Name ___________________________________ Date ____________ Period___ Guided Atomic Mass Directions: Please consult your book (pp. 78-80) to answer the questions. Earlier in the section you have read about mass numbers. Now you are going to read about atomic mass. There is a difference between mass numbers and atomic mass. You know that the mass number of an element is the mass (aka total number of protons and neutrons) for a SPECIFIC isotope of that element. The average atomic mass is the _____________________ _______________________ (1) of the atomic masses of the naturally occurring isotopes of an element. This means that the average atomic mass accounts for ALL of the isotopes of a particular element. (2) What is the difference between the mass number and the average atomic mass of an element? The average atomic mass is what is reported underneath the element symbol on the periodic table. The unit is called the atomic mass unit. 3 Atomic number Li Average atomic mass 6.941 The average atomic mass of an element depends on both the ____________________ (3) and the relative _____________________________ (4) of each of the element’s isotopes. This dependence is because the average atomic mass accounts for ALL of the isotopes of an element. You will be calculating a weighted average in order to calculate the average atomic mass. Which involves the process below. You will be given a percentage for the abundance and an atomic mass of an isotope. For example: Isotope Magnesium – 24 Magnesium – 25 Magnesium – 26 Natural Abundance (%) 78.99 10.00 11.01 Atomic Mass (amu) 23.9850 24.9858 25.9826 (5) Convert the natural abundance percentages to decimals. Fill out the blank parts of the table. Isotope Natural Decimal (Percentage Atomic Mass (amu) Abundance (%) conversion) (6) Multiply mass. Fill out the blank23.9850 parts of the table. Magnesium – 24 the decimal 78.99 by the atomic 0.7899 Magnesium – 25 10.00 24.9858 Magnesium – 26 11.01 25.9826 (6) Multiply the decimal by the atomic mass for each isotope. Fill out the blank parts of the table. Isotope Magnesium – 24 Magnesium – 25 Magnesium – 26 Natural Abundance (%) 78.99 10.00 11.01 Decimal (Percentage Atomic Mass conversion) (amu) 0.7899 23.9850 24.9858 25.9826 Decimal x atomic mass (amu) 18.94 (7) Add all of the decimal x atomic mass. Fill out the blank part of the table. Isotope Magnesium – 24 Magnesium – 25 Magnesium – 26 Natural Abundance (%) 78.99 10.00 11.01 Decimal (Percentage Atomic Mass conversion) (amu) 0.7899 23.9850 24.9858 25.9826 Total Congrats, you have calculated an average atomic mass! Decimal x atomic mass (amu) 18.94