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Determining Periodic Trends Purpose To understand the periodic nature of the periodic table – by knowing an element’s location on the periodic table one can determine the element’s number of valence electrons and have an awareness of its ionization energy, density, electron affinity, electronegativity, atomic radius, and ionic radius, relative to other elements Materials elements from Periodic Trends, periodic table, scissors, tape, white paper, graph paper Procedure Part 1 1. You will be given cards for each of five elements in a certain group. Each card contains information about the physical properties of the element. Use the descriptions in the Periodic Trends section below to place your element cards in what you think is the correct arrangement. You only have representative or main-group elements. The transition metals and noble gases are not used in this activity. 2. Use a periodic table to predict the atomic number and period of each element and write them in the spaces provided on the card. Part 2 1. As a group, organize all the elements into a periodic table. Tape the elements onto white paper when you are sure of their placement. 2. Compare your periodic table with the teacher’s key and make any necessary adjustments. 3. Complete the Analysis and Conclusions section. Terminology of Periodic Trends Valence electron: An electron in an outer shell of an atom that can participate in forming chemical bonds with other atoms. Nonvalence electrons are tightly bound to the nucleus and are called core electrons. Octet rule: Atoms tend to gain, lose, or share electrons to reach eight electrons in their outer electron shells. Ionization energy: The amount of energy required to remove one electron from a neutral atom that is in the gaseous state. Elements on the right side really want to keep their electrons because they are so close to achieving an octet. Elements on the left side don’t mind losing an electron. Going down a group, the shielding effect of additional electrons makes it easier to remove outermost electrons. Atomic radius: One-half the distance of a single bond between two atoms of an element. As we go down a group, the principal energy level increases, causing an increase in the average distance between the electrons and the nucleus. Atoms get bigger as electrons are added to the principal energy levels. All the atoms in a given period have their outermost electrons in the same principal energy level. As we go across a period, we are adding electrons to the same principal energy level. The atoms do not get bigger across a period because we are also adding protons to the nucleus as we move across a period. The additional protons in the positive nucleus increase the pull on the negative electrons resulting in smaller atoms across a period. C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way? Indiana Chemistry Standards Resource, November 2003 Ionic radius: One-half the center-to-center distance between two ions in a crystalline ionic compound. Ions on the left side of the table are smaller than the atoms they come from because they’ve lost an electron. Ions on the right side are bigger than the atoms they come from because they’ve gained an electron. Electronegativity: A measure of attraction of an atom for a pair of shared electrons, a measure of whether an atom “grabs” more than its fair share of the shared electrons. As you go down a group, atoms have more total electrons so they don’t really care that much about their outermost ones. As you go across a period, atoms have more valence electrons and are so close to achieving an octet that they’ll grab another atom’s electrons. Electron affinity: The amount of energy absorbed when an electron is added to a neutral gaseous atom to form a gaseous ion with a 1-charge. It has a negative value if energy is released. Most elements release energy when they gain an electron. Atoms that really want an extra electron have the most negative electron affinity. Atoms that don’t really want another electron do not have very negative values. Analysis and Conclusions 1. Use the words increases or decreases to describe the trends for the following properties of the representative elements, going from left to right across a period: • ionization energy • atomic radius • ionic radius • electronegativity • electron affinity 2. Use the words increases or decreases to describe the trends for the following properties of the representative elements, going from top to bottom down a group: • ionization energy • atomic radius • ionic radius • electronegativity • electron affinity 3. When elements in Group I ionize, are they more likely to gain or lose electrons? Explain your answer. 4. Explain why Group I elements have ionic radii smaller than their atomic radii. C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way? Indiana Chemistry Standards Resource, November 2003 5. When elements in Group VII ionize, are they more likely to gain or lose electrons? Explain your answer. 6. Explain why Group VII elements have ionic radii larger than their atomic radii. 7. Compare how the melting point for metals and nonmetals generally changes as you move down a group. 8. What is the relationship between number of valence electrons and group number? 9. Create a graph of ionization energy versus atomic number and identify the different periods. Compare the ionization energies of metals to nonmetals. 10. Create a graph of atomic radius versus atomic number and identify the different periods. What is the trend going down a group? What is the trend as you go across a period? C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way? Indiana Chemistry Standards Resource, November 2003 Determining Periodic Trends Teacher Directions There are five element cards in a family/group and there are seven families/groups total. When placing the class into groups, make sure there are enough groups to create an integral number of periodic tables. In Part 2, allow students to use the key to confirm their placement of the elements before they attempt to draw conclusions about periodic trends. Answer Key 1. • • • • • ionization energy atomic radius ionic radius electronegativity electron affinity increase decrease increase increase increase 2. • • • • • ionization energy atomic radius ionic radius electronegativity electron affinity decrease increase increase decrease decrease 3. 4. 5. 6. 7. 8. 9. 10. Group I elements lose their one valence electron to achieve a stable octet. The loss of the one valence electron means the electron cloud is smaller. In addition, the positive nucleus has a greater pull on the negative electron cloud, causing the electron cloud to be drawn closer to the nucleus. Group VII elements have seven valence electrons and they will gain an electron to achieve a stable octet. Group VII elements gain electrons while the total positive charge of the nucleus stays the same. The electrons are not attracted to the nucleus as strongly as they were before so the electron cloud spreads out a little. The electron cloud of the ion is also bigger because there is greater repulsion between the increased number of electrons. For metals, the melting point decreases down a group. For nonmetals, the melting point increases down a family. The group number is equal to the number of valence electrons for the main-group elements. Compared to nonmetals, metals have lower ionization energies because they give up electrons easily. As you go down a group, the atoms get larger. As you go across a period, the atoms get smaller. C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way? Indiana Chemistry Standards Resource, November 2003