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19.2 – Masses of Atoms Objectives – LT#5-7 Compute the mass number of an atom. Relate the atomic number to the composition of an element. Identify the components of isotopes. Atomic Mass Nucleus contains majority of mass Protons & neutrons each have a mass equal to 1 atomic mass unit (amu) In reality their mass is 1.67 x 10-24 g Electrons so small their mass is not calculated In reality their mass is 9.11 x 10-28 g Electron Proton/Neutron Atomic Properties Atomic Number = number of protons in an atom Elements on Periodic Table are arranged by atomic number Mass # = sum of protons + neutrons # Neutrons = mass # - # protons (Atomic #) Isotopes Atoms of same element that have different number of neutrons Because # neutrons is different, mass # is different Most elements have more than 1 isotope, each with a different mass Average Atomic Mass Weighted-average mass of all isotopes The # you find on the periodic table Chlorine’s Isotopes Chlorine-35 Chlorine-37 75.53% 24.47% Therefore, the average mass (atomic mass) of chlorine is 35.45. In-Class Assignment/Homework 19.1&19.2 Directed reading 19.2 Enrichment WKT #3 # of moles x mass from periodic table #4 # of grams divided by mass from periodic table