A QUICK REVIEW OF THE ATOM
If you change the It will affect the…
number of…
Protons
Electrons
Neutrons
And the result
will be a…
Identity of the
atom
completely
different atom
Charge
Ion
Mass
Isotope
WHAT ARE ISOTOPES?
They are atoms of the same element that have
different numbers of neutrons.
It’s just a change in the
number of neutrons!!!
Why are you making this
complicated?
AN EXAMPLE
Hydrogen has 3 isotopes
There are 3 different “versions” of a hydrogen atom
Note how the number of
protons never change!!!
AN EXAMPLE
Hydrogen has 3 isotopes
There are 3 different “versions” of a hydrogen atom
Having more neutrons
increases the mass
AN EXAMPLE
Hydrogen has 3 isotopes
There are 3 different “versions” of a hydrogen atom
Mass number = 2
WHY DOES THE ATOMIC MASS ON THE PERIODIC
TABLE HAVE SO MANY DECIMAL PLACES?
ATOMIC MASS
1
H
1.00797
This atomic mass is the one number that best
represents the mass of all three versions of hydrogen.
ATOMIC MASS
1
H
1.00797
No atom of hydrogen anywhere in the universe actually
has this mass.
ATOMIC MASS
1
H
1.00797
This number is a “weighted” average of the masses of
every isotope of hydrogen
CALCULATING A “WEIGHTED” AVERAGE
1. First, you have to know the mass of each isotope
2. Second, you have to know the percent abundance* of
each isotope
*see next slide
PERCENT ABUNDANCE?
Percent abundance is how often that particular isotope
occurs in a sample of the element
A sample of magnesium
collected anywhere in the
universe will contain three
isotopes of magnesium.
Sample of Magnesium
PERCENT ABUNDANCE?
These are the percent abundances of each isotope of
magnesium
78.90% of this magnesium will be
the isotope named “magnesium 24”
10.00% of this magnesium will be
the isotope named “magnesium 25”
Sample of Magnesium
11.10% of this magnesium will be
the isotope named “magnesium 26”
PERCENT ABUNDANCE?
This data will be provided to you in problems or you will
be given a table where you can look it up.
78.90% of this magnesium will be
the isotope named “magnesium 24”
10.00% of this magnesium will be
the isotope named “magnesium 25”
Sample of Magnesium
11.10% of this magnesium will be
the isotope named “magnesium 26”
CALCULATING A “WEIGHTED” AVERAGE
The formula
(Mass of isotope #1)(percent abundance)
100
(Mass of isotope #2)(percent abundance)
100
(Mass of isotope #3)(percent abundance)
100
CALCULATE THE ATOMIC MASS OF MAGNESIUM
FROM THE GIVEN DATA
# protons # neutrons
Isotope 1
Mass
(amu’s)
%
abundance
12
12
23.985045
78.90%
12
13
24.985839
10.00%
12
14
25.982595
11.10%
(magnesium-24)
Isotope 2
(magnesium-25)
Isotope 3
(magnesium-26)
CALCULATING THE ATOMIC MASS OF MAGNESIUM
Using the formula for calculating a weighted average
23.985045(.7890)
+24.985839(.1000)
+25.982595(.1110)
24.306852 amu’s
Note how the %
abundances were
converted to decimals
(just divide each by 100)
Since its a “weighted”
average this number
has more influence since
this mass occurs 79% of
the time
It’s NOT just a straight
average where you add
the masses and divide
by 3
TRY ANOTHER EXAMPLE…
CALCULATE THE ATOMIC MASS OF CHROMIUM
Isotope
Mass
% abundance
#1
49.946046
4.35
#2
51.940510
83.79
#3
52.940651
9.50
#4
53.938882
2.36
CALCULATE THE ATOMIC MASS OF CHROMIUM
49.946046(0.0435)
51.940510(0.8379)
+ 52.940651(0.0950)
53.938882(0.0236)
51.995926 amu’s
VOCABULARY REVIEW
Terms associated with isotopes
VOCABULARY REVIEW
Atomic number = the number of protons
 It’s
the same as the element’s position on
the periodic table
 For example, carbon is the 6th element on the
table, so its atomic number is 6 which means
it has 6 protons (and 6 electrons if it’s a
neutral atom)
VOCABULARY REVIEW
Atomic mass = the “weighted” average
mass of all the different isotopes of an
atom
 This
is the “weird” number on the periodic
table that usually has a bunch of places past
the decimal point
VOCABULARY REVIEW
Mass number = the sum of protons + neutrons
 This
one kind of makes sense because the
electrons essentially don’t have any mass
 This HAS to be a whole number because you can’t
have part of a proton or neutron
 Mass number is NOT given on the periodic table
LET’S PRACTICE THOSE TERMS…
1.
2.
3.
4.
5.
What is the mass number of a carbon atom
that has 6 neutrons? 7 neutrons?
How many neutrons does a nitrogen atom
have if its mass number is 14?
How many neutrons does a chlorine atom
have if its mass number is 36?
What is the atomic number of neon?
What is the atomic mass of neon?
ANSWERS
1.
2.
3.
4.
5.
12 and 13
7
19
10
20.180 amu’s
AX
Z
NOTATION FOR ISOTOPES
A shorthand way to write isotopes
A SHORT WAY TO WRITE ISOTOPES
“A” is the symbol
for Mass Number
(#protons + #neutrons)
“X” will be the
symbol of the
element
AX
Z
“Z” is the
symbol for
Atomic Number
(# of protons)
JAY - Z
Yo! If you’re doin’ chem
problems I feel bad for you
son….
I got 99 problems, and here’s the
first one….HIT ME!!!!
PROBLEM #1
Write 𝐴𝑍𝑋 notation for an isotope that has 17
protons and 18 neutrons.
PROBLEM #1
Write 𝐴𝑍𝑋 notation for an isotope that has 17
protons and 18 neutrons.
35
17𝐶𝑙
PROBLEM #2
Write 𝐴𝑍𝑋 notation for an isotope that has 19
protons and 21 neutrons.
PROBLEM #2
Write 𝐴𝑍𝑋 notation for an isotope that has 19
protons and 21 neutrons.
40
 19𝐾
PROBLEM #3
Write 𝐴𝑍𝑋 notation for an isotope that has
15 protons, 15 electrons, and 16
neutrons.
PROBLEM #3
Write 𝐴𝑍𝑋 notation for an isotope that has
15 protons, 15 electrons, and 16
neutrons.
31
𝑃
15
PROBLEM #4
How many protons, electrons, and neutrons are in
this isotope?
41
Ca
2+
20
PROBLEM #4
How many protons, electrons, and neutrons are in
this isotope?
41
Ca
2+
20
20 protons, 18 electrons, 21 neutrons
MORE EXAMPLES
How many protons, electrons, and neutrons?
1.
56Fe2+
26
2.
27Al3+
13
3.
79Se2-
34
MORE EXAMPLES
How many protons, electrons, and neutrons?
1.
56Fe2+
26
26, 24, 30
2.
27Al3+
13
13, 10, 14
3.
79Se2-
34, 36, 45
34
MORE PRACTICE
Write 𝐴𝑍𝑋 notation for:
1. Isotope with 21 protons, 18 electrons, and 24
neutrons
2.
Isotope with 53 protons, 74 neutrons, and 54
electrons
MORE PRACTICE
Write 𝐴𝑍𝑋 notation for:
1. Isotope with 21 protons, 18 electrons, and 24
neutrons
45Sc3+
2.
21
Isotope with 53 protons, 74 neutrons, and 54
electrons
127I-
53
DIFFERENT FORMS OF 𝐴𝑍𝑋
 35Cl17

is the same as 3517Cl
Might also see “Cl-35” or “chlorine-35”