AP Chemistry Basics of Chemical Bonding Properties of substances are largely dependent on the bonds holding the material together. Basics of Bonding A chemical bond occurs when atoms or ions are strongly attached to each other. Ionic bonds involve the transfer of e– and the subsequent electrostatic attractions. -- “metal/nonmetal” (“cation/anion”) Covalent bonds involve the sharing of e– between two atoms. -- “nonmetal/nonmetal” metallic bonds: each metal atom is bonded to several neighboring atoms -- bonding e– (i.e., valence e–) are free to move throughout the material Lewis symbols show ONLY the valence e– (i.e., the ones involved in bonding). C N S octet rule: atoms “want” 8 v.e– -- several exceptions (a topic for later) Gilbert Lewis (1875–1946) Ionic Bonding Na(s) + ½ Cl2(g) Na + Cl NaCl(s) Na+ + [ Cl ] lattice energy: the energy required to separate 1 mole of solid ionic compound into gaseous ions -- a measure of stability NaCl(s) Na+(g) + Cl–(g) DHlatt = +788 kJ/mol “Salts” are brittle – solids with high melting points. DHfo = –410.9 kJ/mol enthalpy (i.e., heat) of formation In general, ionically bonded substances have... …big, (+) lattice energies. Because lattice energies are electrostatic in nature, two variables are involved in how big they are: 1. the magnitude of the charges (“wears the pants”) 2. the separation between the ions Charge can easily be twice as large (i.e., 2+ and 2– vs. 1+ and 1–), but the separation never varies by that much. Put the following in order of increasing lattice energy: LiBr, FeN, CdO. LiBr < CdO < FeN Now these: MgS, MgCl2, MgO. MgCl2 < MgS < MgO With transition-metal ions, the e– lost first come from the subshell with the largest value of n. e.g., Ni = [Ar] 4s2 3d8 Ni2+ = [Ar] 3d8 Ni3+ = [Ar] 3d7 Recall that polyatomic ions are groups of atoms that stay together and have a net charge. -- e.g., NO3– CH3COO– -- their atoms are held to each other by… covalent bonds Cations are smaller than the neutral atoms from which they are derived. e.g., Li 1s2 2s1 Li+ 1s2 -- orbitals might be completely vacated -- e–/e– repulsion always decreases Fe Fe2+ Fe3+ Anions are larger than the neutral atoms from which they are derived. -- more e–/e– repulsion Cl p+, 17 17 e– Cl– 17 p+, 18 e– An isoelectronic series is a list of species having an identical electron configuration. e.g., N3–, O2–, F–, Ne, Na+, Mg2+, Al3+ (big radius) (small radius) EX. Which has the largest radius? Rb+ Sr2+ (smallest Zeff) Y3+ (largest Zeff)