Ch 8 Basic Concepts of Chemical Bonding Ionic bonds Bonds due to electrostatic attractions btwn oppositely charged ions Covalent bonds Bonds formed by sharing of electrons Metallic bonds Bonds formed by electrons free to move through metal 8 1 Lewis Symbols and the Octet Rule Valence electrons Electrons in the outermost shell G N Lewis 1875 1946 o Lewis electron dot symbols Show valence electrons and track them during bond formation o Lewis symbol Element s symbol and dots to represent valence electrons The Octet Rule Octet rule Atoms tend to gain lose or share electrons until they are surrounded by eight valence electrons Octet has full s and p subshells 8 2 Ionic Bonding Electron transfer Ionic cmpds typically consist metal of low ionization energy and nonmetal of high electron affinity Curved arrow used to indicate transfer of electron Ionic substances usually brittle crystalline able to be cleaved and have high melting points Energetics of Ionic Bond Formation Formation if ionic cmpds is exothermic Attraction btwn ions releases energy and causes ions to form solid array or lattice Lattice energy The energy required to completely separate one mole of a solid ionic cmpd into its gaseous ions o Large positive values indicate strong attraction btwn ions Electrostatic potential energy given by o o Qs are charges on particles Coulombs d is distance between centers meters and k is constant For a given arrangement of ions the lattice energy increases as the charges on the ions increase and as their radii decrease o Follows trend for ionic radius A Closer Look Calculation of Lattice Energies The Born Haber Cycle Born Haber cycle Thermochemical cycle named after German scientists Max Born 1882 1970 and Fritz Haber 1868 1934 Implements Hess s Law Electron Configurations of Ions of the s and p Block Elements Increase in lattice energy is not enough to compensate for energy needed to remove an inner shell electron Ionic cmpds of metals from groups 1 2 and 3 have 1 2 and 3 cations while those of nonmetals of groups 15 16 and 17 have 3 2 and 1 anions Transition Metals Metals of group 13 often occur as 1 ions Generally do not form ions with noble gas configuration In forming ions transition metals lose the valence shell s electrons first then as many d electrons as required to reach the charge of the ion 8 3 Covalent Bonding Covalent bond Chemical bond formed by sharing a pair of electrons Ex Attractions btwn positive nuclei and negative electrons in hydrogen moleucule Lewis Structures Ex Above is a Lewis structure or Lewis dot structure Common to show shared electron pairs as lines and unshared electrons pairs lone pairs or nonbonding pairs as dots For nonmetals of valence electrons in neutral atom same as group Multiple Bonds Single bond Single covalent bond involving one shared electron pair Double bond Bond with two shared electron pairs Triple bond Bond with three shared electron pairs Length of bond btwn two atoms decreases as number of shared electron pairs increases 8 4 Bond Polarity and Electronegativity Bond polarity Measure of how equally or unequally the electrons in a covalent bond are shared Nonpolar covalent bond Bond in which electrons are shared equally Polar covalent bond Bond in which one of the atoms exerts a greater attraction for the bonding electrons than the other Ionic bond formed when difference in attraction is great enough Electronegativity Used to determine kind of bond Electronegativity The ability of an atom in a molecule to attract electrons to itself High electronegativity means negative electron affinity and high ionization energy Linus Pauling 1901 1994 o Developed electronegativity scale Increases across period and decreases down group Electronegativity and Bond Polarity Nonnpolar covalent bond results when electronegativities of bonded atoms are equal Polar covalent bond when atoms differ in electronegativity o Ex o Delta plus and delta minus partial charges Ionic bond when electronegativity difference is very large The greater the difference in electronegativity btwn two atoms the more polar the bond Dipole Moments Polar molecule A molecule in which the centers of positive and negative charge do not coincide Describe both bonds and molecules and polar and nonpolar o Arrow denotes shift in electron density toward fluorine o Crossed end indicates positive side Polar molecules attracted to ions Dipole Established when two electrical charges of equal magnitude but opp sign are separated by a distance Dipole moment The quantitative measure of the magnitude of the dipole o o o o o o Q is charge r is distance The larger the dipole moment the more polar the bond Equals zero for nonpolar molecules Reported in debyes D To convert o Electronegativity difference affects dipole moment more than change in bond length Differentiating Ionic and Covalent Bonding Continuum between extremes of ionic and covalent bonding Metals in high oxidation states form molecular substances rather than ionic cmpds 8 5 Drawing Lewis Structures Procedure 1 Sum the valence electrons from all atoms taking into account overall charge 2 Write the symbols for the atoms show which atoms are attached to which and connect them with a single bond line representing two electrons a Central atom usually less electronegative than other atoms 3 Complete the octets around all the atoms bonded to the central atom 4 Place any leftover electrons on the central atom a Even if it causes more than octet around atom 5 If not enough electrons to give central atom octet try multiple bonds Formal Charge and Alternative Lewis Structures Some instances two or more valid Lewis structures for a molecule that all obey octet rule Formal charge The charge the atom would have if each bonding electron pair in the molecule were shared equally btwn its two atoms For formal charge assign electrons to atom as follows o All unshared electrons atoms assigned to atom on which they are found o Any bond half of bonding electrons assigned to each atom in the bond Formal charges on neutral molecule add to zero Dominant structure o Formal charges closest to zero o Atoms with negative charges on more electronegative atoms are more dominant than those with negative charges on less electronegative atoms Formal charges do not represent real charges on atoms 8 6 Resonance Structures Resonance structures Lewis structures that are equivalent but have different placement of electrons o Double headed arrow to show real molecule is avg of