1. McLeod – Elements, Their Dirty Little Secret l The following Powerpoint presentation was developed by Pearson (see bottom of next 3 rd slide). Where Mr. McLeod has changed or inserted his slides/information, there will be McLeod in the Title. 1

2. 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. 2

3. 3 Chapter 3Atoms and Elements 3.6 Isotopes and Atomic Mass  24 Mg 25 Mg 26 Mg 12 12 12 Copyright © 2009 by Pearson Education, Inc.

4. 4 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

5. 5 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

6. 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 6

7. 7 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 n16 n 35 n 8 e - 15 e - 30 e - Information from Atomic Symbols

8. 8 Naturally occurring carbon consists of three isotopes: 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of the following: 12 C 13 C 14 C 6 6 6 protons ______ ______ ______ neutrons ______ ______ ______ electrons______ ______ ______ Learning Check

9. 9 12 C 13 C 14 C 6 6 6 protons 6 p + 6 p + 6 p + neutrons 6 n7 n 8 n electrons 6 e - 6 e - 6 e - Solution

10. 10 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

11. 11 A. 8 p +, 8 n, 8 e - 16 O 8 B. 17p +, 20 n, 17e - 37 Cl 17 C. 47p +, 60 n, 47 e -107 Ag 47 Solution

12. 12 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. 15 X 15 X 8 7 B. 12 X 14 X 6 6 C. 15 X 16 X 7 8

13. 13 Solution B. 12 X 14 X 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. 15 X 16 X 7 8 These isotopes of nitrogen and oxygen have 8 neutrons.

14. 14 Isotopes of Magnesium

15. 15 Isotopes of Magnesium

16. 16 Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances Isotope % abundance 32 S 95.02 33 S 0.75 34 S 4.21 36 S 0.02 32 S, 33 S, 34 S, 36 S 16 16 Copyright © 2009 by Pearson Education, Inc.

17. 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. 17

18. 18 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. 2 nd 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 11 Na 22.99

19. 19 Isotopes of Some Elements and Their Atomic Mass Most elements have two or more isotopes that contribute to the atomic mass of that element.

20. 20 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: 35 Cl and 37 Cl. So you don’t simply add the two isotope’s masses and divide by 2. Need to do more math.

21. 21 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.