Isotopes and Calculating Atomic Mass

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Presentation transcript:

Isotopes and Calculating Atomic Mass

Elements are made of billions of atoms Remember: an element is made of billions of tiny atoms. They all look alike (same #p) They do not weigh alike (different mass#) Element:

12 12 14 8 12 12 12 12 14 8 12 12 12

Isotopes – Multiple atoms of the same element that have different mass numbers. Carbon – 12 Carbon – 14 © Addison-Wesley Publishing Company, Inc.

Isotopes All elements have isotopes meaning all elements are made of tiny atoms that have different masses.

12 12 14 8 12 12 12 12 14 8 12 12 98% Carbon-12 2% Carbon-14 12

What is Atomic Mass? Atomic mass: the weighted average mass of ALL the isotopes of that element. This number is located on the bottom of each block on the periodic table Not a whole number Different from the Mass #

Mass # vs. Average Atomic Mass Mass # = Individual grade in class - you have many (quiz, test, classwork, etc) Average Atomic Mass = Average grade in May - you have one grade for Chemistry Average Grade HW Quiz Test Lab Quiz

Average Atomic Mass This is a piece of carbon that contains hundreds of billions of atoms, all with different masses. Carbon-12 is found in 98% of the sample. Carbon-14 is found in 2% of the sample. What is the average atomic mass of carbon?

Steps for Calculating Atomic Mass Change percent to decimal Multiply the decimal by the mass # of that isotope Then add the two sums together. Answer must have units 98% = .98 2% = .02 .98 x 12 = 11.76 .02 x 14 = 0.28 11.76 + 0.28 = 12.04 12.04 amu

Determine the atomic mass of the following mixture of isotopes 80% 127 I, 17% 126 I, 3% 128 I 50% 197Au, 50% 198 Au 99% 1 H, 0.8% 2 H, 0.2% 3 H 95% 14 N, 3% 15 N, 2% 16 N

Practice – always use mass with more sig figs *The actual mass of a proton 1.007 and neutron 1.008 Isotope Mass (amu) Neon – 20 19.992 Neon – 21 20.994 Neon - 22 21.991 Practice – always use mass with more sig figs 35 atoms of neon were individually weighed – the following information was found. What is the average atomic mass of Neon? Isotope Quantity Mass (amu) Neon – 20 31 19.992 Neon – 21 1 20.994 Neon - 22 3 21.991