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Chemistry 121 Chapter 7 PERIODIC PROPERTIES OF THE ELEMENTS 7.1 Development of the Periodic Table The periodic table is the most significant tool that chemists use for organizing and recalling chemical facts. Elements in the same column contain the same number of outer-shell electrons or valence electrons . How do we organize the different elements in a meaningful way that will allow us to make predictions about undiscovered elements? Arrange elements to reflect the trends in chemical and physical properties. The periodic table arises from the periodic patterns in the electronic configurations of the elements. Elements in the same column contain the same number of valence electrons. The trends within a row or column form patterns that help us make predictions about chemical properties and reactivity. In the first attempt Mendeleev and Meyer arranged the elements in order of increasing atomic weight. Certain elements were missing from this scheme. For example, in 1871 Mendeleev noted that As properly belonged underneath P and not Si, which left a missing element underneath Si. He predicted a number of properties for this element. In 1886 Ge was discovered; the properties of Ge match Mendeleev’s predictions well. In the modern periodic table, elements are arranged in order of increasing atomic number. 7.2 Effective Nuclear Charge Effective nuclear charge ( Z eff ) is the charge experienced by an electron on a many-electron atom. The effective nuclear charge is not the same as the charge on the nucleus because of the effect of the inner electrons. The electron is attracted to the nucleus, but repelled by electrons that shield or screen it from the full nuclear charge. This shielding is called the screening effect. The nuclear charge experienced by an electron depends on its distance from the nucleus and the number of electrons in the spherical volume out to the electron in question. As the average number of screening electrons ( S ) increases, the effective nuclear charge ( Z eff ) decreases. Z eff = Z – S As the distance from the nucleus increases, S increases and Z eff decreases. S is called the screening constant which represents the portion of the nuclear charge that is screened from the valence electron by other electrons in the atom . The value of S is usually close to the number of core electrons in an atom. Homework Problems: 7.11, 7.14 1 Study material for this chapter: Textbook sections 7.1 through 7.8
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7.3 Sizes of Atoms and Ions Consider a collection of argon atoms in the gas phase. When they undergo collisions, they ricochet apart because electron clouds cannot penetrate each other to a significant extent. The
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This note was uploaded on 05/14/2009 for the course CHEM 121 taught by Professor Wyzlouzil during the Fall '07 term at Ohio State.

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