Unit 2 From Mixtures to Pure Substances Per 7 Agenda 10/1/19 • Quick check • Questions re Research project • Average Atomic Mass (Pogil) • Turn in if finish in class, otherwise tomorrow • Homework - work on research project Quick Check: 1.How many neutrons in an atom of neon-21 (21Ne). State clearly how you figure it out. 2. How many electrons in a neon-21 atom? How do you know? 1.How many neutrons in an atom of neon-21 (21Ne). State how you know. I find neon’s atomic number on periodic table, neon has atomic number 10 which tells me it has 10 protons in its nucleus. The remainder of the particles in the nucleus are neutrons. Mass number - atomic number = number of neutrons 21 - 10 = 11 So Ne-21 has 11 neutrons. 2. How many electrons in a neon-21 atom? How do you know? I know that for an electrically neutral atom, the number of protons in the nucleus is equal to the number of electrons in the electron cloud around the nucleus. For question 1 I found the atomic number of neon-21 is 10, hence the atoms have 10 protons and must also have 10 electrons. Isotope so important - let’s hear it and see it again The Story of the Atom A long story, many characters, egos, arguments twists and turns and it is still developing. Research project with presentation to tell it. Minimum Required Content: ● Full Name of Scientist: ● Dates Alive and something interesting about what was going on in the world at that time outside of scientific world (politics, modes of transportation, fashion, etc.) ● Where Born & early life ● Where studied and what studied ● Where worked and with whom ● Experiment(s) Related to the Understanding of the Atom (Time Period of Work/Date Published) What the experiment was, how it was carried out. What the results were How the results and analysis affected/influence the development of our understanding of matter and the atom ● References in MLA Format Presentations: Begin on Wednesday Oct 9. 5ish mins handwritten cards will be allowed this time Note reading from them will be heavily penalized in the scoring. NO phones. Just NO. Don’t even think about saying you forgot your cards and need to use your phone. NO. Oct 8 looks like it can be a final polish day. Unit 2 Exam will be after the presentations. I will give you a Sample Questions Packet on Oct 7 to work for homework that week. Wednesday October 16 - Multiple choice questions and 1 Structured Response. Thursday October 17 - 4 Structured Response. Average Atomic Mass How are the masses on the periodic table determined? Why? Most elements have more than one naturally occurring isotope. As you learned previously, the atoms of those isotopes have the same atomic number (number of protons), making them belong to the same element, but they have different mass numbers (total number of protons and neutrons) giving them different atomic masses. Average Atomic Mass How are the masses on the periodic table determined? So which mass is put on the periodic table for each element? Is it the most common isotope’s mass? The heaviest mass? This activity will help you answer that question. Average Atomic Mass How are the masses on the periodic table determined? Model 1 - A Strip of Magnesium Metal 1. Write in the atomic number for each Mg atom in Model 1. 24Mg 25Mg 26Mg What, where? Model 1 - A Strip of Magnesium Metal 1. Write in the atomic number for each Mg atom in Model 1. 24Mg 25Mg 26Mg 12 12 12 2. What are the mass numbers of the naturally occurring isotopes of magnesium? Model 1 - A Strip of Magnesium Metal 1. Write in the atomic number for each Mg atom in Model 1. 24 25 26 12 Mg 12 Mg 12Mg 2. What are the mass numbers of the naturally occurring isotopes of magnesium? 24, 25 and 26 3. Do all of the atoms in magnesium in Model 1 have the same atomic mass? Explain. 3. All of the atoms of magnesium in the model do not have the same atomic mass. There are three different isotopes of magnesium shown in the model. The three isotopes each have different mass numbers and so the atomic mass of the atoms of each isotope will also be different. 4. For the sample of 20 atoms, draw a table indicating the mass numbers of the three isotopes and the number of atoms of each isotope present. 24Mg 25Mg 26Mg Number of 16 2 2 atoms 5. Which isotope of magnesium is most common in Model 1? 24Mg 25Mg 26Mg Number of 16 2 2 atoms 6. ...for every 10 atoms of magnesium, approximately how many atoms of each isotope will be found? 24Mg 25Mg 26Mg Number of 16 2 2 atoms 8 1 1 6. ...for every 10 atoms of magnesium, approximately how many atoms of each isotope will be found? approx. one Mg-25, one Mg-26 and eight Mg-24 atoms 24Mg 25Mg 26Mg Number of 16 2 2 atoms 8 1 1 Model 2 - Natural Abundance Information for Magnesium a. 78.99/100 x 20 = 15.8 =16 atoms 10/100 x 20 = 2 atoms 11.01/100 x 20 = 2.2 = 2 atoms b) Is Model 1 accurate…? Model 2 - Natural Abundance Information for Magnesium b) Is Model 1 accurate…? Yes, for the limited number of atoms shown in Model 1, the percentage is close to the actual values for natural abundance of magnesium isotopes (accurate). 8. If you could pick up an atom and put it on a balance, the mass of that atom would most likely be _________ amu because 8. If you could pick up an atom and put it on a balance, the mass of that atom would most likely be 23.9850 amu because there are more Mg-24 in the sample than any other isotope. This isotope is most common (most abundant), so chances are highest that I would pick an atom of that mass. 9. From periodic table mass of Mg is 24.305 amu Does the decimal number shown on the periodic table for magnesium match any of the atomic masses listed in Model 2? No, the decimal number on the periodic table for Mg does not match any of the atomic masses listed in Model 2. 10. The periodic table does not show the atomic mass of every isotope for an element. a. There is not enough room in the boxes on the periodic table to put information about 2, 3, or even more isotopes. 10. The periodic table does not show the atomic mass of every isotope for an element. b. In most cases a scientist will be working with a mixture of all the (naturally occurring) isotopes of an element, so information about individual isotopes in not necessary. 11. What would be a practical way of showing the mass of magnesium atoms in the periodic table given that most elements occur as a mixture of isotopes? 11. What would be a practical way of showing the mass of magnesium atoms in the periodic table given that most elements occur as a mixture of isotopes? Calculate an average value. 12. Propose a possible way to calculate the average atomic mass of 100 magnesium atoms. 12. Add up all the masses of all the 100 atoms in the sample and divide by the number of atoms (100) in the sample. Find the weighted average mass of atoms in the sample. Model 3 – Proposed Average Atomic Mass Calculations 13. Complete the proposed calculations for the average atomic mass of magnesium in Model 3. Model 3 – Proposed Average Atomic Mass Calculations Mary’s method (78.99)(23.9850 amu) + (10.00)(24.9858 amu) +(11.01)(25.9826amu) 100 = 24.305 amu = 24.31 Jack’s Method (0.7899)(23.9850 amu) + (0.1000)(24.9858 amu) + (0.1101)(25.9826amu) = 24.305 amu = 24.31 Alan’s method (23.9850 amu) + (24.9858 amu) + (25.9826 amu) = 24.984 amu 3 Mary’s method (78.99)(23.9850 amu) + (10.00)(24.9858 amu) +(11.01)(25.9826amu) 100 = 24.305 amu Jack’s Method (0.7899)(23.9850 amu) + (0.1000)(24.9858 amu) + (0.1101)(25.9826amu) = 24.305 amu Alan’s method ONLY found average for 3 atoms (23.9850 amu) + (24.9858 amu) + (25.9826 amu) = 24.984 amu 3 assumes equal representation of all isotopes 14.c. show Mary and Jack’s methods are mathematically equivalent models. Mary’s method (78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu) 100 =78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu) 100 100 100 =(0.7899)(23.9850 amu)+(0.1000)(24.9858 amu)+(0.1101)(25.9826amu) =(0.7899)(23.9850 amu) + (0.1000)(24.9858 amu)+(0.1101)(25.9826amu) = 24.305 amu Jack’s Method 15. Use one of the methods in Model (Mary or Jack) to calculate the average atomic mass for oxygen. Isotope information is provided below. Show all of your work and check your answer against the mass listed on the periodic table. Use separate paper - or back of one of the sheets. 15. Calculation of Average atomic mass for oxygen Mary’s method (99.76)(15.9949 amu) + (0.04)(16.9991 amu) + (0.20)(17.9992 amu) 100 = =(0..9976)(15.9949 amu) + (0.0004)(16.9991 amu)+(0.1101)(17.9992amu) = 24.305 amu Jack’s Method 15.9565122 amu 0.00679964 amu + 0.0359984 amu 15.99931024 amu Given 6 sig digs with original measurements 15.9993 amu vs.