A QUICK REVIEW OF THE ATOM
If you change the number of…
It will affect the… And the result 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
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”
Sample of Magnesium
10.00% of this magnesium will be the isotope named “magnesium 25”
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”
Sample of Magnesium
10.00% of this magnesium will be the isotope named “magnesium 25”
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
Isotope 1
(magnesium-24)
# protons # neutrons Mass
(amu’s)
12 12
23.985045
% abundance
78.90%
Isotope 2
(magnesium-25)
12 13
24.985839
10.00%
Isotope 3
(magnesium-26)
12 14
25.982595
11.10%

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
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
Note how the % abundances were converted to decimals
(just divide each by 100)

TRY ANOTHER EXAMPLE…

CALCULATE THE ATOMIC MASS OF CHROMIUM
Isotope
#1
#2
#3
#4
Mass
49.946046
51.940510
52.940651
53.938882
% abundance
4.35
83.79
9.50
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 6 th 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…
4.
5.
1.
2.
3.
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
3.
4.
1.
2.
5.
12 and 13
7
19
10
20.180 amu’s

A X
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
A X
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 a X z notation for an isotope that has 17 protons and 18 neutrons.

PROBLEM #1
Write a X z notation for an isotope that has 17 protons and 18 neutrons.
35
Cl
17

PROBLEM #2
Write a X z notation for an isotope that has 19 protons and 21 neutrons.

PROBLEM #2
Write a X z notation for an isotope that has 19 protons and 21 neutrons.
40 K
19

PROBLEM #3
Write a X z notation for an isotope that has
15 protons, 18 electrons, and 16 neutrons.

PROBLEM #3
Write a X z notation for an isotope that has
15 protons, 18 electrons, and 16 neutrons.
31
P
3-
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.
56 Fe 2+
26
2.
27 Al 3+
13
3.
79 Se 2-
34

MORE EXAMPLES
How many protons, electrons, and neutrons?
1.
56 Fe 2+
26
2.
27 Al 3+
13
3.
79 Se 2-
34
26, 24, 30
13, 10, 14
34, 36, 45

MORE PRACTICE
Write aXz 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 aXz notation for:
1.
Isotope with 21 protons, 18 electrons, and 24 neutrons
2.
45 Sc 3+
21
Isotope with 53 protons, 74 neutrons, and 54 electrons
127 I -
53

DIFFERENT FORMS OF A X
Z

35 Cl
17 is the same as 35
17
Cl

Might also see “Cl-35” or “chlorine-35”