Ch.2 - Atoms, Molecules & Ions

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.2 - Atoms, Molecules & Ions

Problem 122

Chapter 2, Problem 122

Copper metal has two naturally occurring isotopes: copper-63 (69.15%; isotopic mass = 62.93) and copper-65 (30.85%; isotopic mass 64.93). Calculate the atomic weight of copper, and check your answer in the periodic table.

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Identify the isotopes of copper and their respective abundances and isotopic masses: copper-63 with 69.15% abundance and isotopic mass of 62.93, and copper-65 with 30.85% abundance and isotopic mass of 64.93.

Convert the percentage abundances into decimal form by dividing each by 100. For copper-63, it becomes 0.6915, and for copper-65, it becomes 0.3085.

Calculate the contribution of each isotope to the atomic weight by multiplying the isotopic mass by its decimal abundance. For copper-63, multiply 62.93 by 0.6915, and for copper-65, multiply 64.93 by 0.3085.

Add the contributions from each isotope to find the average atomic weight of copper.

Compare the calculated atomic weight with the value listed in the periodic table to verify accuracy.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Isotopes

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. For example, copper has two isotopes, copper-63 and copper-65, which differ in their mass due to the varying number of neutrons. Understanding isotopes is crucial for calculating the average atomic weight of an element based on its isotopic composition.

Atomic Weight Calculation

The atomic weight of an element is calculated as the weighted average of the masses of its isotopes, taking into account their relative abundances. This is done by multiplying the mass of each isotope by its percentage abundance (expressed as a fraction) and summing these values. For copper, the calculation involves using the isotopic masses and their respective percentages to find the overall atomic weight.

Periodic Table

The periodic table is a tabular arrangement of chemical elements, organized by increasing atomic number and grouped by similar properties. Each element's atomic weight is listed, which reflects the average mass of its naturally occurring isotopes. Checking the calculated atomic weight against the periodic table helps verify the accuracy of the calculation and provides a reference for understanding the element's characteristics.

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Periodic Table Classifications

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Textbook Question

Naturally occurring silver consists of two isotopes: 107^Ag (51.84%) with an isotopic mass of 106.9051 and 109^Ag (48.16%) with an isotopic mass of 108.9048. What is the atomic weight of silver? Check your answer in a periodic table.

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Textbook Question

Magnesium has three naturally occurring isotopes: ²⁴Mg (23.985) with 78.99% abundance, ²⁵Mg (24.986) with 10.00% abundance, and a third with 11.01% abundance. Look up the atomic weight of magnesium, and then calculate the mass of the third isotope.

A sample of naturally occurring silicon consists of 28^Si (27.9769), 29^Si (28.9765), and 30^Si (29.9738). If the atomic weight of silicon is 28.0855 and the natural abundance of 29^Si is 4.68%, what are the natural abundances of 28^Si and 30^Si?

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Textbook Question

Germanium has five naturally occurring isotopes: 70^Ge, 20.5%, 69.924; 72^Ge, 27.4%, 71.922; 73^Ge, 7.8%, 72.923; 74^Ge, 36.5%, 73.921; and 76^Ge, 7.8%, 75.921. What is the atomic weight of germanium?

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