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Isotopes & Mass Spectrometry Unit 10 Thank You Scientists! We now have a modern model of the atom!!! Atom Neutral Nucleus Outer Area Center Protons Positive Charge Neutron No Charge Electrons Negative Charge Atomic Mass Unit (AMU) g/mol Atomic number = # of protons The # of protons defines the element. The amount of electrons and neutrons in an element may change, BUT if the number of protons changes, it becomes a new element. In a neutral atom, proton = electrons Atomic mass = protons + neutrons What information do these numbers give us about aluminum? X = symbol of the element. A = atomic or molar mass of the element. Z = atomic number or the number of protons. Elements are defined by the number of protons in the nucleus: Hydrogen has proton. Boron has protons. Thallium has protons. What else contributes to the mass of the of the element? Neutrons! Neutrons contain pretty much the same amount of mass as protons; electrons contain very little mass (small enough that we can ignore their mass) How would we determine the number of neutrons? How would we determine the number of electrons for a neutral atom of this element? What would indicate that the atom is not neutral? 2+ How many electrons does this ion have? Isotopes Isotopes are atoms of the same element (same # protons) with a different # of neutrons. Therefore they have different atomic masses. Because isotopes have different atomic masses, this is usually indicated in the name. Example Hydrogen-2 This isotope has a mass of 2 g/mol. The atomic mass of an element is a weighted average based on relative or percent abundance of all of the isotopes that exist for that element. Example of Calculating Atomic Mass You are given a sample containing 98% carbon12 and 2% carbon-13. What is the atomic mass of the element? 98% of the atomic mass will come from carbon-12. 12 x 0.98 = 11.76 g/mol Carbon-12 will contribute 11.76 g/mol 2% of the atomic mass will come from carbon-13. 13 x 0.02 = 0.26 g/mol Carbon-13 will contribute 0.26 g/mol The atomic mass of the sample is found by adding the contribution of each isotope’s weighted mass. 11.76 g/mol + 0.26 g/mol = 12.02 g/mol How do we measure the relative abundance of isotope? Imagine: You are pushing a shopping cart filled to the max with groceries (heavy ones). You are going to be pretty late for your party if you don’t hurry. A little girl jumps out at the end of the aisle and you have to turn sharply to miss her Would the situation have been different if your buggy was not quite so full? How easily you can change the direction of your movement is affected by how much mass you have. Isotopes have different masses! Maybe we can use this to determine the mass of different isotopes! So how does it work with atoms: We use an instrument called a Mass Spectrometer It measures the atomic mass and the relative abundance of each isotope in a sample. Step 1: Ionization First we need to charge our atoms. What do we call a charged atom? Ions with +1 Charge Step 2: Acceleration The ions are then accelerated so that they are traveling at the same speed. Step 3: Deflection A magnetic force is applied to the ions traveling through the tube. The smaller the mass of the atom, the more the atom is deflected. + Ions with +1 Charge - Step 4: Detection What is the molar mass of the isotope 27 g/mole represented by spectrum A? What are the name and atomic symbol of element A? What are the symbols, including superscripts and subscripts for the isotopes in spectrum B? Based on the experimentally obtained values of atomic mass and percent abundance, calculate the average molar mass of this element. Show your work. (0.787 x 24 ) + (0.103 x 25) + (0.112 x 26) = 24.4 grams/mole