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
N S C Lewis symbols show ONLY the valence e– (i.e., the ones involved in bonding). atoms “want” 8 v.e– octet rule: several exceptions (a topic for later) -- Gilbert Lewis (1875–1946)
Na [ Cl ] Cl enthalpy (i.e., heat) of formation Ionic Bonding Na(s) + ½ Cl2(g) NaCl(s) “Salts” are brittle solids with high melting points. – + Na+ + lattice energy: the energy required to separate 1 mole of solid ionic compound into gaseous ions DHfo = –410.9 kJ/mol -- a measure of stability NaCl(s) Na+(g) + Cl–(g) DHlatt = +788 kJ/mol 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”) 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. Now these: MgS, MgCl2, MgO. LiBr < CdO < FeN 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– covalent bonds -- their atoms are held to each other by…
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 Cl– 17 p+, 18 e– 17 p+, 17 e– An isoelectronic series is a list of species having an identical electron configuration. N3–, O2–, F–, Ne, Na+, Mg2+, Al3+ e.g., (big radius) (small radius) EX. Which has the Rb+ Sr2+ Y3+ largest radius? (smallest Zeff) (largest Zeff)