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Isotopes
Drill
 You completed problem #2 on the Structure of the
Atom WS yesterday.
 Pull out that worksheet.
 We will go over #2.
Objectives
 iWBAT
 determine the number of protons, neutrons and
electrons for isotopes
 Explain how isotopes are different from neutral atoms
and ions.
Guided Notes
 Pick up a copy of the “Isotopes” Guided Notes WS
Isotopes are like twins
ISOTOPES
 atoms that have the same number of protons, but
different numbers of neutrons
 the chemical properties of an element depend
primarily on its electrons and protons
 isotopes of the same element, with different numbers
of neutrons have essentially the same chemical
properties
ISOTOPES
the major difference between 2 isotopes
is their mass
 isotopes with more neutrons have higher mass
than those with fewer neutrons
 mass # = (isotope’s # of p+) + (isotope’s # of n0)
 to identify an isotope you add the mass number
after the element’s name (i.e. chlorine -37)
 number of neutrons = mass # - atomic #
On Your Notes
 Chlorine is used as an example.
 We will look at two chlorine isotopes


Chlorine – 35
Chlorine - 37
 The “35” and “37” represent mass numbers.
 Do you remember how to determine a mass number?
Determine the # of neutrons for
each isotope of chlorine
 Chlorine – 35
 Chlorine – 37
has 18 neutrons
has 20 neutrons
 Both isotopes have the same number of protons and
electrons, but different numbers of neutrons.
Carbon-12
 The entire periodic table is based on the carbon-12
isotope.
 You can write the isotope as:
carbon-12 (the line is a dash not minus sign)
or
12 C (notice how the mass # is written at the top of the symbol)
Protons don’t change
Example
Mass number
p+
e-
nº
Carbon-12
6
6
6
Carbon-14
6
6
8
2 extra neutrons
Electrons don’t change
Iodine-125
 What is the difference between this isotope of iodine
and the iodine shown on the periodic table?
 How many neutrons does iodine-125 have?
Practice
 Pick up a copy of the “Structure of the Atom” WS
 Answer question “5” only
Average Atomic Mass
 Take a look at the Periodic Table.
 Notice that the atomic mass for most of the elements
is not a whole number.
 This is because there are various isotopes for each
element.
 The average atomic mass of the element is calculated
and that is what you see as the mass on the periodic
table.
Avg Atomic Mass
 Pick up a copy of the
Isotopes and Average Atomic Mass WS.
 Let’s look at the example in the box at the top of the
page.
Example
 A sample of cesium is 75% 133 Cs, 20% 132 Cs and
5%
134
Cs. What is its average atomic mass?
 Change the % into a decimal.
 Multiply the decimal times the mass #.
 Add all of the answers.
0.75 x 133 = 99.75
0.20 x 132 = 26.4
0.05 x 134 = 6.7
132.85 amu
You will sometimes
see the mass with
units of amu
“atomic mass units”
Your Grade
 We use a math process like Average Atomic Mass to
calculate your grade.
 50% assessments
 30% labs
 20% home/classwork
Your Grade
 If you currently have…
89.4 % for assessments
73.7 % for labs
84.1 % for home/classwork
What is your overall grade for the marking period?
Your Grade
 Assessments
50% becomes
 Labs
30% becomes
 home/classwork 20% becomes
Assessments
Labs
home/classwork
0.50
0.30
0.20
89.4 % x 0.50 = 44.7
73.7 % x 0.30 = 22.11
84.1 % x 0.20 = 16.82
83.63
Summary
 Explain the difference between a neutral atom, ion and
isotope.
Summary
Protons:
An atom’s identity is determined by the number of
protons. If you change the number of protons, you change
the element.
Neutrons:
Neutrons maintain stability. If you change the number of
neutrons, you have an ISOTOPE.
Electrons:
Electrons determine an atom’s behavior.
If you change the number of electrons, then you have an
ION.