Unit 4 - MoLECULES

Chemical Bonds Continued…

NaCl vs. CO2

What do you already know?

Imagine! Close your eyes and picture a piece of salt. Now zoom

in…..what does it look like at the atomic level? Now imagine carbon dioxide – what does it look like at

the atomic level? How are these two compounds different?

How do non-metals bond with each other?

Recall non-metals have______(high or low) ionization energy when compared to metals. Reason:

Result:

What is electronegativity? Why does it exhibit a distinct periodic trend?

How do nonmetals bond?

Electronegativity - Linus Pauling

Covalent bonding

Atoms are “bonded” because both nuclei (+) are attracted to the same electrons (-).

Covalent vs Ionic

Using electronegativities

Electronegativity Difference /Bond Character

> 1.7 and above / ionic0.4 - 1.7 / polar covalent0 -0.4 /nonpolar covalent

What is a molecule?

Two or more atoms covalently bonded to make a neutral particle is a unit called a molecule.

A polyatomic ion (i.e. NO3-) is very similar to a molecule,

except that it has a charge.

Covalent compounds AND most non-metal elements are composed of molecules.

H2O CO2 O2 P4

MORE KEY TERMS

All diatomic molecules are NONPOLAR COVALENT

Practice lewis dot structure

Electronegativity – tendency of an atom to attract shared electrons to itself! (greatest attraction for electrons)

What is a Lewis symbol?

Lewis symbols… are simple pictures of atoms are used to represents covalent bonds

NeGilbert N. Lewis

• •

• •

• • • •

Lewis SymbolLewis Symbol

Each dot is a valence electron

Each dot is a valence electron

Molecular Representations

Diatomic nonpolar molecules

Polar covalent vs. nonpolar covalent

What determines the structure of molecule?

“Octet rule” - atoms usually form covalent bonds with each other resulting in a total of 8 valence electrons around each atom. What is special about eight e-?

Why usually but not always 8?

What’s the actual REASON that non-metals share electrons?

Drawing Lewis Dot Structure

1. Write symbols of the elements. - if 3 or more elements figure out which one is the central one

( typically C , N, P, S – sometimes O) If all of these are present usually C is the central one.

2. Determine total # of valence electrons Don’t forget about adding or subtracting an electron/s if you are

dealing with the polyatomic ions

3. Use a single bond to connect each atom together

-> then fill in the remaining electrons around the atoms to complete the octet rule

- if total # of electrons won’t fulfill octet rule – double or triple bonds are necessary

Example: Lewis structure for water

Formula of water is H2O

Total valence electrons:Lewis structure to obey octet rule:

Practice on your own: PCl3 and SF2

Lewis structures have limitations

What does a Lewis structure show us about a molecule?

What does it NOT show?

CF4

CF4

NH3

NH3

What about the ammonium ion?

NH4+

N2

N2

nomenclature for molecules

Rules for naming covalent molecules 1. Use prefixes to indicate the # of atoms present

EXCEPTION:• Never use mono for the first element in the molecule

2. Same rules for –ide ending for the last element in molecule

Prefixes: Mono -1 hexa -6 Di- 2 hepta -7 Tri -3 octa -8 Tetra- 4 nona -9 Penta- 5 deca -10

Practice!

Name these compounds: PCl3 H2O

N2O4

SF6

Write formulas for these compounds: Diphosphorus pentoxide

Carbon tetrachloride

VSEPR theory

Linear shape (2/0)

Trigonal planar 3/0

Tetrahedral 4/0

Trigonal Pyramidal(3/1)

Bent: 2/2 or 2/1

Bonds to central atom

#lone pairs on central atom

Molecular geometry

Bond Angle

example

2 0 Linear 180

2 1 Bent 120

2 2 Bent 104.5

3 0 Trigonal Planar

120

3 1 Trigonal Pyramidal

107

4 0 Tetrahedral

109.5

Molecular Structures and the Periodic table

Summary of 4 major structural units of C.

VSEPR video

http://www.youtube.com/watch?v=i3FCHVlSZc4&NR=1

Intermolecular Forces:

1.Hydrogen bonding2.Dipole forces3.London forces (dispersion forces)

Intramolecular Forcesvs. Intermolecular

Intra vs Inter

Intramolecular forces (within a compound) Covalent and ionic bonds

Intermolecular forces (between compounds) IMF’s = Intermolecular Forces

Dipole Forces Hydrogen Bonding

LDF’s = London Dispersion Forces

Bond Polarity vs. Molecule Polarity

Bond Polarity – results from unequal sharing of the electrons in the covalent bond. Use the electronegativity differences to figure out how polar the bond is.

Molecule Polarity – a molecule is polar if it has 1 or more polar bonds and it’s shape does not cancel out the polarity. Nonpolar covalent molecules (nonpolar)

Ex: All diatomics such as N2, H2, etc…

Polar covalent molecules (polar) Ex: water, carbon monoxide, etc…

Intramolecular ForceTutorial Video: Advanced, But Slow and Methodical

Using electronegativities

Electronegativity Difference /Bond Character

> 1.7 and above / ionic0.41 - 1.7 / polar covalent0 -0.4 /nonpolar covalent

Dipole molecule -

Polar Molecule – notations/representations

Intermolecular Forces: Hydrogen bonding

Hydrogen bonding

Hydrogen bonding characteristics

1. Must involve a hydrogen that is bonded to highly electronegative element (O, F, N)

2. The slight positive on the hydrogen is attracted to a neighboring molecule’s nonbonding electron pair.

Dipole Forces (Vander wall)

Dipoles – molecules that have separate centers of partial negative and partial positive charges.

*Note dipole forces are only 1% as strong as an ionic bond attraction.

London dispersion forces (LDFs)

They are small, transient , attractive forces between NONPOLAR molecules

Larger or heavier atoms typically exhibit stronger dispersion forces than smaller, lighter molecules

http://www.youtube.com/watch?v=3t1Jn_jrsQk

http://dl.clackamas.edu/ch104/lesson9molecular_polarity.html

Melting point&

Conductivity

Ionic vs molecular compounds

Intermolecular forces and boiling points!

The GREATER The force of attraction between molecules – the higher the melting or evaporation point will be.

Think about Ionic bonding….. Which would melt the quickest when heated and why?

Aluminum nitride Sodium chloride Calcium oxide

Now what about covalent compounds…..

Boiling points of ionic vs. molecular

Melting Points and Boiling Points of Substances with Similar Formula Weights Substance FW (g/mol) Covalent

moleculesb.p. (Celsius) Ionic

compoundsb.p. (Celsius)

CS2 46.0 NaF 1695

CH4O 64.7 CaCO3 825

propane -42.1 NaCl 801

CO2 -57 MgSO4 1125

ethanol 78.5 TiO 1750

Glucose 146

H2O 100