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Atomic Structure October 03, 2014 Atomic Structure Chapter 4 Democritus's Atomic Philosophy 1. Democritus 2. Dalton 3. Thompson 4. Millikan 5. Moseley 6. Bohr 7. Rutherford 8. Schrodinger Dalton's Atomic Theory Aristotle Dalton transformed Democritus's ideas on atoms into a scientific theory. He studied the ratios in which elements combine in chemical reactions. -Rejected the idea of the atom (Concrete Evidence) There are 4 parts to this theory: -Said matter could be cut continually 1. All elements are composed of tiny indivisible particles called atoms. *most people sided with Aristotle 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can physically mix together or can chemically combine in simple whole- number ratios to form compounds. Atom: the smallest particle of an element that retains its 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, identity in a chemical reaction. are never changed into atoms of another element as a result of a chemical reaction. October 03, 2014 Atomic Size Copper atoms are very small. A pure copper coin the size of a penny contains about 2.4 x 1022 atoms. Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes. Iron Atoms Subatomic Particles Electrons The atoms are now broken down into even smaller, more J. J. Thomson discovered the electron. He thought some fundamental particles, called subatomic particles. smaller particle had to exist. There are 3 subatomic particles Electrons : negatively charged subatomic particle. He used the cathode ray tube to make this discovery. 1. electrons (e-) 2. protons (p+) 3. neutrons (no) How did J. J. Thomson make this discovery? Protons and Neutrons The cathode ray tube has a cathode and anode at each side. The cathode originally is positively charged and the How did we determine the presence of protons and neutrons? anode negatively charged. When the electric current was based through, the cathode became negatively Four ideas about matter and electric charges: charged and the anode became a positive charge. 1. Atoms have no net electric charge. http://www.youtube.com/watch?v=XU8nMKkzbT8 2. Electric charges are carried by particles of matter. 3. Electric charges always exist in whole-number multiples of a Robert A. Millikan single basic unit. (There are no fractions of charges.) *He was then able to calculate the mass of the electron. 4. When a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle is formed. October 03, 2014 Protons and Neutrons The Atomic Nucleus Proton: Positively charged subatomic particles. Confirmed by Eugen Goldstein Scientists wondered how these particles were put together in an atom. Neutron: Subatomic particles with no charge but with a mass nearly equal to that of a proton J. J. Thomson came up with a model known as the Confirmed by James Chadwick "plum-pudding" model. This model stated how electrons were stuck into a lump of positive charge, similar to raisins stuck in dough. This model did not last very long when Ernest Rutherford entered the picture. The Atomic Nucleus http://www.youtube.com/watch?v=5pZj0u_XMbc Rutherford Atomic Model Ernest Rutherford: Gold-Foil Experiment He proposed that the atom is mostly empty space, explaining the lack of deflection of most of the particles. All the positive charge and almost all the mass are concentrated in a small region called the nucleus. Nucleus: Tiny central core of an atom and is composed of protons and neutrons. Atomic Number: The number of protons in the nucleus. Atomic Number Atomic Element Protons Electrons Number K 19 19 5 S 16 V 23 K+ S 2- October 03, 2014 Shorthand Notation Mass Number Mass Number: The total number of protons and neutrons in an atom. 197 79Au How would you then determine the number of neutrons in an atom? Number of neutrons = mass number - atomic number X= Element Symbol Au-197 A= Mass Number Gold-197 Z= Atomic Number Sample Problem 4.1 Practice: How many neutrons are in each atom? How many protons, electrons, and neutrons are in 16 each atom? a) 8 O Atomic Number Mass Number 4 32 A. Beryllium (Be) 9 b) 16S B. Neon (Ne) 10 20 C. Sodium (Na) 11 23 108 c) 47 Ag Answer Protons Electrons Neutrons 80 A. Beryllium (Be) 4 4 5 d) 35 Br B. Neon (Ne) 10 10 10 C. Sodium (Na) 11 11 12 207 e) 82 Pb Practice: Isotopes: Atoms that have the same number of protons Express the following in shorthand notation but different numbers of neutrons a) carbon-12 * Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. b) fluorine-19 c) beryllium-9 October 03, 2014 Uses of Isotopes In nature, most elements occur as a mixture of two or 1. Americum-241: Used in many smoke detectors more isotopes. Each isotope of an element has a fixed 2. Carbon-14: Used in biological research, agriculture, mass and a natural percent abundance. pollution control and archeology. 3. Cesium-137: Used to treat cancerous tumors, control We are then able to determine the isotope that is in liquid flow in oil pipelines. more abundance due to the average (atomic mass) of 4. Iodine-123: Used to diagnose thyroid disorders. the element. 5. Iodine-129: Used to check radioactivity counters in in-bitro diagnostic testing labs. Example: 6. Iodine-131: Used to treat thyroid disorders. (Graves's disease) The two stable isotopes of chlorine are chlorine-35 and 7. Plutonium-238: Has powered more than 20 NASA chlorine-37. spacecraft since 1972. The average SHOULD be 35.969 amu. However, 8. Uranium-234: Used in dental fixtures like crowns and this value is higher than the actual value of 35.453. dentures to provide a natural color and brightness. 9. Uranium-235: Fuel for nuclear power plants So, chlorine-35 is in higher abundance than chlorine-37. Practice: Isotopes Atomic Mass 1) Three isotopes of oxygen are oxygen-16, oxygen-17 It is more useful to compare the relative masses of atoms using a and oxygen-18. Write the symbol for each, including the reference isotope as a standard. This standard is Carbon-12. atomic number and mass number. Atomic mass unit (amu): One twelfth of the mass of a carbon-12 atom. Example: A helium-4 atom, with a mass of 4.0026 amu has about one-third the mass of a carbon-12 atom. 2) Three isotopes of chromium are chromium-50, chromium-52 and chromium-53. How many neutrons Table 4.3 are in each isotope, given that chromium has an atomic number of 24? Average Atomic Mass: a weighted average mass of the atoms in a naturally occurring sample of the element. This average reflects both the mass and the relative abundance of the isotopes as they occur in nature. Practice Problems Calculating Atomic Mass of an Element 1) Boron has two isotopes: boron-10 and boron-11. Which is more abundant given that the atomic mass of boron is Multiply the mass of each isotope by its natural abundance, 10.81? expressed as a decimal, and then add the products. Atomic Mass of Carbon: 2) There are three isotopes of silicon; they have mass C-12: 98.89%, 12.00 amu C-13: 1.11%, 13.003 amu numbers of 28, 29, and 30. The atomic mass of silicon is 28.086 amu. Comment on the relative abundance of these Atomic mass= (12.000 amu x 0.9889) + (13.003 amu x 0.0111) three isotopes. Atomic mass= 12.011 amu October 03, 2014 Calculating Atomic Mass of an Element Calculating Atomic Mass of an Element 1) The element copper has naturally occurring isotopes with 2) Calculate the atomic mass of bromine. The two isotopes mass numbers of 63 and 65. The relative abundance and of bromine have atomic masses and relative abundance of atomic masses are 69.2% for mass-62.93 amu, and 30.8% 78.92 amu (50.69%) and 80.92 amu (49.31%). for mass=64.93 amu. Calculate the average atomic mass of copper..
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