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 Aristotle -Rejected the idea of the atom -Said matter could be cut continually *most people sided with Aristotle Dalton's Atomic Theory Dalton transformed Democritus's ideas on atoms into a scientific theory. He studied the ratios in which elements combine in chemical reactions. (Concrete Evidence) There are 4 parts to this theory: 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atom: the smallest particle of an element that retains its identity in a chemical reaction. 3. Atoms of different elements can physically mix together or can chemically combine in simple wholenumber ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
Atomic Size Copper atoms are very small. A pure copper coin the size of a penny contains about 2.4 x 22 atoms. Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes. Iron Atoms Subatomic Particles The atoms are now broken down into even smaller, more fundamental particles, called subatomic particles. There are 3 subatomic particles 1. electrons (e - ) 2. protons (p + ) 3. neutrons (n o ) Electrons J. J. Thomson discovered the electron. He thought some smaller particle had to exist. Electrons : negatively charged subatomic particle. He used the cathode ray tube to make this discovery. How did J. J. Thomson make this discovery? The cathode ray tube has a cathode and anode at each side. The cathode originally is positively charged and the anode negatively charged. When the electric current was based through, the cathode became negatively charged and the anode became a positive charge. http://www.youtube.com/watch?v=xu8nmkkzbt8 Robert A. Millikan *He was then able to calculate the mass of the electron. Protons and Neutrons How did we determine the presence of protons and neutrons? Four ideas about matter and electric charges: 1. Atoms have no net electric charge. 2. Electric charges are carried by particles of matter. 3. Electric charges always exist in whole-number multiples of a single basic unit. (There are no fractions of charges.) 4. When a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle is formed.
Protons and Neutrons Proton: Positively charged subatomic particles. Confirmed by Eugen Goldstein Neutron: Subatomic particles with no charge but with a mass nearly equal to that of a proton Confirmed by James Chadwick The Atomic Nucleus Scientists wondered how these particles were put together in an atom. J. J. Thomson came up with a model known as the "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 Ernest Rutherford: Gold-Foil Experiment Rutherford Atomic Model 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 Element Atomic Number Protons Electrons K 19 19 5 S 16 V 23 K+ S 2-
X= Element Symbol A= Mass Number Z= Atomic Number Shorthand Notation 197 79Au Au-197 Gold-197 Mass Number Mass Number: The total number of protons and neutrons in an atom. How would you then determine the number of neutrons in an atom? Number of neutrons = mass number - atomic number Sample Problem 4.1 How many protons, electrons, and neutrons are in each atom? A. Beryllium (Be) B. Neon (Ne) C. Sodium (Na) Atomic Number 4 11 Mass Number 9 20 23 Answer A. Beryllium (Be) B. Neon (Ne) C. Sodium (Na) Protons 4 11 Electrons 4 11 Neutrons 5 12 Practice: How many neutrons are in each atom? 16 a) O 8 32 b) 16S 8 c) Ag 47 80 35 d) Br 207 e) Pb 82 Practice: Express the following in shorthand notation a) carbon-12 b) fluorine-19 Isotopes: Atoms that have the same number of protons but different numbers of neutrons * Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. c) beryllium-9
Uses of Isotopes 1. Americum-241: Used in many smoke detectors 2. Carbon-14: Used in biological research, agriculture, pollution control and archeology. 3. Cesium-137: Used to treat cancerous tumors, control liquid flow in oil pipelines. 4. Iodine-123: Used to diagnose thyroid disorders. 5. Iodine-129: Used to check radioactivity counters in in-bitro diagnostic testing labs. 6. Iodine-131: Used to treat thyroid disorders. (Graves's disease) 7. Plutonium-238: Has powered more than 20 NASA spacecraft since 1972. 8. Uranium-234: Used in dental fixtures like crowns and dentures to provide a natural color and brightness. 9. Uranium-235: Fuel for nuclear power plants In nature, most elements occur as a mixture of two or more isotopes. Each isotope of an element has a fixed mass and a natural percent abundance. We are then able to determine the isotope that is in more abundance due to the average (atomic mass) of the element. Example: The two stable isotopes of chlorine are chlorine-35 and chlorine-37. The average SHOULD be 35.969 amu. However, this value is higher than the actual value of 35.453. So, chlorine-35 is in higher abundance than chlorine-37. Practice: Isotopes 1) Three isotopes of oxygen are oxygen-16, oxygen-17 and oxygen-18. Write the symbol for each, including the atomic number and mass number. 2) Three isotopes of chromium are chromium-50, chromium-52 and chromium-53. How many neutrons are in each isotope, given that chromium has an atomic number of 24? Atomic Mass It is more useful to compare the relative masses of atoms using a reference isotope as a standard. This standard is Carbon-12. 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. Table 4.3 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 1) Boron has two isotopes: boron- and boron-11. Which is more abundant given that the atomic mass of boron is.81? Calculating Atomic Mass of an Element Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. Atomic Mass of Carbon: 2) There are three isotopes of silicon; they have mass numbers of 28, 29, and 30. The atomic mass of silicon is 28.086 amu. Comment on the relative abundance of these three isotopes. C-12: 98.89%, 12.00 amu C-13: 1.11%, 13.003 amu Atomic mass= (12.000 amu x 0.9889) + (13.003 amu x 0.0111) Atomic mass= 12.011 amu
Calculating Atomic Mass of an Element 1) The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for mass-62.93 amu, and 30.8% for mass=64.93 amu. Calculate the average atomic mass of copper. Calculating Atomic Mass of an Element 2) Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu (50.69%) and 80.92 amu (49.31%).