chem concept 4-6

Covalent compounds: (aka molecular compounds)

a compound formed when two or more atoms share

e-.

Covalent bonds are formed when nonmetallic elements

share e-.

If there is a ΔEN, a polar

covalent bond forms because

the e- will be unequally shared.

Ex. H2O

If ΔEN = 0, a pure (nonpolar)

covalent bond forms because

the e- are equally shared. Ex. O2

ẟ

+

ẟ

+

ẟ

-

Covalent Compounds

Molecule: a neutral group of atoms held together by

covalent bonds.

A single molecule is an individual unit that can exist on its

own.

One glucose molecule

C6H12O6

One water molecule

H2O

Sodium chloride, aka, table salt

NaCl

Covalent Compounds

Diatomic elements: when 2 atoms of the same

element exist naturally as a molecule.

H2, N2, O2, F2, Cl2, Br2, and I2

Covalent Compounds

Tend to still follow the octet rule.

Bonds can be single, double, or triple.

Single bond = 2 e- shared

Ex. #1: H2

Ex. #2: H2O

H

H

H

H

H

H

O

O

H

H

Covalent Compounds

Tend to still follow the octet rule.

Bonds can be single, double, or triple.

Double bond = 4 e- shared

Ex. #3: O2

O

O

O

O

Ex. #4: CO2

C

O

O

C

O

O

Covalent Compounds

Tend to still follow the octet rule.

Bonds can be single, double, or triple.

Triple bond = 6 e- shared

Ex. #5: N2

N

N

N

N

VSEPR Theory

Valence Shell Electron Pair Repulsion

The tendency for electron pairs to be as far

apart from one another as possible

Why?

Because the valence e- are repulsed by each other!

O

O

O

H

H

N

N

C

O

O

Practice Time!

HF

CCl4

SiO2

Draw Lewis structures to represent the following molecules:

Practice Time! Key

HF

CCl4

Draw Lewis structures to represent the following molecules:

Practice Time! Key

SiO2

Draw Lewis structures to represent the following molecules:

Name the first element with a prefix to indicate

how many there are.

Naming Covalent Compounds

Formula 🡪 Name

Example: P2O5

Diphosphorus pentoxide

Name the second element with a

prefix to indicate how many there

are AND add an –ide ending.

Prefixes

1

mono

2

di

3

tri

4

tetra

5

penta

6

hexa

7

hepta

8

octa

9

nona

10

deca

Exception: Do not use “mono” for “1” on

the first element!!

CO2

P4H10

CH4

H2O2

SF6

Carbon dioxide

Tetraphosphorus decahydride

Carbon tetrahydride

Dihydrogen dioxide

Sulfur hexafluoride

Practice Time!

Write the name of the molecular compounds based on the

chemical formulas below.

Use prefixes to determine the subscripts.

How to determine the Chemical

Formula for a Covalent Compound

Name 🡪 Formula

Example: What is the chemical formula

for dichlorine monoxide?

Cl2O

Prefixes

1

mono

2

di

3

tri

4

tetra

5

penta

6

hexa

7

hepta

8

octa

9

nona

10

deca

A few notes:

If no prefix, assume it means “1”.

Do not simplify!!!

Dicarbon tetroxide

Hydrogen monosulfide

Pentaphosphorus trinitride

Sulfur pentoxide

Silicon tetrafluoride

C2O4

HS

P5N3

SO5

SiF4

Chemical formulas tell us what makes up a molecule.

Ex. C6H12O6

Structural formulas

show a 2D picture of a

molecule’s structure

But molecules are

3-dimensional!

VSEPR Theory

Tendency for electron pairs to be as far

apart from one another as possible,

because valence e- are repulsed by each

other.

Electron domains affect the overall shape of the

atom because every electron domain wants to be

as far from the others as possible!

Electron domain: the space occupied by e-

Could be:

Bonding pair = e- shared in a bond

Lone pair = e- not in a bond

Multiple bonded pairs of e- (like in a double or triple bond)

Molecular Shapes

Linear 🡪 2 electron domains

Ex. O2 and HCN

Trigonal planar 🡪 3 electron domains

Ex. BF3

Tetrahedral 🡪 4 electron domains

Ex. CH4

Trigonal bipyramidal 🡪 5 electron

domains

Ex. PCl5

Octahedral 🡪 6 electron domains

Ex. SF6

Molecular Shapes

Bent 🡪 4 electron domains

2 bonding pairs, 2 lone pairs

Ex. H2O

Trigonal pyramidal 🡪 4 electron domains

3 bonding pairs, 1 lone pair

Ex. NH3

Molecular Shapes

Can be drawn on paper with notation to represent the

3-dimensional nature, but are most easily understood

when built in a 3D model.

Ex. CH4, which is tetrahedral

“Inter” = prefix meaning “between”

or “among”

Think “international” means between

different nations

“Intra” = prefix meaning “within”

Think “intramurals” refers to sports

played within one school

As it relates to bonding, there are:

Forces between different substances

(intermolecular forces) and

Forces within a substance

(intramolecular forces = chemical

bonds)

Overview

The intramolecular forces (bonds) within a

molecule will determine whether or not a

molecule is polar.

The polarity of a molecule will determine the

intermolecular forces (attractions) between

different molecules.

The intermolecular forces (IMF) will

determine the properties we see on the

macro level.

Ex. Boiling point, vapor pressure, melting point,

viscosity, etc.

Molecule Polarity

Polar molecules:

have partial

charges in different

locations within the

overall molecule

Ex. H2O

ẟ

+

ẟ

+

ẟ

-

Nonpolar

molecules: no

partial charges

because the e- are

equally shared

Ex. O2

Molecule Polarity

Polarity can be determined by BEND (Bond

electronegativity difference).

If ΔEN = 0 🡪 nonpolar

If ΔEN is anything BUT 0, there is polarity, but

how polar the molecule is depends on how big

BEND is

ẟ+ is used to mark the atoms with the lower EN (+ side)

ẟ- is used to mark the atoms with the higher EN (- side)

Example: Rank the following molecules from least

polar to most polar based on BEND.

HCl, CO2, HF, O2

Answer: O2, CO2, HCl, HF

Molecule Polarity

Polarity can also be predicted using SNAP.

Symmetrical/Nonpolar, Asymmetrical/Polar

Intermolecular Forces

Intermolecular forces (IMF) = the attractive

forces between particles in a substance

Range in strength, but tend to be weaker than

intramolecular forces (aka, ionic, covalent and

metallic bonds) within compounds.

Polar molecules have the strongest IMF.

Why does this make sense?!

Because they have uneven charges so the partially positive

side (ẟ+) of one molecule is attracted to the partially negative

side (ẟ-) of another. (demo)

Intermolecular Forces

Dipole-dipole force = the short-range

attractive force between polar molecules.

Dipole: formed in a molecule when equal and

opposite charges are separated by a short

distance

Direction is always represented pointing from the

positive pole to the negative pole

Different types of IMF

ẟ

+

ẟ

-

ẟ

+

ẟ

-

Intermolecular Forces

Hydrogen bonding: when a hydrogen atom

bonded to a highly electronegative atom (like N,

O, or F) is attracted to an unshared pair of e- of

an electronegative atom in a nearby molecule.

Represented by

dotted lines

Ex. The hydrogen

atom in H2O

attracted to the e-

pairs of oxygen in

another H2O

ẟ

+

ẟ

-

ẟ

-

ẟ

-

ẟ

+

ẟ

+

ẟ

+

ẟ

+

ẟ

+

Covalent

bond

Hydrogen bond

Different types of IMF

Intermolecular Forces

London dispersion forces: an intermolecular

attraction resulting from an instantaneous and

temporary dipole created from the constant

motion of e-

All atoms and molecules are affected by them.

But they are the ONLY ones that impact noble gases

and nonpolar molecules

Different types of IMF

no dipole

ẟ

+

ẟ

-

temporary dipole

The Effects of IMF

Viscosity = a fluid’s resistance to movement

The stronger the IMF, the higher the viscosity.

Boiling point = the amount of kinetic energy needed

to overcome the force of attraction between a liquid’s

particles

The stronger the IMF, the higher the boiling point.

This applies to melting point, too!

Example: Consider the boiling points of the following substances.

First identify which represents an ionic bond. Then predict which of

the remaining substances have the weakest intermolecular forces,

and which have the strongest.