Isowhat?
Man, you trippin’ nephew.
Isotopes
If you change the It will affect the… And the result
number of…
will be a…
Protons
Electrons
Neutrons
If you change the It will affect the… And the result
number of…
will be a…
Protons
Identity of the
atom
Completely
different atom
Electrons
Charge
Ion
Neutrons
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
neutrizzle fo shizzle!!!
An Example
•
Hydrogen has 3 isotopes
–
There are 3 different “versions” of a hydrogen atom
1. 1 proton and 0 neutrons (mass number = 1)
2. 1 proton and 1 neutron (mass number = 2)
3. 1 proton and 2 neutrons (mass number = 3)
Note how the number of
protons never change!!!
Ever Wonder Why The Masses
of Atoms Are Such Weird
Numbers?
• For example, carbon’s mass is 12.01115 amu
– amu means atomic mass unit
– amu’s are a unit created specifically for measuring the
extremely small masses of atoms
• Another example, neon’s mass is 20.183 amu
Why All the Decimal Places?
• Atomic mass is the “weighted” average of
all the different isotopes of that element.
Example: Magnesium
• Has 3 isotopes
# protons
# neutrons Mass
(amu’s)
%
abundance
Isotope 1
12
12
23.985045
79%
Isotope 2
12
13
24.985839
10%
Isotope 3
12
14
25.982595
11%
% abundance??????
• Every isotope is found in nature in a fixed
percentage.
• In other words, 79% of the magnesium
atoms in any “scoop” of magnesium found
anywhere in the universe will be
magnesium atoms with 12 neutrons, 10% of
the atoms in the scoop will have 13
neutrons, and 11% will have 14 neutrons.
– These percentages are different for each
element…you just have to look them up to find
out what they are.
Calculating Atomic Masses
• For magnesium, the atomic mass is given
by…
23.985045(.79)
+24.985839(.10)
+25.982595(.11)
24.30486 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
them up and divide
by 3
The General Formula for
Calculating Atomic Masses Is…
(mass of isotope 1)(% abundance/100)
+(mass of isotope 2)(% abundance/100)
+(mass of isotope 3)(% abundance/100)
+’keep going if there are more isotopes’
O.K. nephew…try these
examples, then we can
chill
‘till the next episode…
Calculate the Atomic Mass
of Carbon
Isotope
Mass
% abundance
#1
12.000000
98.90
#2
13.003355
1.10
For Carbon…
12.000000(98.90 / 100)
+ 13.003355(1.10 / 100)
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
For Chromium…
49.946046(4.35/100)
51.940510(83.79/100)
+ 52.940651(9.50/100)
53.938882(2.36/100)
The Vocabulary Section
Terms associated with isotopes
A Few Important Terms
Ya Gotta Know
• 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)
A Few Important Terms
You Gotta Know
• 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
A Few Important Terms
You Gotta Know
• Mass number = the sum of protons + neutrons
– This one kind of makes sense because remember we
said the electrons essentially don’t have any mass
– This HAS to be a whole number!!!!!
– It has to be, because you can’t have part of a proton
or neutron
– Mass number is NOT given on the periodic table
Answers
1.
2.
3.
4.
12 and 13
7
19
10
A
ZX
Notation for Isotopes
A shorthand way to write isotopes
A Short Way to Write Isotopes
A
Z
X
“X” will be the
symbol of the
element
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
1. Write az X notation for an isotope that has
17 protons and 18 neutrons.
Problem #2
• Write azX notation for an isotope that has 19
protons and 21 neutrons.
Problem #3
• Write azX notation for an isotope that has
15 protons, 18 electrons, and 16 neutrons.
31
15
Answer: P3-
Problem #4
• How many protons, electrons, and neutrons
are in this isotope?
41
• 20
Ca2+
Answer: 20 protons, 18 electrons, 21
neutrons
Problem #5
•
How many protons, electrons, and
neutrons are in:
56Fe2+
1. 26
2.
27Al3+
3.
79Se2-
13
34
Problem #5 Answers
1. 26, 24, 30
2. 13, 10, 14
3. 34, 36, 45
Problem #6
Write the proper notation for:
1. Isotope with 21 protons, 18 electrons, and
24 neutrons
2. Isotope with 53 protons, 74 neutrons, and
54 electrons
Problem #6 Answers
1.
2.
45Sc3+
21
127I53
Different Forms of
•
35 Cl
17
AX
Z
is the same as 35Cl17
• Might also see “Cl-35” or “chlorine-35”