Download Chapter 3: Atoms: The Building Blocks of Matter

The Lion and the Gazelle
Every morning in Africa, a gazelle
wakes up. It knows it must run
faster than the fastest lion or it
will be killed.
Every morning a lion wakes up. It
knows it must outrun the
slowest gazelle or it will starve
to death.
It doesn't matter whether you are a
lion or a gazelle. When the sun
comes up, you better start
running.
-African Proverb
Section 3-3
Counting Atoms
Coach Kelsoe
Chemistry
Pages 77–87
Atomic Number


Atoms of different elements
have different numbers of
protons.
The atomic number of an
element is the number of
protons in the nucleus of
each atom of that element.
Atomic Number



Atoms of hydrogen have 1
proton; atoms of helium have
2 protons, and so on.
The atomic number identifies
an element.
Since atoms are neutral, an
atom will have the same
number of protons as
electrons.
Isotopes




Isotopes are atoms of the same element that
have different masses.
Isotopes will have the same number of
protons and electrons, but will differ in the
number of neutrons.
Tin has the most isotopes (10)
For example, Hydrogen has 3 isotopes:



Protium- one proton, no neutrons
Deuterium- one proton, one neutron
Tritium- one proton, two neutrons.
Mass Number

The mass number of
an atom is the total
number of protons and
neutrons in the nucleus
of an isotope.
Designating Isotopes


Only hydrogen has different names for its
isotopes.
There are two ways we designate isotopes:



Hyphen notation: hydrogen-1, uranium-235
Nuclear symbol: mass # top, atomic # 11H, 235
92 U
We find the number of neutrons by
subtracting the atomic number from the mass
number- Ex: 235
92 U  235-92=143 neutrons
Sample Problem

How many protons, neutrons,
and electrons are found in an
atom of Helium-4?


Protons-2, Neutrons-2,
Electrons-2
How many protons, neutrons,
and electrons are found in an
atom of 120
50Sn?

Protons-50, Neutrons-70,
Electrons-50
Nuclides

Nuclide is a general
term for any isotope
of any element.

Nuclides for
hydrogen are
protium, deuterium,
and tritium.
Relative Atomic Masses



Masses of atoms in
grams are very small.
When comparing
masses of atoms, we
use atomic mass
units.
An atomic mass unit is
equal to 1/12 the
mass of a carbon-12
atom.
Average Atomic Masses


Average atomic mass is the weighted
average of the atomic masses of the naturally
occurring isotopes of an element.
To calculate average atomic mass, we
multiply the mass of each isotope by the
decimal fraction representing its percentage
in the mixture.
Sample Calculation

If we had 3 red styrofoam balls
with a mass of 4.0 grams and 5
black styrofoam balls with a
mass of 5.0 grams, what would
it’s average mass be?



(3/8)(4.0 g)  (.375)(4) = 1.5 g
(5/8)(5.0 g)  (.625)(5) = 3.125 g
1.5 g + 3.125 g = 4.625 g average
Bellringer

List how many protons, neutrons, and electrons
are in each of the following atoms.

Nickel-60

31P
15

Barium-138

201Hg
80

Krypton-86

42Ca
20

Nitrogen-14

184W
74
Relating Mass to Numbers of Atoms

There are three
important concepts
that provide the
basis for relating
masses in grams to
numbers of atoms:



The mole
Avogadro’s number
Molar mass
The Mole



The mole is the SI unit for
amount of substance.
A mole is the amount of a
substance that contains
as many particles as
there are atoms in exactly
12 grams of carbon-12.
The mole is a counting
unit, like dozens.
Not a real mole!
Avogadro’s Number


The number of atoms in a mole has been
determined to be 6.0221367 x 1023.
Avogadro’s number – 6.022 x 1023 – is the
number of atoms in exactly one mole of a
pure substance.
Molar Mass



The mass of one mole of a pure substance is
called the molar mass of that substance.
Molar mass is written in g/mol and is
numerically equal to the atomic mass of the
element.
Even though the numbers are the same,
molar mass and atomic mass aren’t exactly
the same. Molar mass is measured in g/mol,
atomic mass is measured in amu or g.
Gram/Mole Conversions

We can make a conversion factor to convert
units from grams to moles and vice versa.
For example:
Sample Problems

What is the mass in grams of 2.85 mol of the
element potassium, K?
2.85 mol K x 39.10 g K = 111 g K
1 mol K

What is the mass in grams of 5.10 mol of the
element oxygen, O?
5.10 mol O x 16.00 g O = 81.6 g O
1 mol O
Sample Problems

How many moles of iron, Fe, are in 7.00 g of
iron?
7.00 g Fe x 1 mol Fe = 0.125 mol Fe
55.85 g Fe

How many moles of gold, Au, are in 234.0 g
of gold?
234.0 g Au x 1 mol Au = 1.188 mol Au
196.97 g Au
Conversions with Avogadro’s Number


We can make a conversion factor to convert
units from moles to atoms and vice versa.
Here’s a GREAT conversion factor:
- 11 National
Championships + 5
NCAA probations x
smelly cow college =
Sample Problems

How many moles of sulfur, S, are in 89700
atoms of sulfur?
89700 atoms x 1 mol S =
6.022 x 1023 atoms

How many moles of sodium, Na, are in 6.2 x
104 atoms of sodium?
6.2 x 104 atoms x 1 mol Na =
6.022 x 1023 atoms
Sample Problems

How many atoms of lead, Pb, are in 0.325
mol of lead?
0.325 mol x 6.022 x 1023 atoms =
1 mol

How many atoms of fluorine, F, are in 0.018
mol of fluorine?
0.018 mol x 6.022 x 1023 atoms =
1 mol