Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Standard Model wikipedia , lookup
Photoelectric effect wikipedia , lookup
Compact Muon Solenoid wikipedia , lookup
Electric charge wikipedia , lookup
Introduction to quantum mechanics wikipedia , lookup
Electron scattering wikipedia , lookup
Elementary particle wikipedia , lookup
Chapter 7 1. Definitions: - electron - proton - neutron - subatomic particle - radioactivity - nucleus - electron cloud - electron shell - Bohr-Rutherford Model - Atomic number - Atomic mass unit - Mass number - Isotope 2. 3. 4. 5. Note- Models of atomic structure read pg 234- Thomson- and questions pg 240-244 Bohr-Rutherford- and questions 246- 248 and questions ( Atomic number…etc) Chapter 7: Models of Atomic Structure Investigating the Atom During the 19th century, when many chemists were studying elements, other scientists were interested in the way electricity interacts with matter. Because they knew that air conducts electricity better if it is trapped and its pressure is reduced, they used gas discharge tubes in their investigations. Gas discharge tubes are made of glass and can trap air and other gases. They are equipped with electrodes at either end: The positive charged electrode is called the anode, and the negatively charged electrode is called the cathode. The pressure inside the tube can be reduced by pumping air out of the tube. (Modern gas discharge tubes are used in television picture tubes.) Scientists found that when they reduced the pressure in the tube, the gases inside began to glow! The reason was that cathode rays from the cathode were traveling through the tube toward the anode and were carrying a negative charge. The search for the electron began! (Electron is the word they used to describe tiny, negatively charged bits of matter). Scientists were puzzled. If atoms contain negatively charged electrons, why did they usually have no charge? Joseph John Thomson (1856-1940) inferred that atoms must possess something with a positive charge to balance the negatively charged electrons. He experimented with tubes of hydrogen gas and concluded that anode rays must be made up of positive particles. Thomson called them protons. Thomson concluded the following: -all atoms contain both protons and electrons -all protons are identical, and all electrons are identical. Electrons differ from protons, however. -an electron has a negative charge. A proton has a positive charge. A proton has a positive charge. -an electron has the same amount of charge as a proton, even though the charges are opposite in kind. -a proton has much more mass than an electron. 1938-The first electron microscope was made by Albert Prebus and James Hillier at the University of Toronto. This gave scientists more knowledge about the structure of the atom. In 1803, Dalton had said that atoms were indivisible. By Thomson’s time (1900’s), it was evident that this was not true! Atoms could be torn apart by high-energy electricity. Protons and electrons were known to be sub-atomic particles Thomson’s view of subatomic particles: Negative electrons were like raisins in a muffin: they could be pulled out of the positively charged dough In 1895, x-rays were discovered. This led to the study of radioactivity. During that time, a student of Thomson’s named Ernest Rutherford was teaching at McGill University. He won the Nobel Peace Prize in Chemistry for his work in radiation. In 1909, Rutherford probed atoms with radioactive particles. He discovered that atoms must also contain: -a nucleus: a tiny core that is very small in volume, dense compared to the rest of the atom, and intensely positive. -an electron cloud: an “envelope” that is very large in volume, light compared to the nucleus, and negatively charged. Because the protons in an atom’s nucleus account for less than half the mass calculated for the nucleus, Rutherford inferred that the nucleus must also contain additional, uncharged (neutral) particles. He called them neutrons and concluded that each neutron would have roughly the same mass as a proton. His ideas would not be confirmed until the 1930’s! In 1912, a Danish scientist, Niels Bohr, began working with Rutherford in Manchester, England. He was trying to explain the question put forth by doubters of Rutherford’s ideas: if positive and negative charges attract, why didn’t the negatively charged electrons crash into the positively charged nucleus? Bohr came up with a model that compared the nucleus to the sun and the electrons to the planets. Why doesn’t the gravitational pull of the sun cause the planets to spiral inward and crash? Because they revolve at just the right speed to remain in orbit. Similarly, the atom’s positively charged nucleus exerts a strong force of attraction on negative electrons. The electrons don’t spiral inward and crash because they are moving rapidly in fixed regions around the nucleus. These 3-D, sphere-shaped regions are called electron shells. Bohr concluded that the more energy an electron had the more distant shell (energy level) it could occupy. Bohr-Rutherford Model of Atomic Structure 1st shell 2nd shell 3rd shell This was the first modern view of the atom! In 1913, Henry Mosely began working in Rutherford’s laboratory. He discovered a very regular pattern in the way different elements responded to x-rays. This pattern provided information about the atomic nucleus of each atom. He determined an increasing number of protons as you move through Mendeleeve’s table. #of protons=Atomic # Atomic Number: number of protons in an atom. Example: Fluorine (9) –has nine protons in the nucleus Therefore, it also has 9 electrons How many neutrons does it have? You cannot tell by the atomic number. You must look at the mass number (based on atomic mass): gives the total # of protons and neutrons. Ex: the atomic mass of F = 19 Atomic # - = 9 ____ 10 neutrons Proper notation: F Atomic Mass Units: because atomic mass is not always a whole number, it is stated in atomic mass units (u). Atomic mass is not the same as the mass expressed in grams. It is a ratio. All atomic masses are defined relative to the mass of carbon12. Thomson’s Inferences about the Proton (P234) Questions: 1. Who is JJ Thomson referred to as? 2. What did he infer about the protons and electrons of the atom? 3. What is meant by the term subatomic particle? 4. What was one idea about the atom that was changed in Dalton’s Theory by Thomson’s time? 5. Draw Thomson’s model of the atom. 6. What did he compare his model to? Answers: 1. The Father of the Electron. 2. The atom must have something with a positive charge to balance the negatively charged electron. 3. Particles smaller then an atom 4. By Thomson’s time it was clear that atoms could be torn apart by high-energy electricity; therefore atoms were divisible after all. 5. Diagram: 6. Raisin bread/raisin muffin The Bohr- Rutherford Model Page 238 to 244 Questions: 1. 2. 3. 4. What does Rutherford conclude about the model of the atom? How did Bohr contribute to Rutherford’s findings? What is the Bohr- Rutherford model? Draw the diagram of the atom that came from the conclusions of Bohr- Rutherford. ( Fig.7.20) What is an electron shell? Do the activity on “Summarizing Atomic Structure” of Scientific Inquiry on page 244. 5. 6. Answers: 1. Rutherford concluded that: a. An atom contains a nucleus that is a tiny core, very small in volume, dense compared to the rest of the atom and intensely positive. b. The atom has an electron cloud that is an envelop very large in volume, light compared to the nucleus and negatively charged. c. He also inferred that the protons were not alone in the nucleus and realized the nucleus must contain additional (uncharged) particles. These particles were identified as neutrons which have roughly the same mass as a proton but no charge. 2. Borh’s contribution to the model states the atom’s positive nucleus exerts a strong force of attraction on the negative electrons. This fixed positioning of the electrons was identified as an electron shell. 3. The Bohr-Rutherford model has a central nucleus and electron shells. It can be called the first “modern” view of the atom. Diagram page 243 Electron shell: In Bohr’s model of an atom, a fixed, three dimensional sphere-like region around the atom’s nucleus; electrons move rapidly within the shell. 4. 5. Atomic Number, Mass Number and Atomic Structure: Page 246Isotopes pg 248 Questions: 1. 2. 3. 4. 5. 6. 7. What is the role of a neutron? What does the mass number represent? How do you find the number of neutrons in an atom? Is the atomic mass and the mass number the same? Why or why not. Define atomic mass unit. What is an isotope? Due to the different mass numbers of certain elements, why is the atomic mass of an element on the periodic table constant? Answers: 1. Neutrons hold the nucleus together and keep the protons from flying apart. 2. Mass number is the total number of protons and neutrons in the nucleus. 3. Subtracting the atomic number from the mass number. 4. No, mass number is the number of protons and neutrons in the nucleus; atomic mass is an average mass of an element. 5. The unit used to measure the mass of atoms relative to 1/12 the mass of the atom of carbon 12. 6. Any of two forms of an element that have the same number of protons but a different number of neutrons. 7. The atomic mass in the periodic table is an average of the isotopes.