Covalent Model

Covalent Bond

A type of strong chemical bond in which two non-metal atoms share one or more pairs of valence electrons.

Types of atoms in a covalent bond

Non-metals

Bond length and strength pattern

Single bonds < Double bonds < Triple Bonds

Electronegativity

The ability of an atom to attract electrons when it is in a compound

Nonpolar (pure covalent) bonding

Equal sharing of electrons

The electronegativity difference between nonpolar bonding atoms

Low (^EN <=0.4)

polar covalent (permanent dipole) bond

Unequal sharing of electrons

The electronegativity difference between polar bonding atoms

^EN 0.4 -> 1.7

Less electronegative atom in polar covalent bond become...

Partial positive (delta (d) +)

Dipole

a molecule that has two poles, or regions, with opposite charges

dipole moment

a measure of the separation and magnitude of the positive and negative charges in polar molecules

Greater dipole moment =

Stronger polar bond

Molecular Polarity

uneven distribution of molecular charge (one end is more positive overall)

Signs of a polar molecule

- Polar bonds exist

- Lone pairs

- All bonded atoms are not the same

- Assymetrical polarities (dipoles =/= 0)

Signs of non-polar molecule

- No polar bonds exist

- Central atom has no lone pairs

- All bonded atoms are the same

- Molecular shape is symmetrical (dipoles = 0)

Properties of polar molecules

- Dissolves in polar solvents

- Have dipole-dipole forces

- May have hydrogen bonds

properties of non polar molecules

-doesnt conduct electricity

-volatile

-dissolves in non polar solvents

- Only have london dispersion forces

Allotropes

different molecular structures of the same element

Allotropes of carbon

diamond, graphite, buckminsterfullerene, nano tubes

Diamond structure

Each carbon atom forms four covalent bonds in a very rigid giant covalent structure

3D Tetrahedral arrangement

sp^3 hybridised

Properties of diamond

Hard

High melting point

Does not conduct electricity

insoluble in polar solvents

Graphite structure

Giant covalent lattice bonded in flat layers (hexagons) held by London dispersion

Each carbon is bonded to 3 others, leaving a delocalised outer shell e- on each atom free to move along the layers

2d Network (trigonal planar)

Properties of graphite

slippery, layered structure, bonds in layers are tight but between layers are weak, conducts electricity, high MP

Silicon dioxide (SiO2) structure

3D network (tetrahedral)

Si bonds to 4 oxygen

O bonds to 2 Si

Properties of SiO2

Very high MP

Hard

Does not conduct electricty

Buckminsterfullerene structure (C60)

Bonds to 3 other carbons (strong covalent bonds)

Made up of pentagons and hexagons (soccer ball)

sp^2 hybridised

one delocalized electron

london dispersion between molecules

Properties of buckminsterfullerene

Lower melting point than diamond

Soft - molecules slide past eachother

Poor conductor - limited electron delocalisation

Traps molecules inside a cage

Nanotubes structure

Cylindrical tube of single layer graphite

Carbon forms 3 covalent bonds

sp^2 hybridised

Properties of nanotubes

High strength, good conductors of electricity and heat, high MP

Molecular solids

solids that are composed of molecules held together by intermolecular forces

Intermolecular forces

weak forces of attraction between molecules

Other name for intermolecular forces

van der Waals

London dispersion forces

the intermolecular attraction resulting from the uneven distribution of electrons and the creation of temporary dipoles on surrounding molecules

How to make london dispersion stronger

more electrons

larger atoms

greater SA

Dipole-dipole forces

Permanent dipoles attract oppositley charged dipoles

Hydrogen bonding

H molecules around high electronegative atom - becomes so positive, it attracts other molecules dipoles

Hydrogen bonding occurs between

H, N, O, F

Importance of hydrogen bonding

Ice is less dense than water - hexongonal open lattice arrangement

water has high BP

Solubility

The ability to dissolve in another substance

Like dissovles like refers to

Similiar strength in IMF

Process of saturation for solubility

water attracts to dissolved ions - surrounds and breaks apart