Chem 101 Exam 3

Electronegativity

The ability of an atom to attract bonding electrons to itself.

Electronegativity trend

Electronegativity increases across a period and decreases down a group.

Most electronegative element

Fluorine.

Least electronegative element taught in slides

Francium.

Effect of larger electronegativity difference

Produces a more polar bond.

General bond polarity rule used in class

If ΔEN > 0.4, treat the bond as polar.

Ionic bond

Usually formed between a metal and a nonmetal.

Polar covalent bond

Formed between nonmetals with different electronegativities.

Nonpolar covalent bond

Formed between nonmetals with similar electronegativities.

Central atom in a Lewis structure

Usually the least electronegative atom except hydrogen.

First step in drawing a Lewis structure

Choose the skeletal structure.

Second step in drawing a Lewis structure

Count total valence electrons.

Third step in drawing a Lewis structure

Place single bonds.

Fourth step in drawing a Lewis structure

Complete terminal atom octets.

Fifth step in drawing a Lewis structure

Place remaining electrons on the central atom.

Sixth step in drawing a Lewis structure

Form multiple bonds if necessary.

Final step in drawing a Lewis structure

Verify the electron count.

How are anions handled in Lewis structures?

Add electrons equal to the negative charge.

How are cations handled in Lewis structures?

Subtract electrons equal to the positive charge.

How are ions shown in Lewis structures?

Place brackets around the structure and include the charge.

Resonance structures

Different valid Lewis structures with the same atom connectivity and same total electrons.

What changes between resonance structures?

Only electron placement changes.

Can resonance structures have different atom connectivity?

No, atom connectivity remains the same.

Do resonance structures have the same total number of electrons?

Yes.

Resonance hybrid

The actual structure represented as an average of resonance contributors.

What is the actual structure of a molecule with resonance?

The resonance hybrid, not any single resonance structure.

Effect of charge delocalization

Stabilizes a molecule or ion.

Formal charge

An electron bookkeeping system used to evaluate Lewis structures.

Formal charge formula

FC = Valence electrons − [Nonbonding electrons + 1/2(Bonding electrons)].

Instructor shortcut for formal charge

FC = Valence electrons − (dots + bonds).

Sum of formal charges in a neutral molecule

0.

Sum of formal charges in an ion

Equal to the ion charge.

What is the total formal charge of a neutral molecule?

0.

What is the total formal charge of a polyatomic ion?

Equal to the ion charge.

Preferred formal charge arrangement

Formal charges should be as small as possible.

Location of negative formal charge when possible

On the more electronegative atom.

Best resonance contributor

Usually the structure that minimizes formal charges and places negative charge on more electronegative atoms.

Three exceptions to the octet rule

Odd-electron species, incomplete octets, and expanded octets.

Odd-electron species

Molecules with an odd number of electrons that cannot give all atoms complete octets.

Example of an odd-electron species

NO.

Hydrogen octet exception

Hydrogen is stable with 2 electrons.

Beryllium octet exception

Beryllium commonly has 4 electrons around it.

Boron octet exception

Boron commonly has 6 electrons around it.

Expanded octet

A central atom with more than 8 electrons.

Which elements can commonly have expanded octets?

Period 3 and heavier nonmetals.

Why are expanded octets often favored?

They can reduce formal charge.

VSEPR theory

Electron groups repel each other and arrange themselves as far apart as possible.

What counts as one electron group?

A lone pair, single bond, double bond, or triple bond.

How many electron groups does a double bond count as in VSEPR?

One electron group.

How many electron groups does a triple bond count as in VSEPR?

One electron group.

Electron geometry

The arrangement of all electron groups around a central atom.

What determines electron geometry?

The total number of electron groups around the central atom.

Molecular geometry

The arrangement of only atoms around a central atom.

What determines molecular geometry?

The number of bonding groups and lone pairs around the central atom.

Difference between electron geometry and molecular geometry

Electron geometry includes lone pairs; molecular geometry describes the arrangement of atoms only.

Electron geometry with 2 electron groups

Linear.

Bond angle for linear geometry

180°.

Hybridization for 2 electron groups

sp.

Electron geometry with 3 electron groups

Trigonal planar.

Bond angle for trigonal planar geometry

120°.

Hybridization for 3 electron groups

sp².

Molecular geometry with 3 electron groups and 1 lone pair

Bent.

Electron geometry with 4 electron groups

Tetrahedral.

Bond angle for tetrahedral geometry

109.5°.

Hybridization for 4 electron groups

sp³.

Molecular geometry with 4 electron groups and 1 lone pair

Trigonal pyramidal.

Molecular geometry with 4 electron groups and 2 lone pairs

Bent.

Electron geometry with 5 electron groups

Trigonal bipyramidal.

Bond angles for trigonal bipyramidal geometry

90° and 120°.

Hybridization for 5 electron groups

sp³d.

Molecular geometry with 5 electron groups and 1 lone pair

Seesaw.

Molecular geometry with 5 electron groups and 2 lone pairs

T-shaped.

Molecular geometry with 5 electron groups and 3 lone pairs

Linear.

Electron geometry with 6 electron groups

Octahedral.

Bond angle for octahedral geometry

90°.

Hybridization for 6 electron groups

sp³d².

Molecular geometry with 6 electron groups and 1 lone pair

Square pyramidal.

Molecular geometry with 6 electron groups and 2 lone pairs

Square planar.

Effect of lone pairs in VSEPR theory

They repel more strongly than bonding pairs.

Why do lone pairs decrease bond angles?

They occupy more space and cause greater repulsion.

Straight line in a 3D molecular drawing

Bond lies in the plane of the paper.

Solid wedge in a 3D molecular drawing

Bond comes out of the page.

Dashed or hatched wedge in a 3D molecular drawing

Bond goes behind the page.

Requirements for a polar molecule

Polar bonds and an asymmetrical arrangement of dipoles.

What must be true for a molecule to be nonpolar?

Its bond dipoles cancel so the net dipole is zero.

Can a molecule with polar bonds be nonpolar?

Yes, if molecular geometry causes the bond dipoles to cancel.

Why is HCl polar?

It contains a polar bond.

Why is H₂O polar?

It has polar O-H bonds and a bent geometry so dipoles do not cancel.

Why is CO₂ nonpolar?

The molecule is linear so bond dipoles cancel.

Procedure for determining molecular polarity

Draw Lewis structure, determine geometry, determine bond polarity, draw dipoles, add dipoles, and determine whether they cancel.

Molecular geometries typically identified as polar in the slides

Bent, trigonal pyramidal, seesaw, T-shaped, and square pyramidal.

Effect of polarity on intermolecular forces

Polar molecules generally have stronger intermolecular attractions.

Effect of polarity on boiling point

Polar molecules generally have higher boiling points.

Solubility of polar substances in water

Polar substances tend to dissolve in water.

Solubility of ionic compounds in water

Ionic compounds tend to dissolve in water.

Why does oil not mix with water?

Oil is nonpolar and water is polar.

Bond length trend with bond order

Higher bond order gives a shorter bond.

Relationship between bond order and bond length

As bond order increases, bond length decreases.

Relationship between bond order and bond strength

As bond order increases, bond strength increases.

Relative bond lengths of single, double, and triple bonds

Triple bonds are shortest, then double bonds, then single bonds.