Engineering Materials

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CHAPTER 2

ATOMIC STRUCTURE AND INTERATOMIC BONDING

PROBLEM SOLUTIONS

Fundamental Concepts Electrons in Atoms

2.1 Cite the difference between atomic mass and atomic weight.

Solution Atomic mass is the mass of an individual atom, whereas atomic weight is the average (weighted) of the atomic masses of an atom's naturally occurring isotopes.

2.2 Silicon has three naturally occurring isotopes: 92.23% of 28Si, with an atomic weight of 27.9769 amu, 4.68% of 29Si, with an atomic weight of 28.9765 amu, and 3.09% of 30Si, with an atomic weight of 29.9738 amu. On the basis of these data, confirm that the average atomic weight of Si is 28.0854 amu.

Solution The average atomic weight of silicon is computed by adding fraction-of-occurrence/atomic weight products for the three isotopes—i.e., using Equation 2.2. (Remember: fraction of occurrence is equal to the percent of occurrence divided by 100.) Thus

2.3 Zinc has five naturally occurring isotopes: 48.63% of 64Zn with an atomic weight of 63.929 amu; 27.90% of 66Zn with an atomic weight of 65.926 amu; 4.10% of 67Zn with an atomic weight of 66.927 amu; 18.75% of 68Zn with an atomic weight of 67.925 amu; and 0.62% of 70Zn with an atomic weight of 69.925 amu. Calculate the average atomic weight of Zn.

Solution The average atomic weight of zinc is computed by adding fraction-of-occurrence—atomic weight products for the five isotopes—i.e., using Equation 2.2. (Remember: fraction of occurrence is equal to the percent of occurrence divided by 100.) Thus

Including data provided in the problem statement we solve for as

= 65.400 amu

2.4 Indium has two naturally occurring isotopes: 113In with an atomic weight of 112.904 amu, and 115In with an atomic weight of 114.904 amu. If the average atomic weight for In is 114.818 amu, calculate the fraction-of-occurrences of these two…...

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