# ENG 2002 Lecture Notes - Electronvolt, Phosphide, Electronegativity

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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.

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2.2 Chromium has four naturally-occurring isotopes: 4.34% of 50Cr, with an atomic weight of 49.9460

amu, 83.79% of 52Cr, with an atomic weight of 51.9405 amu, 9.50% of 53Cr, with an atomic weight of 52.9407 amu,

and 2.37% of 54Cr, with an atomic weight of 53.9389 amu. On the basis of these data, confirm that the average

atomic weight of Cr is 51.9963 amu.

Solution

The average atomic weight of silicon

(A

C

r

) is computed by adding fraction-of-occurrence/atomic weight

products for the three isotopes. Thus

A

Cr = f50C

r

A50C

r

+ f52C

r

A52C

r

f53C

r

A53C

r

f54C

r

A54C

r

(0.0434)(49.9460 amu) + (0.8379)(51.9405 amu) + (0.0950)(52.9407 amu) + (0.0237)(53.9389 amu) = 51.9963 am

u

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2.3 (a) How many grams are there in one amu of a material?

(b) Mole, in the context of this book, is taken in units of gram-mole. On this basis, how many atoms

are there in a pound-mole of a substance?

Solution

(a) In order to determine the number of grams in one amu of material, appropriate manipulation of the

amu/atom, g/mol, and atom/mol relationships is all that is necessary, as

# g/amu = 1 mol

6.022 1023 atoms

1 g / mol

1 amu / atom

= 1.66 10-24 g/amu

(b) Since there are 453.6 g/lbm,

1 lb - mol = (453.6 g/lb

m

) (6.022 10 23 atoms/g - mol)

= 2.73 1026 atoms/lb-mol

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