Chem 330 Midterm 2

formal charge

FC = #VE - # electrons in lone pairs - # bonds

sum of formal charges should be the charge of molecule

structures with lower formal charges are more stable

negative FC goes on most electronegative atom for stability

exceptions to octet rule

odd #VE → unpaired electron goes on most electronegative atom

B and Al can have less than 8 electrons

elements with n ≥ 3 can have more than 8 electrons

molecular geometry nomenclature

A- central atom

X- bonding atom

E- lone pair

steric number

SN = # atoms bonded to central + # lone pairs

relative repulsion strength

LP/LP > LP/bond > bond/bond

molecule w lone pair vs bonding atom

when have equivalent structure, bond angles are smaller in molecules with LPs compared to only bonding electrons

relationship between atomic size and bond angle

as size increases, LPs occupy larger spaces → bond angles smaller

sigma bond

single bond, cylindrically symmetric, no nodal plane along bond axis

pi bond

two lobes on each side of bond axis, nodal plane on bond axis, weaker interaction

electron promotion

electron moves up to a higher energy orbital to make more electrons available for bonding

pi bond

formed by unpaired electrons in 2p orbitals

determining hybridization

number hybrid orbitals = # lone pairs + # bonded atoms

2 → sp

3 → sp²

4 → sp³

don’t hybridize terminal, single bonded atoms

energy of antibonding vs. bonding orbitals

antibonding higher

bond order

BO = ½ (# bonding electrons - # antibonding electrons)

larger BO → shorter bond length → higher dissociation energy

sigma 2pz vs pi 2px and y relative energy

if Z < 8, pi < sigma

if z >=8, sigma < pi

relative energy of antibonding orbitals doesn’t change with z

diamagnetic vs. paramagnetic

diamagnetic → no unpaired electrons

paramagnetic → has unpaired electrons, will be magnetic

non-bonding MO’s

large energy difference between atomic orbitals

depending on shape, constructive and destructive interference can cancel (2px or y + 1s)

electrons not counted for bond order

combining models

use localized model for sigma bonds, left over electrons in unhybridized orbitals go to pi bonds

draw energy diagram for pi bonds, destructive interference = node + higher energy

energy of breaking/forming bonds

breaking - endothermic

forming - exothermic

ΔHb

standard bond enthalpy

indicates reactants and products in standard state (1 atm, 25 C)

ΔHr

enthalpy of reaction

pos - endo, neg - exo

reaction enthalpy calculation

ΔHr = sum of ΔHb(reactants) - sum of ΔHb(products)

ΔHr = sum of ΔHf(products) - sum of ΔHf(reactants)

low bond order

less stable/more reactive molecule

molecular vs. electron domain geometry

molecular- takes into consideration lone pairs and atoms

electron domain- what geometry would be if every e- domain was an atom

heteronuclear MO’s

s of atom with lower n mixes with pz of atom with higher n

px and py of higher n nonbonding

s of atom with higher n nonbonding