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Unit 3 History of the Atom 400 B.C. – Democritus & Leucippus Beach sand smaller piece of sand atomos (indivisible) Everything is composed of imperishable, indivisible elements called atomos. Aristotle’s views All substances are made of 4 elements Fire, Air, Earth, Water Blend these elements in different proportions to get all substances So who was right? Greeks settled argument by… Debating! Aristotle was a better speaker so he won. His views carried on through the Middle Ages. 1808 - Dalton English schoolteacher Recognized that elements were made of atoms Combined LOTS of research Dalton’s Atomic Theory Dalton’s Atomic Theory 1. All matter is made of tiny, indivisible particles called atoms. 2. Atoms of a given element are identical in their physical and chemical properties. 3. Atoms of different elements differ in their physical & chemical properties Dalton’s Atomic Theory 4. Atoms of different elements combine in simple whole # ratios to form compounds 5. In a chemical reaction, atoms are combined, separated, & rearranged, but never created, destroyed, or changed. From Dalton’s Theory… Law of Definite Proportions (#4) 2 samples of a compound have the same proportions by mass 500 kg NaCl = 60.66% Cl & 39.34% Na 2 mg NaCl = 60.66% Cl & 39.34% Na Law of Conservation of Mass (#5) Mass of reactants = mass of products From Dalton’s Theory… Law of Multiple Proportions If 2 or more compounds are composed of the same elements, the ratio of the mass of the elements is always a small, whole # NO 1.14 g O: 1 g N NO2 2.28 g O: 1 g N O [NO]: O [NO2] = 1.14 : 2.28 = 1 : 2 Another way to look at it… Water (H2O) has 8 g of oxygen per g of hydrogen. Hydrogen peroxide (H2O2) is 16 g of oxygen per g of hydrogen. 16 to 8 is a 2 to 1 ratio of oxygen. Always whole #s because you have to add a whole atom --- you can’t add a piece of an atom. Dalton’s Atomic Model The Solid Sphere Model The research continues… 1897 – JJ Thomson – cathode ray tube Movie Pumped air out of glass tube and applied voltage to metal electrodes at either end of the tube Anode = positive charge Cathode = negative charge Thomson’s Experiment Voltage source - + Metal Disks Thomson’s Experiment Voltage source Passing + an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + By adding an electric field he found that the moving pieces were negative Also placed a paddle wheel in the center – and it turned! So they must have mass! Thomson’s Conclusions The cathode ray consists of particles that have mass & a negative charge Particles called ELECTRONS Plum pudding model Negative electrons in a ball of positive charge The research continues… We know that an electron has a negative charge and atoms are neutral Mass of an electron MUCH less than an atom Something must be missing… Rutherford - 1909 Beam of small, positively charged particles called ALPHA particles Aimed at thin gold foil (a few atoms thick) Measured the angles of deflection Movie Lead block Uranium Flourescent Screen Gold Foil What Rutherford Expected The Plum Pudding Model Alpha particles would pass through without changing direction very much. Because… The positive charges were spread out evenly & would not stop the positive alpha particles. What he expected Because Because, he thought the positive charges were evenly distributed in the atom Because, he thought the positive charges were evenly distributed in the atom What he got Rutherford’s Explanation Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get hit the dense positive center. + + Conclusions from Rutherford The small dense, positive place at the center is called the NUCLEUS Radius of nucleus is less than 1/10000 the radius of the atom PROTON – positive charged particle in nucleus Charge is exactly equal but opposite to an electron BUT still not enough mass Still More Research… NEUTRONS Found in nucleus with protons Do not have a charge Same mass as protons The Nuclear Model - Rutherford Electrons revolve around nucleus in elliptical orbits Also called planetary model The Bohr Model Electrons are found in certain levels or shells around the nucleus More research… 1924 – de Broglie - Electrons behave like waves around the nucleus Heisenberg’s Uncertainty Principle A ceiling fan Planck & Einstein – quantum theory Electrons found in clouds instead of strict orbitals Today’s model – Quantum model Historical models of the atom (solid sphere, plum pudding, nuclear model, & planetary model all predicted exact locations of particles. The modern quantum theories combine all of this research with more recent findings that suggest a certain level of unpredictability. Subatomic particles Name Relative Symbol Charge mass Actual mass (g) Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0 0 1 1.67 x 10-24 Most of the mass is in the nucleus! The Atom All atoms have protons and electrons Most atoms have neutrons Elements differ from each other in the number of protons in an atom Protons & neutrons made of quarks Atomic Number The number of protons in an atom Same in all atoms of an element Example – Hydrogen=1 Atomic number also reveals the number of electrons in a neutral atom Mass Number Number of particles in the nucleus = # of Protons + # of neutrons Example – Neon has a mass # of 20 Can vary among atoms of an element Different elements can have the same mass number Not specifically on periodic table Using Atomic Symbols Each element has a name & a symbol Examples – Sulfur = S Sodium = Na The subscript to the right tells you how many atoms are present S8 = 8 sulfur atoms Using Atomic Symbols Contain the symbol of the element, the mass number, and the atomic number. Mass number Atomic number X Symbols/Notation Find the … Atomic number Mass Number number of protons number of neutrons number of electrons Name 24 11 Na Symbols/Notation Find the … –Atomic number –Mass Number –number of protons –number of neutrons –number of electrons –Name 80 35 Br Isotopes All atoms of an element have the same number of protons but not necessarily the same # of neutrons ISOTOPE – Atoms of the same element with different numbers of neutrons Naming Isotopes Name – mass number Examples Helium-3, Helium-4 Symbols/Notation 3 2 4 He 2 He Name That Element if an element has an atomic number of 34 and a mass number of 78 what is the –number of protons –number of neutrons –number of electrons –Complete symbol – Name Name That Element if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol – Name Name That Element if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Complete symbol –Name Atomic Mass There are different isotopes of each element that all have different masses Therefore we look at AVERAGE atomic mass for an element Based on abundance of each isotope in nature Atomic Mass Example You have five rocks -- four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? (50 + 50 + 50 + 50 + 60) / 5 260/5 = 52 g Atomic Mass Atoms are so small that even picograms aren’t useful 1 g = 10-12 pg So… we use a different unit Atomic Mass Units (amu) AKA a Dalton (Da) Defined as 1/12 the mass of a carbon- 12 atom. Very close to mass number Example – Oxygen (O) Mass number = 16 (Average) atomic mass = 15.999 amu Calculating averages Average atomic mass = (% as decimal x mass) + (% as decimal x mass) Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. Atomic Mass of Compounds Add the masses of the parts! EXAMPLE H2O = 2 hydrogen atoms + 1 oxygen H2O = 2 (1.0079) + 15.999 H2O = 18.015 amu in 1 molecule Another number problem… Most samples of elements have LOTS of atoms We want things to be simple! Scientists created a new unit The MOLE # of atoms in exactly 12 g of Carbon-12 Avogadro’s number = the number of particles in one mole of an element Avogadro’s number = 6.022 x 1023 particles (atoms or molecules) Avogadro as a conversion factor 0.30 mol F = _________ atoms F 0.30 mol F 6.022 x 1023 atoms F 1 mol F Molar Mass Mass in grams of 1 mole of an element Units are g/mol Molar mass of 1 mole = atomic mass Copper (Cu) atomic mass = 63.55 amu Therefore molar mass = 63.55 g/mol Molar mass as a conversion factor 3.50 mol Cu = _________ g Cu 3.50 mol Cu 63.55 g Cu 1 mol Cu Molar mass as a conversion factor 2 step conversions – 3g Cu = _________ atoms Calculating compounds – molar mass of H2O? 2 step conversions with compounds – 4g H2O = _____ atoms Atomic Nuclei Review Nucleus made of protons & neutrons Isotope – nucleus with same # of protons & different # neutrons Why don’t nuclei repel each other? 1935 Yukawa – attractive force between neutrons & protons stronger than proton repulsion Only works when particles very close together Not as strong in large nucleus Spontaneous Nuclear Change AKA transmutation All to increase stability 1. Fission – large nucleus splits 2. Fusion – small nuclei combine 3. Radioactive decay – release of particles Radioactive Decay 1. Convert neutrons to protons Emission of high energy negative particles called BETA PARTICLES If a neutron loses its negative charge, it becomes a proton Increases atomic number, no change to mass number Radioactive Decay 1. Convert neutrons to protons Example 234 90 Th 234 Pa 91 + 0 -1 β Radioactive Decay 2. Convert protons to neutrons Nucleus captures an electron If a proton combines with an electron, it forms a neutron Atomic number decreases by 1, no change in mass number 238 92 U 238 91 Pa Radioactive Decay 3. Losing ALPHA PARTICLES An alpha particle is a positive particle identical to a helium-4 4 nucleus ( 2 He) Example 238 92 U 234 90 4 Th + He 2 Radioactive Decay Also capable of producing a high energy photon called GAMMA RAY EMISSION Does not change nucleus Types of Radiation ALPHA BETA GAMMA Symbol How change the nucleus Penetration Low Medium High Shielding provided by Skin Paper, clothing Lead Danger Low Medium High Nuclear reactors Uranium atoms split (fission) and releases neutrons Neutrons trigger more uranium to split nuclear chain reaction Boron absorbs neutrons so is used to control the speed of the chain reaction Fission products like iodine & cesium also produce alpha, beta, & gamma particles to stabilize Nuclear reactors Heat must be removed to allow decay to continue (decay = stabilization) so cooling systems are vital Fukushima, Japan Power failed from earthquake Diesel generators failed from tsunami Release of radioactive isotopes of cesium & iodide Half Life Rate of decay of a radioactive sample Constant – not influenced by temperature, pressure, etc. Often used to determine age – RADIOACTIVE DATING Half Life Most Carbon on Earth is C-12 All animals have C-14 from plants All living plants & animals have fixed ratio of C-14 to C-12 Once dead, C-14 levels decrease Measure ratio & compare to known Half-Life Example C-14 half life = 5715 years You find an artifact with a C-14:C-12 ratio that is 1/8 the modern ratio. How old is the artifact? Half-Life Example 1. Determine the number of half- lives that the artifact has undergone. 2. Multiply the # of half-lives by the length of the half-life Half-Life Example Ratio is one eighth the modern ratio 1 ½ ½ ¼ ¼ 1/8 3 half lives 5715 years x 3 half lives =17,145 years old More Half-Life Examples The half-life of Ra-226 is 1599 years. How many years are needed for the decay of 15/16 a given amount? Assume the half-life of a substance is 3.824 days. How much time will it take for ¼ of a sample to remain? More Half-Life Examples Assume the half-life of a substance is 3.0 minutes. How long will it take for 16 mg to become 1.0 mg?