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Concept Check 2.1
Atomic Size
PROBLEM 2.1: The diameter of a US dime is 17.9 mm, and the diameter of
a silver atom is 2.88 Å. How many silver atoms could be arranged side by side
across the diameter of a dime?
(NOTE: 1 Å = 10-10 m)
SOLUTION:
17.9 mm 10-3 m 1 A 1 Ag atom
7
Ag atoms =
=
6.22x10
Ag atoms
-10
1 mm 10 m 2.88 A
Concept Check 2.2
Atomic Size
PROBLEM 2.2: The diameter of a carbon atom is 1.54 Å. (a) Express this
diameter in picometers. (b) How many carbon atoms
could be aligned side by side across the width of a pencil line that is 0.20 mm
wide?
Concept Check 2.3 & 2.4
Determining the Number of Subatomic Particles in Atoms
PROBLEM 2.3: How many protons, neutrons, and electrons are in (a) an
atom of 197Au, (b) an atom of strontium-90?
SOLUTION:
(a) atomic number = protons = 79
neutral atom, protons = electrons = 79
mass number – protons = neutrons = 197 – 79 = 118
(b) atomic number = protons = 38
neutral atom, protons = electrons = 38
mass number – protons = neutrons = 90 – 38 = 52
PROBLEM 2.4: How many protons, neutrons, and electrons are in (a) a
138Ba atom, (b) an atom of phosphorus-31?
Concept Check 2.5 & 2.6
Writing Symbols for Atoms
PROBLEM 2.5: Magnesium has three isotopes with mass numbers 24, 25,
and 26. (a) Write the complete chemical symbol (superscript and subscript)
for each. (b) How many neutrons are in an atom of each isotope?
SOLUTION:
24
12
Mg
25
12
Mg
26
12
Mg
neutrons = mass number – protons
Mg-24 12 neutrons
Mg-25 13 neutrons
neutrons
Mg-26 14
PROBLEM 2.6: Give the complete chemical symbol for the atom that
contains 82 protons, 82 electrons, and 126 neutrons.
Concept Check 2.7
Calculating the Atomic Weight of an Element from Isotopic
Abundances
PROBLEM 2.7: Naturally occurring chlorine is 75.78% 35Cl (atomic mass
34.969 amu) and 24.22% 37Cl (atomic mass 36.966 amu).
Calculate the atomic weight of chlorine.
SOLUTION:
Atomic weight = (0.7578)(34.969 amu) + (0.2422)(36.966 amu)
= 26.50 amu + 8.953 amu
= 35.45 amu
Concept Check 2.8
Calculating the Atomic Weight of an Element from Isotopic
Abundances
PROBLEM 2.8: Three isotopes of silicon occur in nature: 28Si (92.23%),
atomic mass 27.97693 amu; 29Si (4.68% ), atomic mass 28.97649 amu; and
30Si (3.09%), atomic mass 29.97377 amu. Calculate the atomic weight of
silicon.
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