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Lecture 9

CHEM H90 Lecture 9: Bonding (04/21/17)

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University of California - Irvine

LECTURE 9 Friday, April 21, 2017 Bonding Outline • Basic types of bonds • Strong bonds: o Covalent o Ionic o Metallic • Weak bonds: o Hydrogen bonds o Van der Waals interactions • Reference material: o Cartoon guide: chapter 3 o Context: 1.7, 1.8, 2.3 o Context: 5.2, 5.7 Covalent bonds • Loosely, the bonds between non-metals • Electrons are shared between atoms • Most common in molecules • Classic example, H 2 • All homonuclear diatomics, such as O ,2N 2 • Usually covalent for organic molecules o (e.g.) methane, CH 4 Ionic bonds • Loosely, the bonds between metal and non-metal • An electron is transferred from one atom to another - • Classic example is NaCl, where e goes from Na to Cl • Usually, a solid o (e.g.) a salt is solid under normal conditions Metallic bonds • In metal solids, electrons are shared over the entire lattice of nuclei o Electrons can move very easily ▪ This is why metals conduct heat and electricity very well • Classic example is bulk Li • Each atom gives one e to the entire solid, so have 10 e shared by 10 ions • This sea of electrons flows very easily, so metals are very good conductors Weak bonds • Not usually called chemical bonds • Strongest are hydrogen bonds, crucial to all special properties of water o It’s a weak ionic bond between H in one water molecule and O in another ▪ Makes life possible on this planet o Also get H-bonding in a few other liquids • Van der Waals bonds are weaker still, and allow noble gases to bind o Always attractive, always add, and that’s why you can get strong effects but ONLY when you have many many atoms Strength of bonds • Energy needed to pull atoms completely apart • Use units of kcal/mol o 1 kcal/mol is 0.05 eV • Covalent, ionic, and metallic bonds are few hundred kcal/mol (i.e., up to about 5 eV) • Hydrogen bonds are about 10 kcal/mol • Van der Waals are about 1 to 5 kcal/mol Octet rule • Main-group atoms “want” to close their outermost shells, which contains 8 electrons o If only a few valence electrons, get rid of them o If close to full, grab some electrons • For H, it’s a duet rule o It’s by itself in that first row and there’s only room for two Common names for elemental ions • Number of electrons is fixed • “-ide” in the name of the compound • The halogens grab one electron, oxygen grabs two electrons, etc. What charge does the ion have? o The nonmetals and main-group will form only one ion o Heavier atoms and especially transition metals will often form more than one common ion due to the energy difference • Type I: o Cation (metal) always has the same charge (Na , Ca ) 2+ • Type II: 3+ 2+ o Cation (metal) exists in different charge forms (Fe , Fe ) Type II cations • More than one typical ion Polyatomic ions: Like irregular verbs • There are combinations but always have the same ionic form • Most are
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