Lots of multisyllabic words, I know

Taking a Step back: PolarityCovalent Bonding

atoms “bonded” through sharing of electrons

Draw H2, Cl2, and H2O

H2 and Cl2 – have an equal sharing of electrons Non Polar Covalent

H2O– unequal sharing of electrons Polar Covalent

Polar CovalentF2 vs. HF

The only time a bond is completely nonpolar is when identical elements are bonded together.

In the above example, Fluorine pulls harder on the electrons than the Hydrogen… the Fluorine end of the molecule has more electron density than the hydrogen end. Fluorine is δ- and Hydrogen is δ+

ElectronegativityIs the ability of an atom in a

molecule to draw electrons to itselfIs a combination of

Ionization Energy and Electron Affinity

Is a MADE UP NUMBER

ElectronegativityOn the periodic chart,

electronegativity increases as you go……from left to right

across a row.…from the bottom to

the top of a column.

Electronegativity and PolarityYou can use the difference in the electronegativities of

two elements to determine whether the bond they form will be nonpolar, polar, or ionic

If the difference in electronegativities is > 1.7, the bond is considered ionic.

Bond Electronegativity Difference

Type of Bond

H and H 2.1 – 2.1 = 0 Non-polar covalent

N and H 3.0 – 2.1 = 0.9 Polar Covalent

Na and Cl 0.9 – 3.0 = 2.1 Ionic

Polar Covalent BondThe greater the difference in electronegativities, the

more polar the bond is

Polar Covalent BondWhen two atoms share electrons unequally, a bond dipole

results.The dipole moment, , produced by two equal but opposite

charges separated by a distance, r, is calculated: = Qr

Polar MoleculesPredicting whether or not a molecule will be polar or

nonpolar depends on both the bonds and the geometry

To determine whether or not a molecule will be nonpolar:All terminal atoms are identicalAll of the terminal atoms are arranged symmetrically

Polar MoleculesDraw CH4 , BF3, Cl2CO, NH3

Which ones have a net dipole moment and which ones have no net dipole?CH4 and BF3 = no net dipole

Cl2CO and NH3 = net dipole

Overlap and BondingWe think of covalent bonds forming through the

sharing of electrons by adjacent atoms.In such an approach this can only occur when

orbitals on the two atoms overlap.

HybridizationAtomic orbitals (s, p, d, f) cannot adequately explain

the bonding in molecules

Consider CH4 … how do we get the tetrahedral shape out of the above orbitals

HybridizationAn averaging of atomic orbitals into a new set of

“hybrid orbitals”Consider beryllium:

In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.

HybridizationConsider beryllium:

if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.

Hybrid OrbitalsMixing the s and p orbitals yields two degenerate

orbitals that are hybrids of the two orbitals.These sp hybrid orbitals have two lobes like a p orbital.One of the lobes is larger and more rounded as is the s

orbital.

Hybrid OrbitalsThese two degenerate orbitals would align themselves 180

from each other.This is consistent with the observed geometry of beryllium

compounds: linear.

Hybrid OrbitalsWith hybrid orbitals the orbital diagram for beryllium

would look like this.The sp orbitals are higher in energy than the 1s orbital but

lower than the 2p.

Hybrid OrbitalsThink about Boron

What’s its electron configuration?How many bonds does it normally form?How?

Hybrid Orbitals…three

degenerate sp2 orbitals.

Hybrid OrbitalsFor Carbon:

Hybrid Orbitals…four degenerate

sp3 orbitals.

Hybrid OrbitalsFor geometries involving expanded octets on the central

atom, we must use d orbitals in our hybrids.

Hybrid OrbitalsThis leads to five degenerate sp3d orbitals…

…or six degenerate sp3d2 orbitals.

Hybrid OrbitalsOnce you know the electron-domain geometry, you know

the hybridization state of the atom.

Short cut… count the bonded electron domains… that’s the number of hybridizations.

# Hybrid Orbitals

Hybridization Bond Angles

Electron Pair Geometry

2 sp 180 Linear

3 sp2 120 Trigonal planar

4 sp3 109 Tetrahedral

5 sp3d 120,90 Trigonal bipyramidal

6 sp3d2 90 octahedral

Practice ProblemsSpecify the electron-pair and molecular geometry for

each underlined atom in the following list. Describe the hybrid orbital set used by this atom in each molecule or ion.

a) BBr3

b) CO2

c) CH2Cl2

d) CO3-2

Practice Problems AnswersElectron-Pair Molecular HybridGeometry Geometry Orbital

Set

a)Trigonal Planar Trigonal Planar sp2

b)Linear Linear sp

c)Tetrahedral tetrahedral sp3

d)Trigonal planar trigonal planar sp2