Transcript Isotope
All about Isotopes Mrs. Pittaluga – 8th Grade Science Atom Review • Atoms – basic building block of matter – Protons • Identifies the element (atomic number) • Positive charge • Found in the nucleus – Electrons • Determine reactivity of the element • Negative charge • Found in the electron cloud with relatively no mass – Neutrons • No charge • Found in the nucleus • Counts for part of the mass of the atom Changing Particles If you change the It will affect the… number of… And the result will be a… Protons Identity of the atom A completely different atom Electrons Charge Ion Neutrons Mass Isotope Atomic Mass • The mass of an atom is found entirely in the nucleus • Value is given as “amu”, which means atomic mass units • Decimal number because it is an AVERAGE of the all the isotopes of the element • Isotope is a different version of the same element What is an Isotope? • An atom with the same number of protons and a different number of neutrons • Atomic mass will be slightly different between isotopes due to the change in neutrons • All elements have isotopes – Some occur more abundantly than others – Some occur naturally and some are created in a lab • Atomic mass is called a “weighted average” to account for the difference in abundance Representing Isotopes • Elements are represented using a chemical symbol. – For example: C = carbon; He = helium; Pb = lead – First letter is always capital and second is lower case • 2 ways to represent an isotope – Element name along with the mass number – Chemical symbol along with the mass number and atomic number Some Examples Element Name • Carbon-14 – A neutral carbon atom has 6 protons and 6 neutrons – Carbon-14 has 6 protons and 8 neutrons Chemical Symbol Calculating Atomic Mass • What is a weighted average? – Not calculated the same as a regular average – Need to know the mass of each isotope – Need to know the percent abundance • Percent abundance is how often that particular isotope occurs in a sample of an element • The formula: – (Mass of Isotope #1)(Percent abundance) + (Mass of Isotope #2)(Percent abundance) + (Mass of Isotope #3)(Percent abundance) +… – The percent values should ALWAYS be converted to a decimal first (i.e. 90% = .90, 74.9% = .749) Let’s Practice! • Silver is found to have two stable isotopes, one has an atomic mass of 106.904 amu and the other mass is 108.905 amu. The first isotope represents 51.82% of the mass of the element and the second represents 48.18%. What is the atomic mass of the element silver? – Using the formula for a weighted average, we simply plug in the values: – (106.904 amu)(.5182) + (108.905 amu)(.4818) = ??? • Atomic mass = 107.868 amu •You can check your answer by checking the periodic table More Practice • Calculate the atomic mass of chromium using the following table: Isotope Mass % Abundance #1 49.946046 4.35 #2 51.940510 83.79 #3 52.940651 9.50 #4 53.938882 2.36 • Did you get 51.996? Does this answer match the periodic table? One More! How to Solve • The problem is set up the same way but in place of abundance in the formula, we are going to us “x”. – – – – – – – – – – – Rubidium-85 is calculated as (84.9118 amu)(x) Rubidium-87 is calculated as (86.9092 amu)(1-x) Set the whole equation equal to the average atomic mass: (84.9118 amu)(x) + (86.9092 amu)(1-x) = 85.47 84.9118x + 86.9092 – 86.9092x = 85.47 -1.9974x + 86.9092 = 85.47 -1.9974x = -1.4392 x = .7205 Substitute the value for x into 1-x Rubidium-85 is 72.05% abundance and Rubidium-87 is 27.95% abundant Do those abundance values make sense?