Chapter 7 Lecture Notes

Chapter 7 Lecture Notes - Periodic Properties of the...

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Unformatted text preview: Periodic Properties of the Elements Chapter 7 Periodic Properties of the Elements Periodic Properties of the Elements Development of Periodic Table • Elements in the same group generally have similar chemical properties. • Properties are not identical, however. Periodic Properties of the Elements Development of Periodic Table Dmitri Mendeleev and Lothar Meyer independently came to the same conclusion about how elements should be grouped. Periodic Properties of the Elements Development of Periodic Table Mendeleev, for instance, in 1871 predicted germanium (which he called eka-silicon) to have an atomic weight between that of zinc and arsenic, but with chemical properties similar to those of silicon. Periodic Properties of the Elements Development of Periodic Table Mendeleev’s prediction was on the money But why? (Mendeleev had no clue). Periodic Properties of the Elements Periodic Trends • In this chapter we’ll explain why • We’ll then rationalize observed trends in Sizes of atoms and ions. Ionization energy. Electron affinity. Periodic Properties of the Elements Effective Nuclear Charge • In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons. • The nuclear charge that an electron “feels” depends on both factors. • It’s called Effective nuclear charge. • electrons in lower energy levels “shield” outer electrons from positive charge of nucleus. Na atom looks like this: Periodic Properties of the Elements Effective Nuclear Charge The effective nuclear charge, Z eff , is: Z eff = Z − S Where: Z = atomic number S = screening constant, usually close to the number of inner (n-1) electrons. Na Periodic Properties of the Elements • Example: Which element’s outer shell or “valence” electrons is predicted to have the largest Effective nuclear charge? Kr, Cl or O? Effective Nuclear Charge Periodic Properties of the Elements • Example: Which element’s outer shell or “valence” electrons is predicted to have the largest Effective nuclear charge? Kr, Cl or O? • Cl: Z eff ≈ 17 - 10 = 7 • O: Z eff ≈ 8 - 2 = 6 • N: Z eff ≈ 7 - 2 = 5 • Ca: Z eff ≈ 20 - 18 = 2 Effective Nuclear Charge Periodic Properties of the Elements Valence electrons Many chemical properties depend on the valence electrons. Valence electrons: The outer electrons, that are involved in bonding and most other chemical changes of elements. Rules for defining valence electrons. 1. In outer most energy level (or levels) 2. For main group (representative) elements (elements in s world or p world) electrons in filled d or f shells are not valence electrons 3. For transition metals, electrons in full f shells are not valence electrons. Periodic Properties of the Elements Valence electrons Many chemical properties depend on the valence electrons....
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Chapter 7 Lecture Notes - Periodic Properties of the...

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