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Chapter 3
Atoms: The Building
Blocks of Matter
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Objectives
•Define the terms atomic mass and molar mass.
•Define the terms mole and Avogadro’s number.
•Write the name for common elements, given the symbol,
or the symbol, given the name.
•Calculate the molar mass of an element or compound,
given its formula.
•Calculate the mass of an element or compound given the
number of moles, or the number of moles of a given
mass of an element or compound.
•Calculate the number of atoms or molecules of an
element or compound given the number of moles, or the
number of moles given the number of atoms or
molecules.
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Chapter 3
Section 1
The Atom
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The modern definition of an element is a
substance that cannot be further broken
down by ordinary chemical means – H, C, O
Elements also combine to form compounds
that have different physical and chemical
properties than those of the elements that
form them – H2O.
The transformation of a substance into one
or more new substances is a chemical
reaction.
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Relative Atomic Masses
Masses of atoms expressed in grams are
very small and not useful. (O: 2.66 x 10-23 g)
It is more convenient to use relative atomic
masses.
Therefore the standard used to govern units
of atomic mass is the carbon-12 atom.
It has been assigned a mass of exactly
12 atomic mass units, or 12 amu.
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The atomic mass of any other atom is
determined by comparing it with the mass
of the carbon-12 atom.
Examples:
The hydrogen atom has an atomic mass of
1/12 that of the carbon-12 atom or 1 amu.
Oxygen has an atomic mass of 16/12 the
mass of a carbon-12 atom or 16 amu.
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Relating Mass to Numbers of
Atoms
Introduction of three very important concepts:
1) The mole
2) Avogadro’s number
3) Molar mass
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The Mole
Mole – the amount of carbon atoms that
are in exactly 12 g of carbon.
One mole of carbon weighs 12 grams.
The mole is the SI unit for amount of
substance. It is a counting unit.
Mole is related to the counting term dozen.
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What is a counting unit?
You’re already familiar with one counting unit…a
“dozen”
A dozen = 12
“Dozen”
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12
A dozen doughnuts
12 doughnuts
A dozen books
12 books
A dozen cars
12 cars
A dozen people
12 people
A Mole of Particles
Contains 6.02 x 1023 particles
Avogadro’s Number
1 mole C = 6.02 x 1023 C atoms
1 mole H2O = 6.02 x 1023 H2O molecules
1 mole NaCl= 6.02 x 1023 NaCl molecules
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Avogadro’s Number – 6.02 x 1023
is the number of particles in exactly one
mole of a pure substance.
1 mole of gold = 6.02 x 1023 particles
1 mole of uranium = 6.02 x 1023 particles
1 mole of water = 6.02 x 1023 particles
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How big is a mole?
Enough soft drink cans to cover the surface
of the earth to a depth of over 200 miles.
If we were able to count atoms at the rate of
10 million per second, it would take about 2
billion years to count the atoms in one
mole.
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Molar Mass
Molar Mass – The mass (in grams) of one
mole of a pure substance.
Molar masses are written in units g/mol.
The molar mass of an element is equal to
the atomic mass of the element.
Look on the periodic table for the atomic
masses.
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Other terms commonly used for
the same meaning of molar mass:
Molecular Weight
Molecular Mass
Formula Weight
Formula Mass
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Molar Mass
A molar mass of an element contains one
mole of atoms.
1mole = 4.00 g helium = 6.02 x 1023 atoms.
1mole = 6.94 g lithium = 6.02 x 1023 atoms.
1mole = 200.6 g mercury = 6.02 x 1023
atoms.
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Molar Mass
Examples:
Molar mass of oxygen (O) = 15.99 g/mol
Molar mass of iron (Fe) = 55.85 g/mol
Molar mass of gold (Au) = 197 g/mol
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One mole of carbon (12 grams) and
one mole of copper (63.5 grams)
Both contain 6.02 x 1023 atoms
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Molar Mass for Compounds
The molar mass for a compound = the
sum of the molar masses of all the
elements in the compound.
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Molar Mass
A molar mass of a compound is the sum
of the molar masses of the elements.
Example: Water, H2O:
2 H = 2 x 1g/mole = 2g/mole
1 O = 1 x 16g/mole = 16g/mole
molar mass of H20 =18g/mol
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Molar Mass
A molar mass of a compound is the sum
of the molar masses of the elements.
Example: methane, CH4:
4 H = 4 x 1 g = 4 g/mole
1 C = 1 x 12 g = 12 g/mole
molar mass of CH4 =16 g/mol
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Example: Molar Mass &
Parenthesis
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Sr(NO3)2
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Example
Example:
Find the
molar
mass for
Al(OH)3
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Homework
Worksheet C.5 – molar masses of
compounds
Due:
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Gram/Mole Conversions
How many roses are in 3 ½ dozen
roses?
Relationship: 1 dozen roses = 12 roses
3.5 dozen x 12 roses =
1 dozen
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42 roses
Gram/Mole Conversions
Molar masses can be used as a conversion
factor in chemical calculations.
Example: The molar mass of helium is 4.00
g/mol. How many grams of helium are in 2
moles of helium:
amount of He in moles
amount of He in grams
4.00 g He
2.00 mol He x 1 mol He
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= 8.00 g He
Gram/Mole Conversions
Molar masses can be used as a conversion
factor in chemical calculations.
Amount in moles x molar mass (g/mol) = mass in grams
Example: What is the mass in grams of
2.50 mol of oxygen gas?
2.50 mol O2 x
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32.00 g O2
1 mol O2
= 80.0 g O2
Gram/Mole Conversions
A chemist produced 11.9 g of aluminum, Al.
How many moles of aluminum were
produced?
mass of Al in grams
1 mol Al
11.9 g Al x
27 g Al
27
amount of Al in moles
= 0.44 mol Al
Gram/Mole Conversions
How many moles are present in 352 g of
iron(III) oxide, Fe2O3?
mass of Fe2O3 in grams
amount of Fe2O3 in moles
Calculate the molar mass of Fe2O3 = 160 g/mol
352 g Fe2O3 x 1 mol Fe2O3 = 2.2 mol Fe2O3
160 g
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Gram/Mole Conversions
Review sample problem – page 82
Practice problems – Top of page 83, 1-4
(worksheet)
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Gram/Mole Conversions
Practice problems – page 83 (bottom), 1-2
(worksheet)
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Classwork
Worksheet: C-7
Grams to mole conversions
Moles to gram conversions
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Homework
Worksheet: C-13
(skip)
Grams to mole conversions
Moles to gram conversions
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Atoms / Gram Conversions
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Atoms/Molecules and Grams
Since 6.02 X 1023 molecules = 1 mole
AND
1 mole = molar mass (grams)
You can convert atoms/molecules to
moles and then moles to grams! (Two
step process)
You can’t go directly from atoms to
grams!!!! You MUST go thru MOLES.
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Calculations
molar mass
Grams
Avogadro’s number
Moles
atoms
Everything must go through
Moles!!!
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Atoms/Molecules and Grams
How many atoms of Cu are
present in 35.4 g of Cu?
35.4 g Cu
1 mol Cu
63.5 g Cu
6.02 X 1023 atoms Cu
1 mol Cu
= 3.4 X 1023 atoms Cu
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Problem
How many atoms of K are present in
78.4 g of K?
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Atoms/Molecules and Grams
How many atoms of K are present
in 78.4 g of K?
78.4 g K
1 mol K
39.0 g K
6.02 X 1023 atoms K
1 mol K
= 12.1 X 1023 atoms K
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Problem
What is the mass (in grams) of 1.20 X
108 atoms of copper (Cu)?
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Problem
What is the mass (in grams) of 1.20 X
108 atoms of copper (Cu)?
1.20 x 108 atoms
1 mole Cu
63.5 g Cu
6.02 x 1023 atoms 1 mole Cu
= 1.27 x 10-14 grams copper
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Problem
What is the mass (in grams) of 1.20 X
1024 molecules of glucose (C6H12O6)?
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Problem
What is the mass (in grams) of 1.20 X
1024 molecules of glucose (C6H12O6)?
1.20 x 1024 mol.
1 mole glucose
6.02 x 1023 mol.
= 359 grams glucose
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180 g glucose
1 mole glucose
Classwork
Problems – page 88
Questions: 21, 24 (a, c, e and f),
28 (a-e)
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