# Isotopes Atoms of the same element with different mass numbers. Mass # Atomic # Nuclear symbol: Hyphen notation: carbon-12 Courtesy Christy Johannesson.

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Isotopes Atoms of the same element with different mass numbers. Mass # Atomic # Nuclear symbol: Hyphen notation: carbon-12 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Isotopes + + + + + + Nucleus Electrons Nucleus Neutron Proton Carbon-12 Neutrons 6 Protons 6 Electrons6 Nucleus Electrons Carbon-14 Neutrons 8 Protons 6 Electrons6 + + + + + + Nucleus Neutron Proton

Isotopes Chlorine-37 –atomic #: –mass #: –# of protons: –# of electrons: –# of neutrons: 17 37 17 20 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Cl 37 17

Average Atomic Mass weighted average of all isotopes on the Periodic Table round to 2 decimal places Avg. Atomic Mass Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Avg. Atomic Mass Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. 16.00 amu Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Avg. Atomic Mass Average Atomic Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. 35.40 amu Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Isotopes Dalton was wrong. Atoms of the same element can have different numbers of neutrons different mass numbers called isotopes California WEB C-12 vs. C-14

Naming Isotopes Put the mass number after the name of the element carbon- 12 carbon -14 uranium-235 California WEB

Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. More concerned with average atomic mass. Based on abundance of each element in nature. Don’t use grams because the numbers would be too small California WEB

Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass we need the average from percent abundance. California WEB

Calculating averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = (4 x 50) + (1 x 60) = 260 g Average mass = (4 x 50) + (1 x 60) = 260 g 5 5 Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 California WEB

Calculating averages Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 Average mass =.8 x 50 +.2 x 60 80% of the rocks were 50 grams 20% of the rocks were 60 grams Average = % as decimal x mass + % as decimal x mass + % as decimal x mass + California WEB

Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHT Boron is 20% B-10 and 80% B-11. That is, B-11 is 80 percent abundant on earth. For boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

Atomic Mass Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium? If not told otherwise, the mass of the isotope is the mass number in amu. California WEB IsotopePercentAbundanceMass Mg-2478.9923.985018.94575 Mg-2510.0024.95852.49585 Mg-26 11.0125.98262.86068 24.304 amu

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. Cu 29 63.548 IsotopePercentAbundanceMass Cu-6369.162.9343.48463 Cu-6530.964.9320.06337 63.548

Given the average atomic mass of an element is 118.21 amu and it has three isotopes (“A”, “B”, and “C”): isotope “A” has a mass of 117.93 amu and is 87.14% abundant isotope “B” has a mass of 120.12 amu and is 12.36% abundant Find the mass of isotope “C”. Show work for credit.

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