mcleod – elements, their dirty little secret l the following powerpoint presentation was developed...
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McLeod – Elements, Their Dirty Little Secret
l 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
l So what is the “Dirty Little Secret of Elements?
l Answer: They do not exist. All the elements on the Periodic Table (i.e. the elements carbon, oxygen etc) do not actually exist.
l So what does exist? l 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
3.6Isotopes and Atomic Mass
24Mg 25Mg 26Mg 12 12 12
Copyright © 2009 by Pearson Education, Inc.
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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.
McLeod - Isotopes
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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
McLeod - Atomic Symbol for Isotopes
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-).
Information from Atomic Symbols
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Examples of number of subatomic particles for atoms
Atomic symbol
16 31 65
O P Zn 8 15 30
8 p+ 15 p+ 30 p+
8 n 16 n 35 n 8 e- 15 e- 30 e-
Information from Atomic Symbols
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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 13C 14C 6 6 6
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______
Learning Check
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12C 13C 14C 6 6 6
protons 6 p+ 6 p+ 6 p+
neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-
Solution
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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- ___________
Learning Check
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A. 8 p+, 8 n, 8 e- 16O
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B. 17p+, 20 n, 17e- 37Cl 17
C. 47p+, 60 n, 47 e- 107Ag 47
Solution
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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 15X 8 7
B. 12X 14X 6 6
C. 15X 16X 7 8
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Solution
B. 12X 14X
6 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 16X
7 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 naturallyoccurring sulfur containsseveral isotopes with thefollowing abundances
Isotope % abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02
32S, 33S, 34S, 36S16 16 16 16
Copyright © 2009 by Pearson Education, Inc.
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. 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
11Na22.99
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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.