CHAPTER 8 AP CHEMISTRY BONDS Bond energy l

CHAPTER 8 AP CHEMISTRY

BONDS ¡ Bond energy l l ¡ Ionic bonds l l ¡ very stable, high melting point Na. Cl, 800 o. C electrostatic attractions of closely packed opposite charged ions metal and nonmetal usually form these bonds coulomb’s law l l ¡ energy required to break a bond forms ions because it will have lowest energy this way energy of interactions E = 2. 31 x 10 -19 J. nm (Q 1 Q 2) r Q 1 and Q 2 numerical charge distance between the Na+ and Cl- is 2. 76 Å

CONTINUE A bond will form when the energy in a compound is lower than as separate atoms ¡ this will minimize the sum of positive (repulsive) energy and negative (attractive) energy terms and is called the BOND LENGTH ¡

ELECTRONEGATIVITY ¡ ¡ Ability of an atom to attract shared electrons to itself page 334 polarity dipole moment +-------> б. Б+ б+ б- show the direction of bond polarities and which have dipole moment HCl, Cl 2 , SO 3, CH 4, H 2 S б- б-

PREDICTING IONIC COMPOUNDS ¡ ¡ ¡ ¡ Ca + O <=> Ca 2+ + O 2 calcium is transferring 2 electron‘s to the oxygen Ion size determines structure and stability of ionic solids, properties of ions in aqueous solution, and the biological effects of the ions positive ions are smaller than parent atoms negative ions are larger than parent atoms Isoelectronic ions have the same electron configuration as the noble gas it is close to. N 3 -, O 2 -, F-, Ne, Na+, Mg 2+, Al 3+ page 341

LATTICE ENERGY ¡ ¡ ¡ ¡ ¡ change in energy when gaseous ions form ionic solids page 256 lattice energy = k(Q 1 Q 2), k = proportionality ( r ) constant page 357 -358 covalent bonds with ionic characteristics dipole moment gives a covalent bond a percent of ionic character (measured dipole moment X-Y) X 100 (calculated dipole moment X+ Y-) compounds with more than 50% ionic characteristics are considered ionic compounds If the compound when melted can conduct an electrical current it is called a SALT

COVALENT BONDING forces which hold nonmetal atoms together ¡ because this bond consists of sharing electrons it is very stable ¡ Why should the sharing of two electrons between two nuclei result in stability? ¡ l l when two hydrogen atoms approach each other the electron of one atom is attracted to the other nucleus, this lowers the electrostatic energy of the system by overlapping orbitals, energy can be exchanged and overall energy is reduced

LEWIS STRUCTURES ¡ eight electrons in the valence shell are more valence electrons are in the outer most energy level stable ¡ use dot diagrams to show this number of valence electrons = 1 to 8 In the lewis structure a covalent bond is a line. single bond = 2 electrons shared double bond = 4 electrons shared triple bond = 6 electrons shared 8 electrons will never be shared, too much repulsion ¡ ¡ ¡

WRITING LEWIS STUCTURES ¡ ¡ ¡ ¡ count the number of valence electrons NH 3 C 2 H 4 l 5+3(1) = 8 2(4) + 4(1) = 12 CH 3 OH l 4 + 3(1) + 6 + 1 = 14 draw a skeleton structure by joining the atoms by single bonds central atom written first in the formula the following are usually terminal atoms l H, Br, F, I, Cl, O make sure all bonds are accountable determine the number of valence electrons that have not been accounted for

CONTINUE draw the Lewis structures for ¡ hydroxide ion OH¡ hydroxyl amine NH 2 OH ¡ chlorate ion Cl. O 3¡ Cl 2 CO ¡ HCN ¡

RESONANCE FORMS ¡ ¡ ¡ in certain cases, the Lewis structure does not adequately describe the properties of the ion or molecule that it represents NO 2 the concept of resonance is involved whenever a single Lewis structure does not explain all bonds write three resonance forms for the nitrate ion Formal charge l ¡ ¡ ¡ the charge on an atom in a molecule or ion that is calculated by assuming that all lone pairs of electrons shared by an atom, belong to the atom Cf = Ev - (Eu + 1/2 Eb) Cf = formal charge, Ev = # of valence electrons Eu= # of unshared electrons, Eb= # of bonded electrons

CONTINUE ¡ ¡ ¡ the most likely Lewis structure is one where the formal charge is close to zero any negative formal charge is on the most electronegative atom (not the central atom) determine the formal charge in nitrogen (I) oxide N-O-N N-N-O

EXCEPTIONS TO THE OCTET RULE ¡ ¡ ¡ ¡ ¡ molecules that contain an odd number of valence electrons NO(11 valence electrons) NO 2 (17 valence electrons) species that contain an odd number of electrons are paramagnetic (attracted by a charge) less than a full octet of electrons BF 3 Be. F 2 expanded octet surrounded by more than four pairs of valence electrons Xe. F 2, Xe. F 6, Xe. F 4, SF 6 draw the Lewis structure for ICl 4 -

BOND ENERGY ¡ ¡ ¡ ¡ ¡ enthalpy change, ΔH, to break a bond in a mole of gaseous substance go over page 365 endothermic if ΔH > 0 exothermic if ΔH < 0 ΔH = (bond energies broken) -(bond energies formed) Cl-Cl(g) + H-CH 3 <=> H-Cl(g) + Cl-CH 3(g) ΔH = Σ(239 + 413) - Σ(427 + 339) -114 k. J estimate ΔH H-N-N-H (g) N-N (g) + 2 H-H (g) H H

CONTINUE ¡ ¡ ¡ bond strength and length l as the number of bonds increase the length decreases making then stronger bond polarity l nonpolar bond atoms share electrons equally (small electronegative difference) l polar bond atoms share electrons unequally (large electronegative difference) l ionic bond atoms do not share electrons (very large electronegative difference) review oxidation numbers what is the oxidation state of the underlined element P 2 O 5, Na. H, Cr 2 O 72 -, Sn. Br 4, Ba. O 2