Basics of Chemical Bonding AP Chemistry 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). octet rule: atoms “want” 8 v.e – -- several exceptions (a topic for later) SCN Gilbert Lewis (1875–1946)

Ionic Bonding Na(s) + ½ Cl 2 (g) NaCl(s) “Salts” are brittle solids with high melting points.  H f o = –410.9 kJ/mol 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)  H latt = +788 kJ/mol enthalpy (i.e., heat) of formation Na Cl +Na + + [ Cl ] 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 2. the separation between the ions (“wears the pants”) Put the following in order of increasing lattice energy: LiBr, FeN, CdO. LiBr < CdO < FeNMgCl 2 < MgS < MgO Now these: MgS, MgCl 2, MgO. Charge can easily be twice as large (i.e., 2+ and 2– vs. 1+ and 1–), but the separation never varies by that much.

With transition-metal ions, the e – lost first come from the subshell with the largest value of n. e.g., Ni = [Ar] 4s 2 3d 8 Ni 2+ =[Ar] 3d 8 Ni 3+ =[Ar] 3d 7 Recall that polyatomic ions are groups of atoms that stay together and have a net charge. -- their atoms are held to each other by… covalent bonds -- e.g., NO 3 – CH 3 COO –

Cations are smaller than the neutral atoms from which they are derived. e.g., Li 1s 2 2s 1 Li + 1s 2 -- orbitals might be completely vacated -- e – /e – repulsion always decreases FeFe 2+ Fe 3+

Anions are larger than the neutral atoms from which they are derived. -- ClCl – 17 p +, 17 e – 17 p +, 18 e – An isoelectronic series is a list of species having an identical electron configuration. more e – /e – repulsion e.g., EX.Which has the Rb + Sr 2+ Y 3+ largest radius? N 3–, O 2–, F –, Ne, Na +, Mg 2+, Al 3+ (small radius)(big radius) (smallest Z eff )(largest Z eff )