Chapter Seven Learning Objectives. Developing the Periodic Table

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1 Chapter Seven Learning Objectives understand effective nuclear charge and its role in explaining periodic trends understand and predict periodic trends in atomic and ionic radii, ionization energy, and electron affinity describe the chemistry of some main-group elements Chemistry 1A Periodic Properties 1 Developing the Periodic Table The Russian chemist Dmitri Mendeleev is credited with the arrangement of the periodic table: his arrangement in which elements with similar properties were listed in the same columns was a huge success and a true mark of his genius. Chemistry 1A Periodic Properties 2 1

2 Effective Nuclear Charge Many periodic trends can be rationalized by the concept of effective nuclear charge, Z eff : the ability of electrons to shield or screen other electrons in an atom. The notion of Z eff explains why, in a many-electron atom, the orbitals for a given value of n increase in energy with increasing l value. Chemistry 1A Periodic Properties 3 Explaining Periodic Trends Most periodic trends are influenced by two factors: the increase in Z eff across a period and the increase in n down a group. The Z eff increases across a period because the effect of each additional proton is more significant than the effect of each additional electron. As n increases down a group, electrons have a greater probability of being farther from the nucleus. Chemistry 1A Periodic Properties 4 2

3 Periodic Trends in Atomic Radii For main-group elements, atomic radii increase down a group and decrease across a period. Chemistry 1A Periodic Properties 5 Periodic Trends in Ionic Radii For ions with the same charge, ionic radii increase down a group. Ion Formation For isoelectronic ions, such as O 2, F, Na +, Mg 2+, and Al 3+, what is the trend in ionic radii? Chemistry 1A Periodic Properties 6 3

4 Ionization Energy The ionization energy, I, is the energy required to remove an electron from an atom or ion in the gas phase. The energy required to remove the first electron from a neutral atom is called the first ionization energy, I 1. Why is the second ionization energy of an atom always greater than its first ionization energy? Successive Ionization Energies Chemistry 1A Periodic Properties 7 Periodic Trends in First Ionization Energies For main-group elements, I 1 values decrease down a group and increase across a period. What are the anomalies? Chemistry 1A Periodic Properties 8 4

5 Anomalous First Ionization Energies How can we use electron configurations to explain the drop in I 1 from Mg to Al and again from P to S? Chemistry 1A Periodic Properties 9 Electron Affinity Some atoms have an affinity or liking for electrons and can acquire one or more electrons to form a negative ion. The electron affinity of an atom quantifies the energy released when an atom or ion in the gas phase gains an electron. An element with a high ionization energy generally has a high affinity for an electron. Chemistry 1A Periodic Properties 10 5

6 Periodic Trends in Electron Affinities The larger the affinity of an atom for an electron, the more negative the energy value. What are the trends? Electron Affinity Periodic Trends Chemistry 1A Periodic Properties 11 Explaining Trends in Electron Affinities Why do the halogens have the most negative electron affinity values? Why do the alkaline earth metals have no affinity for an electron? What other group of elements is predicted to have highly positive electron affinities? How can you explain the observed electron affinities for the group 5A elements? Chemistry 1A Periodic Properties 12 6

7 Trends in the Properties of Metals and Nonmetals metal oxide + water base SrO (s) + H 2 O (l) Sr(OH) 2 (aq) K 2 O (s) + H 2 O (l) 2 KOH (aq) nonmetal oxide + water acid P 4 O 10 (s) + 6 H 2 O (l) 4 H 3 PO 4 (aq) SO 3 (g) + H 2 O (l) H 2 SO 4 (aq) metal oxide + acid salt + water nonmetal oxide + base salt + water Write the balanced chemical equation for the reaction between solid copper (II) oxide and sulfuric acid: Chemistry 1A Periodic Properties 13 Group Trends for the Active Metals: Groups 1A and 2A Why are the alkaline earth metals less reactive than the alkali metals? Which two alkaline earth metals do not react with water to produce hydrogen gas and an aqueous metal hydroxide? metal + water hydrogen gas + metal hydroxide Ca (s) + 2 H 2 O (l) H 2 (g) + Ca(OH) 2 (aq) Chemistry 1A Periodic Properties 14 7

8 Group 7A: The Halogens The halogens are among the most reactive elements in the periodic table. The most reactive, fluorine, forms compounds with every element except three of the noble gases. What atomic property of the halogens is most responsible for their high reactivity? Chemistry 1A Periodic Properties 15 Group 8A: The Noble Gases The noble gases are neither metals like most elements nor reactive nonmetals like the halogens. A few compounds have been made by reacting krypton and xenon with fluorine. Why are these larger members of the group more likely to form compounds? Chemistry 1A Periodic Properties 16 8