips are the electrons mixed throughout the dough. The electrons (as we know them today), he guessed, were positioned throughout the atom and there was a cloud of positive charge. Questions: 1. In J.J. Thomson s model of the atom, what represented plums and what represented pudding? 2. What is the charge on an electron? 3. Why do you think J.J. Thomson called his model of the atom, the plum- pudding model? Use evidence from the text to support your answer. 2

3 Theory: Rutherford s Nucleus Model Scientist: Ernest Rutherford Rutherford was a scientist who liked to play with radioactive material. His favorite radioactive thing to play with was alpha particles, which are positively charged. In a famous experiment conducted in 1911, he decided to shoot a bunch of alpha particles at a really thin piece of gold foil. Imagine this to look like a sort of laser or beam of light. When he did this, he found that most of the particles went through the foil in a straight line, while some of them bounced off at irregular angles. He thought, why is this? His idea was to come up with a model of the atom in which most of the atom is empty space with electrons floating around in it. The protons, however, are all concentrated in the middle of the atom (called the nucleus) according to this model, the positively charged alpha particles would go straight through the atom most of the time and only be deflected on the rare occasions when they passed very close to the tiny nucleus. For this discovery, we will always know Rutherford as the man with the gold foil. Check for Understanding 1. What is the name of the radioactive particle used in Rutherford s experiment? 2. What happened to the particles when Rutherford shot them at a piece of gold foil? 3. What did this tell Rutherford about the structure of the atom? Explain. 3

4 Theory: Electron Cloud Atom Scientist: Niels Bohr In 1913, a few years after Rutherford and Thomson, Niels Bohr used Rutherford s experiment to develop his own idea of the structure of the atom. His idea was that electrons traveled in rings around the center of the atom and were not even mixed like Thomson believed. The electrons moved only in certain circular paths around the nucleus (center of the atom), like the running lanes around a track. Electrons have to stay in their lane unless a bunch of energy is added to the atom. When energy is added to the electrons, the electrons jump from their normal orbit to a higher energy orbital farther from the center. Essentially they move from an inner ring to an outer ring. Since the world likes to exist at low energy more than at high energy, the electrons eventually return to their ground state orbitals. When this happens, the energy that they absorb is given off as light. Since the color of light is very closely related to its energy, you only see very particular colors of light being given off by the very particular energy differences between the ground state and the excited state. Check for Understanding 1. In the Bohr Model of the atom, how do electrons move? 2. What do we see as a result of electrons releasing energy? 4

5 Summary of Atomic Models John Dalton Ernest Rutherford Key Discoveries JJ Thomson Niels Bohr Key Discoveries 5

6 Homework Due Tomorrow: 1. On the line below, label the year when for each scientist described in class today and write a sentence that outlines their main contribution to the discovery of the structure of the atom 2. How did Rutherford s experiment help disprove Thomson s plum- pudding model of the atom? [Hint: what did Rutherford discover about the structure of the atom?] 3. Water can exist as a solid (ice), liquid (water) or gas (steam). In all three states, there is made of oxygen and hydrogen. Does anything change about an atom of oxygen in ice vs. water? If no, why not? If yes, what changes? 6