Chemical Bonding!! Coulomb?s law: What causes atoms to stick together to form molecules? 2 21 r qq F ? But the electrons are not standing still? Still worse, they are not behaving as little tiny particles but rather as waves? so instead we make approximations! H (1s 1 ) + H (1s 1 ) 2 protons 2 electrons Noble Gases Almost never react to form compounds because of their stable electron configurations Noble gases (Group 8A) have filled principal shells First compound containing a noble gas (XeF 4 ) was formed in 1962 at Argonne National Lab First compound of Ar formed in 2000 (HFAr) Historical Notes: XeF 2 Lewis Dot Diagrams Bonding theory based on the number of electrons in the outermost "valence" shell of the atom ?Core electrons? do not participate Noble gas configurations represent extra stability G. N. Lewis Journal of the American Chemical Society v38 p762 (1916) Lewis Symbols Dots represent the valence electrons First four dots are placed singly on each of four sides, then paired (similar to Hund?s rule) Example: Give Lewis symbols for magnesium, silicon, and phosphorus. Ionic Bonds Consider the reaction between Na and Cl to form table salt The two ions have opposite charges which are attracted to each other and form an ion pair We call this ionic bonding because it derives from electrostatic interaction of an ion pair Ionic Crystals Ionic compounds also derive stability from their extended structures (all the ions interact with the other ions) Example: Use Lewis symbols to show the formation of ionic bonds between magnesium and nitrogen. What are the name and formula of the compound that results? Covalent Bonds Quantum mechanical calculation of the electron density for H 2 Electron density increase between the nucleus leads to attractive force Covalent bonds result from the sharing of electrons between atoms Electrons are shared so that each element can obtain a noble gas configuration (octet rule and duet rule) Lewis Structures Shared pairs of electrons are called bonding pairs (often denoted by a solid line ?) Electron pairs which are not shared are called lone pairs Example: Give the Lewis symbols for the elements nitrogen and fluorine. Draw the Lewis structure of the stable binary (two element) compound formed when N reacts with F. Second Row Hydrides Lewis structures do not predict shape (Chapter 9) Multiple Bonds Sometimes atoms share more than one electron pair to obtain an octet. This results in double (two pairs shared) and triple (three pairs shared) bonds. NN triple bond CO double bond Bond Polarity Bond polarity describes the sharing of electrons between atoms Electrons shared equally Electrons shared unequally Low e - density (net positive) High e - density (net negative) ?? ?? nonpolar covalent polar covalent Electronegativity Electronegativity is a measure of the ability of an atom in a molecule to attract electrons to itself Related to trends in ionization energy and electron affinity Ionic/Covalent Character of Bonds Example: Arrange the following sets of atoms in the expected order of increasing electronegativity. (a) Cl, Mg, Si (b) As, N, Sb (c) Na, P, O Example: Use electronegativity values to arrange the following bonds in order of increasing polarity: Br?Cl, Cl?Cl, Cl?F, H?Cl, I?Cl Jon Camden Microsoft PowerPoint - lecture14_chap8.ppt
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