Weighted-average atomic mass

The weighted-average atomic mass of the element iridium is just slightly more than 192 u. The mass of the first isotope is a bit less than 191 u. Hence, the mass of the second isotope must more than 192 u that isotope must be 193 Ir. 

We use the expression for determining the weighted-average atomic mass. 

Then the expression for the weighted-average atomic mass is used, with the percent abundances converted to fractional abundances by dividing by 100. The average atomic mass of potassium is 39.0983 u. 

Using the data given by the graph, calculate the weighted average atomic mass of element X. Identify the unknown element. 

Calculating the Weighted Average Atomic Mass of Chlorine... 

To determine the weighted average atomic mass of chlorine, the mass contribution of each of the two isotopes is calculated, and then those two values are added together. 

In nature, elements come to us as nonchanging mixtures of isotopes, and each isotope has its own atomic mass. In order to have an atomic mass that is accurate for an element and accounts for the natural mixture of its isotopes, it is necessary to determine what is called a weighted-average atomic mass. This sounds complicated, but it is the best way to get exact atomic masses of the elements. 

The weighted-average atomic mass takes into account the fact that about 76% of the chlorine atoms are of the lighter isotope, Cl-35, and about 24%, the heavier. Here is how the atomic mass of chlorine is calculated, taking into account the fact that it is composed of two isotopes ... 

Consider a hypothetical element, X, that has three isotopes. Calculate the weighted-average atomic mass of the element from the following data. 

The weighted-average atomic mass equals 85.47 amu (to 4 sig. figs.). 

Background Most elements in nature occur as a mixture of isotopes. The weighted average atomic mass of an element can be determined from the atomic mass and the relative abundance of each isotope. In this activity, you will model the isotopes of the imaginary element "Snackium." The measurements you make will be used to calculate a weighted average mass that represents the average atomic mass of "Snackium."... 

Calculate Use the isotopic percent abimdance of the snack pieces and the mass to calculate the weighted average atomic mass for your element Snackium. ... 

SECTION Z4 The atomic mass scale is defined by assigning a mass of exactly 12 amu to a atom. The atomic weight (average atomic mass) of an element can be calculated from the relative abundances and masses of that elements isotopes. The mass spectrometer provides the most direct and accurate means of experimentally measuring atomic (and molecular) weights. 

Only two isotopes of copper occur naturally, Cu (atomic mass = 62.9296 amu abundance 69.17%) and (atomic mass = 64.9278 amu abundance 30.83%). Calculate the atomic weight (average atomic mass) of copper. 

Sometimes a qualitative understanding of the relationship between isotopic masses, percent isotopic abundances, and weighted-average atomic mass is all that we need, and no calculation is necessary, as illustrated in Example 2-5. Example 2-6 and the accompanying Practice Examples provide additional applications of equation (2.3). 

EXAMPLE 2-5 Understanding the Meaning of a Weighted-Average Atomic Mass... 

PRACTICE EXAMPLE A The masses and percent isotopic abundances of the three naturally occurring isotopes of silicon are Si, 27.9769265325 u, 92.223% Si, 28.976494700 u, 4.685% °Si, 29.973377017 u, 3.092%. Calculate the weighted-average atomic mass of silicon. 

Each element is listed in the periodic table by placing its symbol in the middle of a box in the table. The atomic number (Z) of the element is shown above the symbol, and the weighted-average atomic mass of the element is shown below its symbol. Some periodic tables provide other information, such as density and melting point, but the atomic number and atomic mass are generally sufficient for our needs. Elements with atomic masses in parentheses, such as plutonium, Fu (244), are produced synthetically, and the number shown is the mass number of the most stable isotope. 

The weighted-average atomic mass of carbon was calculated on page 48. 

Magnesium has three naturally occurring isotopes. Their masses are 23.985042 u, 24.985837 u, and 25.982593 u. What is the weighted-average atomic mass of magnesium in a sample for which the percent isotopic abundances of these three isotopes are 78.99%, 10.00%, and 11.01%, respectively ... 

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