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Chapter 21 Main Groups Week 1 2009

Chapter 21 Main Groups Week 1 2009 - Chapter 21 Properties...

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Chapter 21. Properties of the Elements (We will also add the metals from Chapter 20) First, let’s look at Periodic Table and Group numbering. 21.2 Hydrogen . The simplest atom, ~90% of all atoms in the universe. 1 H isotope (protium) = proton surrounded by e - . 2 H (deuterium) or 3 H (tritium) contain 1 or 2 neutrons as well. Deuterium ( 2 H) was produced in the Big Bang—too “fragile” to survive fusion conditions in the stars (which produce the lighter elements) or supernovas (which produce heavier elements). Stable molecular form is H 2 i.e. H–H. Hydrogen is the exception in the periodic table — cannot be satisfactorily classified in any group: similarities to certain metals and non-metals (groups 1 and 17, respectively). Brief Summary of Hydrogen Chemistry - almost always forms covalent compounds; ionic ones rare. - high ionization energy (e - close to nucleus without other e’s to shield it) and low electronegativity (only one proton to attract e - s). unlike groups 1 and 17 in that H + and H - ions are rare because they usually bond covalently to other things instead of being found as ions e.g., H 3 O + , OH - , NH 4 + , etc Very rare exception is the hydride ion, H - Ionic Hydrides (H - ) . Result from H2 (g) being reduced to H - ions, rather than oxidized to H + It is very difficult to reduce H 2 (g) H 2 (g) + 2 e - 2 H - (g) Eº = -2.23 V but very strong reducing agents (Na, Ca, Li, etc, with half-cell potentials below -2.23 V) can do it to give ionic salts containing H - , called the hydride ion.
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2 Na (s) + H 2 (g) 2 NaH (s) (ionic salt) The hydride ion is very reactive and will either: (1) find a H + and go to H 2 (g): NaH (s) + H 2 O (l) Na + (aq) + OH - (aq) + H 2 (g) or (2) reduce something and go to H 2 (g): TiCl 4 (l) + 4 LiH (s) Ti (s) + 4 LiCl (s) + 2 H 2 (g) Covalent Hydrogen Compounds . Common and stable: CH 4 , NH 3 , H 2
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