Transcript IsotopesofElements

McLeod – Elements, Their Dirty Little Secret
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The following Powerpoint presentation was
developed by Pearson (see bottom of next 3rd
slide). Where Mr. McLeod has changed or
inserted his slides/information, there will be
McLeod in the Title.
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McLeod
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So what is the “Dirty Little Secret of
Elements?
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Answer: They do not exist. All the elements on the
Periodic Table (i.e. the elements carbon, oxygen etc)
do not actually exist.
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So what does exist?
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Answer: Isotopes of elements. What we call elements
are really several different isotope that we “say” are
the same thing called elements. We will discuss in
more detail in another slide.
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Chapter 3 Atoms and Elements
3.6
Isotopes and Atomic Mass
24Mg
25Mg
26Mg
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Copyright © 2009 by Pearson Education, Inc.
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McLeod - Isotopes
Isotopes
• are atoms of the same element that have different
mass numbers (and different actual mass).
• have the same number of protons and electron, but
different numbers of neutrons.
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McLeod - Atomic Symbol for Isotopes
An atomic symbol of an Isotope
•
•
represents a particular isotope of an element.
gives the mass number in the upper left corner and the atomic number in the
lower left corner.
Example: An atom of sodium with atomic number 11 and a mass number 23 has
the following atomic symbol:
mass number
23
Na
atomic number
11
Atomic mass (Z) – number of protons
Mass Number (A) – number of protons and neutrons
Other ways to represent the above isotope is Sodium-23 or Na-23
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Information from Atomic Symbols
The atomic symbol for a specific atom of an element
gives the
• number of protons (p+),
• number of neutrons (n),
• and number of electrons (e-).
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Information from Atomic Symbols
Examples of number of subatomic particles for atoms
Atomic symbol
16
8
31
O
8 p+
8n
8 e-
P
15
15 p+
16 n
15 e-
65
Zn
30
30 p+
35 n
30 e-
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Learning Check
Naturally occurring carbon consists of three isotopes:
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of the following:
12C
6
13C
14C
6
6
protons
______
______
______
neutrons
______
______
______
electrons ______
______
______
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Solution
12C
6
13C
14C
6
6
protons
6 p+
6 p+
6 p+
neutrons
6n
7n
8n
electrons 6 e-
6 e-
6 e-
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Learning Check
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
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Solution
A. 8 p+, 8 n, 8 e-
16O
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B. 17p+, 20 n, 17e-
37Cl
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C. 47p+, 60 n, 47 e-
107Ag
47
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Learning Check
1. Which of the pairs are isotopes of the same element?
2. In which of the pairs do both atoms have 8 neutrons?
A.
15X
8
B.
C.
12X
15X
7
14X
6
6
15X
16X
7
8
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Solution
B.
12X
6
14X
6
The atomic symbols in “B.” represent isotopes of
carbon with 6 protons each, but one has 6 neutrons
and the other has 8.
C.
15X
7
16X
8
These isotopes of nitrogen and oxygen have 8
neutrons.
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Isotopes of Magnesium
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Isotopes of Magnesium
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Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
95.02
33S
0.75
34S
4.21
36S
0.02
32S, 33S, 34S, 36S
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Copyright © 2009 by Pearson Education, Inc.
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McLeod – What is up with Elements
How is it OK to say there is only a Sulfur “element” if it is made up of 3 different
isotopes?
Answer: This requires 2 conditions (which the element/isotopes fulfill) :
a) All isotopes “act” or are chemically the same
Since all isotopes have same # of electrons, they “act” chemically
the same (remember chemistry is study of electron movement).
b) Distribution of the numbers of each isotope must be
known and constant. Called Percent abundance of an isotope.
Example is a package of M&Ms: Even though they are different colors, each
M&M tastes the same, and they always come in the same number ratio
in each bag. So we are OK with buying bags (1 thing) instead of
individual M&Ms.
Therefore, just like the package of M&M, even though what we actually have are
isotopes, we will all “play the game” and say there is only one thing, the Element.
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McLeod - Atomic Mass
The atomic mass of an element
• Definition: weighted (by number of isotope) average of
the mass of all the isotopes in an element.
• 2nd definition: It is the mass (in amu) of the “fake”
thing we call the element.
• Each block on the Periodic Table represents an element.
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Na
22.99
There are always at least 2 numbers in the block.
• Whole number – Atomic number
• Never whole number (has decimal) - Atomic mass
• Units for Atomic mass – amu (atomic mass units)
• Based on mass of a proton
• Really just a “fake” unit that we will make “real” later
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Isotopes of Some Elements and
Their Atomic Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
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McLeod - Atomic Mass for Cl
The atomic mass of chlorine is
• due to all the Cl isotopes.
• not a whole number.
• the weighted average of two
isotopes: 35Cl and 37Cl. So
you don’t simply add the
two isotope’s masses and
divide by 2. Need to do
more math.
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McLeod - Calculating Atomic Mass
The calculation for atomic mass requires the
• percent(%) abundance of each isotope.
• atomic mass of each isotope of that element.
• sum of the weighted averages. To do this, we will use a
table to do the calculating. Actual calculations are
explained in a separate location.
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