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If you change the number of…
It will affect the… And the result will be a…
Protons
Electrons
Neutrons
Identity of the atom
completely different atom
Charge Ion
Mass Isotope
They are atoms of the same element that have different numbers of neutrons.
It’s just a change in the neutrizzle fo shizzle!!!
Hydrogen has 3 isotopesThere are 3 different “versions” of a
hydrogen atom
Note how the number ofprotons never change!!!
Hydrogen has 3 isotopesThere are 3 different “versions” of a
hydrogen atom
Having more neutrons increases the mass
Hydrogen has 3 isotopesThere are 3 different “versions” of a
hydrogen atom
Mass number
= 1
Mass number = 2
Mas
s nu
mbe
r
= 3
6
C12.01115
10
Ne
20.180
13
Al26.982
1
H1.00797
This atomic mass is the one number that best represents the mass of all three versions of
hydrogen.
1
H1.00797
No atom of hydrogen anywhere in the universe actually has this mass.
1
H1.00797
This number is a “weighted” average of the masses of every isotope of hydrogen
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 is how often that particular isotope occurs in a sample of the element
Sample of Magnesium
A sample of magnesium collected
anywhere in the universe will contain
three isotopes of magnesium.
These are the percent abundances of each isotope of magnesium
Sample 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”
11.10% of this magnesium will be the isotope named
“magnesium 26”
This data will be provided to you in problems or you will be given a table where you can look
it up.
Sample 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”
11.10% of this magnesium will be the isotope named
“magnesium 26”
The formula(Mass of isotope #1)(percent abundance)
100
(Mass of isotope #2)(percent abundance)100
(Mass of isotope #3)(percent abundance)100
# protons # neutrons Mass (amu’s)
% abundance
Isotope 1(magnesium-24)
12 12 23.985045 78.90%
Isotope 2(magnesium-25)
12 13 24.985839 10.00%
Isotope 3(magnesium-26)
12 14 25.982595 11.10%
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 sincethis mass occurs 79% of
the time
It’s NOT just a straightaverage where you addthe masses and divide
by 3
Note how the %abundances were
converted to decimals(just divide each by 100)
Isotope Mass % abundance
#1 49.946046 4.35
#2 51.940510 83.79
#3 52.940651 9.50
#4 53.938882 2.36
49.946046(0.0435) 51.940510(0.8379)
+52.940651(0.0950) 53.938882(0.0236) 51.995926 amu’s
Terms associated with isotopes
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)
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
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
1. What is the mass number of a carbon atom that has 6 neutrons? 7 neutrons?
2. How many neutrons does a nitrogen atom have if its mass number is 14?
3. How many neutrons does a chlorine atom have if its mass number is 36?
4. What is the atomic number of neon?5. What is the atomic mass of neon?
1. 12 and 132. 73. 194. 105. 20.180 amu’s
A shorthand way to write isotopes
AXZ
“A” is the symbolfor Mass Number
(#protons + #neutrons)“Z” is thesymbol for
Atomic Number(# of protons)
“X” will be thesymbol of the
element
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!!!!
Write aXz notation for an isotope that has 17 protons and 18 neutrons.
Write aXz notation for an isotope that has 17 protons and 18 neutrons.
35Cl17
Write aXz notation for an isotope that has 19 protons and 21 neutrons.
Write aXz notation for an isotope that has 19 protons and 21 neutrons.
40K19
Write aXz notation for an isotope that has 15 protons, 18 electrons,
and 16 neutrons.
Write aXz notation for an isotope that has 15 protons, 18 electrons,
and 16 neutrons.
31 P3- 15
This isotope is
also an ion!!!
How many protons, electrons, and neutrons are in this isotope?
41
Ca2+
20
How many protons, electrons, and neutrons are in this isotope?
41
Ca2+
20
20 protons, 18 electrons, 21 neutrons
How many protons, electrons, and neutrons?
1. 56Fe2+26
2. 27Al3+13
3. 79Se2-34
How many protons, electrons, and neutrons?
1. 56Fe2+26 26, 24, 30
2. 27Al3+13 13, 10, 14
3. 79Se2-34 34, 36, 45
Write aXz notation for:1. Isotope with 21 protons, 18 electrons,
and 24 neutrons
2. Isotope with 53 protons, 74 neutrons, and 54 electrons
Write aXz notation for:1. Isotope with 21 protons, 18 electrons,
and 24 neutrons
45Sc3+21
2. Isotope with 53 protons, 74 neutrons, and 54 electrons
127I-53
35Cl17 is the same as 3517Cl
Might also see “Cl-35” or “chlorine-35”