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Determining an Isotope
Mass Numbers
• Atomic Mass=Protons + Neutrons
• The identity is based on the number of protons.
• Given: Iodine-131, Iodine-129, Iodine-127
• Let’s determine the number of neutrons.
Protons
• Where is Iodine on the periodic table?
• Element number 53.
• All Iodine isotopes have the same exact number of protons, 53.
• All Iodine isotopes also have 53 electrons.
Neutrons
• Let’s determine the number of neutrons.
• First, Iodine 131.
• 131 is protons + neutrons.
• 131-53 protons=78 neutrons.
• Iodine 130.
• 130-53=77 neutrons
• Iodine 127
• 127-53=74 neutrons
Finding the number of protons
• Given a substance with a mass of 42 amu.
• We are told this atom has 22 neutrons.
• What is the identity?
• Take the total mass of 42. This is 42 protons and neutrons.
• 42-22 neutrons=20 protons
How to solve for the identity
• The atomic number is also the same as the number of protons.
• The atomic number is like a permanent address. No matter what you do to the house, it will always have the same foundation (protons) and always have the same location (atomic number).
• 20 protons=Atomic # 20, Calcium