CYCLE 6 Developing Ideas ACTIVITY 4: Atoms and the Periodic Table-- KEY Purpose Mendeleev developed his periodic table based on a limited number of macroscopic observations and measurements—atomic weights (now called atomic mass) and a few known physical properties and chemical reactivity. Mendeleev’s table was revised as new elements were discovered and as experimental evidence about atom structure emerged. The modern periodic table now organizes the more than 110 elements according to those same trends or periodicity in physical and chemical properties described by Mendeleev, but also accounts for atom structure. In this activity you will investigate how the organization of Mendeleev’s periodic table relates to the structure of the atom. This activity will help you answer the question: How does the structure of the atom relate to the organization of the periodic table? Collecting and Interpreting Evidence Exploration #1: How does atom structure relate to the organization of the periodic table? Your instructor will provide you with an element card mat, similar to the one you used in Activity 3. This element card mat gives the relationship between the organization of elements and atom structures, rather than the relationship between organization of elements and their observable properties. In the element card mat, the atom is depicted using a modified Bohr model: the number of protons (p) and neutrons (n) in the nucleus of the element’s most abundant isotope is shown in the center of the atom; the orbits (energy levels) are shown as concentric circles around the nucleus; the electrons that reside within each energy level are shown as dots. Even though scientists believe the Bohr model to be inaccurate, it is often used in science books to help students study atom structure and the relationship to the periodic table. © 2007 PSET 6-57 Cycle 6 A small version of this element card mat is shown below. Examine the element card mat provided by your instructor, or the image below, and answer the questions. 1A 2A 3A 4A 5A 6A 7A 1 2 3 4 What general trend do you observe in the atomic number as you move from left to right across the table? From top to bottom? Increases left to right and increases top to bottom. How is the atomic number of an element different from the element to its left? To its right? Greater by 1 than element on left, less by 1 than element on right. 6-58 8A Activity 4: Atoms and the Periodic Table What is the relationship between atomic number and number of protons in the nucleus? Atomic number = # of protons What is the relationship between atomic number and number of electrons in the atom? Atomic number = # of electrons For elements with atomic number 1-20, is there a definite relationship between the number of protons and the number of neutrons in the nucleus of the most abundant isotope? For elements with atomic number greater than 20? For elements 1-20, some have 1:1 ratio, but no definite relationship between # protons and # of neutrons for any. What is the relationship between the number of occupied energy levels (concentric circle(s) containing dots) and the period (horizontal) number? # occupied energy levels = period # What is the maximum number of electrons that can occupy the first energy level? The second energy level? The third energy level1? 1st = 2, 2nd = 8, 3rd = 18 What is the relationship between the number of electrons in the outermost energy level and the group (vertical) number? # electrons in outermost energy level = group # The following points summarize additional scientists’ ideas that relate atom structure to the organization of the periodic table: 1 You may be wondering why gallium (and Period 4 elements to its right) have has an additional 10 electrons in the third energy level as compared to potassium and calcium. Recall that 10 transition elements separate calcium and gallium. For each of those elements, one electron is added to the third energy level instead of to the fourth energy level. To understand why, more advanced models of atom structure and electron arrangement are needed. 6-59 Cycle 6 a) Moving across the periodic table, elements increase by one atomic number. An atom of an element with an atomic number of ‘Z’ will have ‘Z’ protons in the nucleus and ‘Z’ electrons moving around the nucleus. The atomic number is the whole number listed on the periodic table. b) Isotopes of an element differ in their number of neutrons in the nucleus. The mass number ‘A’ of a particular isotope is equal to the sum of the protons and the neutrons in the isotope’s nucleus. The mass number is not given on the periodic table. For many elements, the mass number for the most abundant isotope can be predicted from the atomic mass of the element. See if you can figure out the relationship! c) The atomic mass is a weighted average of the mass numbers of the various isotopes of an element. The atomic mass is the decimal number listed on the periodic table and has units of amu (atomic mass units). An amu is equivalent to the mass of one proton. Use the information provided (ideas a, b, and c) and a periodic table to complete the table below. One element has been completed for you. Table 1: Atomic Structure for Elements (Most Abundant Isotope) Element Carbon Sodium Nitrogen Chlorine Calcium Iron Chromium Iodine Platinum Gold 6-60 Chemical Atomic # of # of # of Mass Element Symbol Number Protons Electrons Neutrons Number Atomic Mass C Na N Cl Ca Fe Cr I Pt Au 6 11 7 17 20 26 6 11 7 17 20 26 6 11 7 17 20 26 6 12 7 18 20 30 53 78 79 53 78 79 53 78 79 72 117 118 12 23 14 35 40 56 52 127 195 198 12.01 22.98 14.00 35.45 40.08 55.84 126.90 195.08 197.97 Activity 4: Atoms and the Periodic Table In the table above you were provided and completed information for the most abundant isotope of each element listed. Scientists differente between different isotopes of the same element by writing the mass number after the name or as a superscript before the symbol. For example, the most abundant isotope of carbon has 6 neutrons and a mass number of 12. Therefore, scientists call it carbon-12 or 12C. Carbon-13, or 13C, is an isotope of carbon that is used in a variety of applications, including breath tests for the helicobacter pylori bacteria which causes stomach ulcers, as well as air pollution and climate change studies. How many neutrons does carbon-13 have? 7 neutrons Carbon-14, or materials. 14C, is radioactive isotope of carbon that is used for dating How many neutrons does carbon-14 have? 8 neutrons Only non-radioactive isotopes are used when determining the atomic mass of an element. For carbon, 99% of the non-radioactive isotopes are carbon-12 and 1% are carbon-13. Thus, 99 of 100 carbon nuclei will have a mass number of 12 and 1 of 100 carbon nuclei will have a mass number of 13. The calculation of the atomic mass accounts for the abundance of each isotope 99 nuclei are 12C (99 x 12) 1 nucleus is 13C + (1 x 13) 100 = 12.01 Calculate the atomic mass of boron if 80% of the non-radioactive isotopes are boron-11 and 20% are boron-10. Show your work. 80 nuclei are 11B (80 x 11) + 100 20 nuclei are 10B (20 x 10) = 10.8 6-61 Cycle 6 Exploration #2: How does electron arrangement relate to trends in chemical behavior? Using the orbit model we are reminded of the relationship between an electron’s distance from the nucleus and its energy. The further away from the nucleus an electron is, the higher the energy level the electron occupies, and therefore the greater the potential energy possessed by an electron. The electrons that are furthest from the nucleus, or that reside in the highest energy level, are called the valence electrons. All other electrons of an atom are called the core electrons. Your instructor will provide you with a different element card mat. This element card mat depicts a particular chemical behavior of atoms, the formation of ions. Ions are charged atoms that form when atoms gain or lose electrons. We will explore how ions interact with each other in the homework. A small version of this element card mat is shown below. Examine the element card mat provided by your instructor, or the image below, and answer the questions. 1A 1 2 3 4 6-62 2A 3A 4A 5A 6A 7A 8A Activity 4: Atoms and the Periodic Table Complete the table based on patterns you observe in the element card mat. One column has been completed for you as an example. Table 2: Valence Electrons and Ion Formation2 Group 1A How many valence electrons initially? Atom gains or loses valence electrons? 3A 4A 5A 6A 7A 8A 2 3 4 5 6 7 8 Loses Loses Loses Gains Gains Gains -- 2 Lost 3 Lost 4 Lost 3 Gained 2Gained 1Gained -- 2+ 3+ 4+ 3- 2- 1- -- 1 Loses How many valence electrons are either lost or gained? Charge on ion formed? 2A 1 Lost +1 Which electrons are lost or gained in the formation of ions—core electrons or valence electrons? Valence electrons Why are ions positive when they lose electrons and negative when they gain electrons? (Hint: compare the number of positive protons versus the number of negative electrons after losing and gaining electrons) # (+) protons > # (-) electrons when they lose electrons = net positive charge; # (+) protons < # (-) electrons when they gain electrons = net negative charge 2 Elements in Group IV and other metalloids can form positive or negative ions depending on the other elements they are bound to. We have elected to only display the ion that gives the clearest relationship between position and ion formation for later activities. 6-63 Cycle 6 What is the relationship between the number of electrons lost and the number value of the charge on a positive ion? Explain why this is. They are the same. If 1 is lost, then 1+ charge, if 2 are lost, then 2+ charge, etc. Same reason as previous question: take away an electron and there is one more (+) proton than (-) electrons, so 1+. What is the relationship between the number of electrons gained and the number value of the charge on a negative ion? Explain why this is. They are the same. If 1 is gained, then 1- charge, if 2 are gained, then 2- charge, etc. Same reason as previous question: add an electron and there is one more (-) electron than (+) protons so 1-. What kind of charge—positive or negative—are you more likely to find on metal ions? Positive—they lose electrons What kind of charge—positive or negative—are you more likely to find on nonmetal ions? Negative—they gain electrons After an atom loses or gains valence electrons, the arrangement of electrons in its ion resembles the arrangement of electrons in which group (1A-8A) of elements? The noble gases, group 8A. 6-64 Activity 4: Atoms and the Periodic Table Summarizing Questions Discuss these questions with your group and note your ideas. Leave space to add any different ideas that may emerge when the whole class discusses their thinking. S1. Describe the organization of the elements in the periodic table based on atom structure (i.e. protons, neutrons, electrons)? Atomic number (and # protons, # electrons) increases by 1 going from left to right. The # neutrons generally increases from left to right, but with no set pattern. Energy levels closest to the nucleus fill until they reach their “maximum” capacity, then the next energy level up begins to fill. First energy holds a maximum of 2, 2nd holds up to 8, and 3rd holds up to 18. S2. Describe the organization of the elements in the periodic table based on basic arrangement of electrons. In same group, each element has same number of valence electrons. In same period, each element increases by one valence electron from left to right and valence electrons occupy the energy level that is equal to the period #. S3. Which electrons—core or valence—might have more power in predicting similar physical properties and chemical reactivity of elements in the same group? Explain your reasoning. Valence electrons. Elements with the same number of valence electrons form compounds in the same ratios, combining power, have similar chemical reactivity (i.e. toward water, oxygen, metals, etc.), and form ions with the same magnitude of positive or negative charge, etc. 6-65 Cycle 6 S4. Positron Emission Spectroscopy (PET) is a medical procedure used to study the chemical processes involved in the working of healthy or diseased human brains. When using PET, molecules that our bodies normally use (like oxygen, water, and glucose) are labeled with isotopes such as 15O and 18F, and then injected into the patient. The PET Scanner then records the signals that those isotopes emit as they journey through the brain. In the space below, draw modified Bohr models (like those used in Exploration #1) for 15O and 18F. Label them clearly. S5. Using only the modern periodic table to help you, complete the table for 55Fe2+ ion: Table 3. 55Fe2+ ion 6-66 Atomic number 26 Number of protons 26 Ion charge 2+ Number of electrons 24 Mass number 55 Number of neutrons 29
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