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Isotopes and Mass Number Isotope Atoms of the same element with: Same number of protons BUT Different number of neutrons ELEMENT IS TO ISOTOPE AS DOG IS TO DOG BREED Mass Number: Mass of an atom FLASHBACK: Protons are what animal? Neutrons are what animal? Electrons are what animal? Mass # = mass of the nucleus Nucleus = protons + neutrons Therefore: mass# = protons + neutrons Nomenclature Carbon - 14 Mass Number 14 6 Atomic Number C Practice 1. Carbon – 12 12 6 6 C Oxygen - 17 2. 17 8 9 O Draw Diagram p+ en0 6 Mass # 12 p+ 8 e- 8 n0 9 Mass # 17 6 6 Calculating Atomic Mass Atomic Mass is the average of all an element’s masses You need two values Mass of each isotope % abundance of each isotope Percent abundance is how much of the isotope is represented in the world Example: The following is data for the unknown element X. For element 6 X mass = 6.015 abundance = 7.50% For element 7 X mass = 7.016 abundance = 92.5% Atomic Mass Chart Isotope Mass (amu) % Abundance X-6 6.015 x 7.50 45.1 X-7 7.016 x 92.5 649 + 694 100 = 6.94 amu Find the atomic mass on the periodic table What is element X? 6.94 amu The element is Lithium!