2. Bonding, Molecules, Intermolecular Forces

Which electrons play a role in bonding?

Valence electrons (in the outermost shell)

Octet rule

G.N. Lewis; atoms will behave to gain a full octet/valence

Lewis Structure

Lone pairs count as 2 electrons; single bond counts as 2 electrons

Exceptions to Octet Rule - Incomplete Octets

• Hydrogen, He are stable with 2 valence

• Li stable with 3 valence

• Be is stable with 4 valence

• B is stable with 6 valence

Exceptions to Octet Rule - Expanded Octets

Atoms in the 3rd period and higher (P, Si, S, etc.) can have more than 8 valence because they aren’t limited to s and p subshells

When will octet rule not work?

When there is an odd # of valence in a molecule, radical is required on lewis structure

Ionic Bonding

Strongest; between cations/anions (nonmetal/metal); electrons are completely TRANSFERRED

Ionic Compound Properties

Highly ordered crystal lattice, high MP, brittle, “electrolytes”

Why are ionic compounds electrolytes?

When they dissociate, they conduct electricity

Covalent Bonding

Sharing of electron pairs; electrons are attracted by the nucleus of both atoms

Nonpolar Covalent

Between the same or very similar electronegative atoms (less than .5 difference)

Polar Covalent

Between a very electronegative and non-EN atom (.5-1.7 difference); more EN will gain partial positive charge (dipole)

More electronegative =

Uneven electron distribution due to dipole

Dipole moment

Electrons flow towards the more EN atom, conferring partial ± charges; calculated by adding up ALL dipoles

Isoelectronic Pair

Atom and ion with the same electron configuration (B- and C)

Can polar bonds exist in a nonpolar molecule?

Yes, since dipoles can cancel each other out

Coordinate Covalent Bond

Both shared electrons originate from the SAME atom (NH3 bonded to BF3, lone pair nitrogen is used)

Bond Order

Number of bonds between 2 atoms (Single - 1, Double - 2, Triple - 3)

Sigma Bond

End-to-end overlap of atomic orbitals along axis

Pi Bond

Parallel atomic orbitals perpendicular to the internuclear axis

Are sigma or pi bonds stronger?

Sigma

How many pi bonds in a triple bond?

2 (always 1 sigma bond)

Can bond rotation occur in triple/double bonds?

No, the pi bond would break.

Bond energy and bond length:

Bond order increase → Bond energy increasing → Bond length decreasing

Metallic Bonding

Weakest bonding due to delocalization (electrons are attracted to multiple nuclei)

Intermolecular Forces

Forces that act between two or more molecules; not within a molecule (intra)

Are intermolecular forces stronger or weaker than intramolecular forces?

Inter are WEAKER than intra (easily disrupted by heat)

Four types of Intermolecular Forces (Decreasing Strength):

1. Ion-Dipole (Strongest)

2. Hydrogen Bonding

3. Dipole-Dipole

4. London-Dispersion (Weakest)

Van der Waals Forces are in these IMFs:

1. Hydrogen bonding

2. Dipole-dipole

3. LDF

London Dispersion Force (VDW)

Weakest IMF found in any pair of molecules; result in temporary dipoles that induces one in another molecule

LDF Strength increases as molecular size…

Increases; more likely to form temporary dipole

Dipole-Dipole Interactions

Occur between polar molecules (- side of dipole of one is attracted to the + side of the other dipole)

Important Dipole-Dipole Interaction

Carbonyls; C=O bond (+C, -O) allows for many reactions

Hydrogen Bonding

Partial positive H attached to O, N, or F is attracted to O, N, or F of another molecule

Affect of IMFs Boiling Point/Melting Point

Stronger IMFs = Higher MP/BP

Ion-Dipole Force

Occurs between ions and molecules having a dipole

Lewis Structures

Valence electrons are shown by dots, covalent bonds shown by lines (add up valence for each atom in a molecule)

Resonance

Compounds that have more than one Lewis Structure due to differing distribution of electrons (Ex. NO2)

True Resonance Structure

Hybrid of all possible resonance that resembles the most stable one (partial double bonds)

Bond order of hybrid double bond due to resonance:

1.5

Why does resonance make a molecule more stable?

Distribution of electrons across multiple atoms

Formal Charge =

Valence Electrons - Number of Bonds - Lone Pair Electrons

Orbital Hybridization

When atoms combine to form a molecule, their orbitals overlap (sp, sp2, sp3 hybridized)

Number of hybrid orbitals =

Number of s and p superscripts (s¹p² = 3 sp2 orbitals)

How to determine hybridizatoin:

Count the number of groups around the central atom (include lone pair as 1 group)

VSEPR Theory

Use of Lewis structure and electronic relationships to predict shapes of molecules

Electronic vs. Molecular Geometry

Electron geometry considers lone pairs, molecular geometry does NOT (even if there are lone pairs in the molecule)

How to determine electron geometry:

Base on hybridization (sp = linear, sp2 = trigonal planar, sp3 = tetrahedral)

2 bonds, no lone pairs =

Linear, 180^o

3 bonds, no lone pairs =

Trigonal planar, 120^o

4 bonds, no lone pairs =

Tetrahedral, 180^o

3 bonds, 1 lone pair =

Trigonal pyramidal, 107^o

2 bonds, 2 lone pairs =

Bent, 104.5^o

5 bonds, no lone pairs =

Trigonal bipyramidal; 90^o, 120^o, 180^o

6 bonds, no lone pairs =

Octahedral; 90^o, 180^o

Why do bond angles decrease with lone pairs?

Lone pairs are stronger and push bonded atoms closer together