Masses of atoms

1. Masses of atoms 17.2

2. Warm up • Label the different parts of an atom. • Complete the table with the appropriate term or number.

3. Objectives • Compare between the atomic number and the mass number of an atom • Calculate the number of electrons, protons, and neutrons in an atom given its mass and atomic numbers and vise verse • Explain the existence of the majority of the mass of an atom in its nucleus • Define isotopes • Explain why isotopes have different mass number and similar chemical properties • Define the average atomic mass • Calculate the average atomic mass of an element based on the natural abundances rate of its isotopes

4. Introduction • Can you guess what is the mass of your laptop in Kg? • Can you predict the mass of the laptop after you place a paperclip on top of it? • How can you compare the mass of the laptop with and without the paperclip to the mass of the atom and its particles? • Which of the following: laptop of paper clip refers to the mass of the electrons in an atom?

5. Atomic mass Complete the following table

6. Atomic mass • The unit of measurement used for atomic particles is the atomic mass unit (amu) • Mass of p+ = mass of No = 1 amu • Mass of e- <<<<<<<<<<< mass of p+ or No • can you conclude where is the majority of the mass of an atom is located?

7. Atomic mass • Atomic mass is the mass of the atom (mass of protons and neutrons) in amu

8. Protons identify the elements • Each type of atom of an element has a different number of protons.

10. Compare number of P+ and e-

11. Application • Find, using the periodic table, which element has an atom with the following number of protons: 1- six protons 2- 8 protons 3- 12 protons 4- 20 protons What is the atomic number of hydrogen, helium, argon, lithium, magnesium, chlorine, sulfur, phosphorous (use the periodic table)

12. Mass number • The mass number is the sum of the number of the protons and the neutrons in the nucleus of an atom. • Number of neutrons= mass number – atomic number ( # of P+)

13. Atomic and mass number

14. Application • Find the Atomic number (Z) and the mass number (A) for the following. • Protons are colored red and neutrons are blue

15. Application

16. What is the difference and the similarity between the following pictures?

17. Isotopes • Atoms of the same element with different number of neutrons can have different properties. • Ex: C-12 and C-14 • Isotopes are atoms of the same element that have the same atomic number (number of protons) but different mass number ( number of neutrons)

18. Identifying isotopes • An isotope can be identified by writing the name or symbol of the atom followed by its mass number. For example: Carbon-14 or C-14 & Chlorine-35 or Cl-35

19. Application • Naturally occurring elements of Boron have mass number of 10 or 11. Calculate the number of neutrons of B-10, and B-11. • Uranium-238 has 92 protons. How many neutrons does it have?

20. Application • Find the number of protons, electrons and neutrons for each isotope of Lithium. • Write the name or the symbol of the three isotopes of Lithium.

21. Application Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #P _______ _______ _______ #N _______ _______ _______ #E _______ _______ _______

22. An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 2) 35 3) 65 B. Number of neutrons in the zinc atom 1) 30 2) 35 3) 65 C. What is the mass number of a zinc isotope with 37 neutrons? 1) 37 2) 65 3) 67

23. Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- __________ C. 47p+, 60 n, 47 e- ___________

24. An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is 1) 14 2) 16 3) 34 C. The element is 1) Si 2) Ca 3) Se D. Another isotope of this element is 1)34X 2)34X 3)36X 16 14 14

25. Atomic mass Na 22.99 • Listed on the periodic table • Gives the mass of “average” atom of each element • Average atomic mass is based on all the isotopes and their abundance % • Atomic mass is not a whole number

26. Application Using the periodic table, specify the atomic mass of each element: A. calcium __________ B. aluminum __________ C. lead __________ D. barium __________ E. iron __________

27. Calculating average • Calculate the weighted average of the student quiz scores if the following results were obtained: 5 students scored 80%; 19 students scored 90%

28. Calculating average atomic mass • The atomic mass of an element is the weighted average mass of all of its naturally occurring isotopes • Weighted average = mass isotope1(%) + mass isotope2(%) + … 100100

29. Application Isotopes Mass of Isotope Abundance 24Mg =24.0 amu 78.70% 25Mg = 25.0 amu 10.13% 26Mg = 26.0 amu 11.17% Atomic mass (average mass) Mg = 24.3 amu Mg 24.3

30. Application Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the average atomic mass of gallium?

31. Wrap up • What is meant by atomic number? • What is the mass number? • How can we calculate the number of protons, neutrons and electrons of an element? • What are isotopes? • How can we identify an isotope? • How can we calculate the average atomic mass of an element?

32. Home work • Section review questions Page 515 # 1 to 7 Take a snap shot of the complete periodic table and save it on your desktop.