2.2 Covalent Bonding

covalent bond

electrostatic force of attraction between one or more pairs of shared electrons and positively charged nuclei

covalent bonding occurs between...

non-metals

Covalent bonding and Voltility:

• Covalent network: solids are RTP- vapourising them requires lots of energy as strong copvalent bonds

• Covalent molecular: weak intermolecular forces that can be easily overcome- generally volitaile

octet rule

the tendency of atoms to gain a valence shell with a total of eight electrons

Lewis diagram

two-dimensional diagram of an atom where the valence shell electrons are represented as dots/crosses and covalent bonds are shown, don't show geometric shape

structural diagram

non-polar covalent bonds occur when...

two atoms have the exact same electronegativity

polar covalent bonds occur when...

two atoms have different electronegativity

partial charge

when one atom has a higher electronegativity so draws the electron pair closer to its nucleus

the charge of non-bonding (lone) electron pairs is always...

partially negative

bond length

measure of the distance between two bonded nuclei

bond strength

measure of the energy required to break the bond (in terms of bond enthalpy)

as bond length increases...

bond strength decreases (bond enthalpy goes down)

as bond length decreases...

bond strength increases (bond enthalpy goes up)

as we go down a group and the atomic radius increases, bond length should... and why?

bond length increases as there are more electron shells

having multiple covalent bonds will mean that bond lengths ... and bond strength ...

bond length decreases and bond strength (enthalpy) increases

dative covalent bond

covalent bond formed between two atoms, only one of which has provided electrons for the bond

in dative bonds (coordinate bonds), the arrow that represents the bond points towards the ...

atom accepting electrons (beneficiary)

the exceptions to the octet rule are... (incomplete octet)

beryllium (4 outer shell electrons) and boron (6 outer shell electrons)

electron domain

location of a pair or several pairs of electrons (charge centres)

the shape of the covalent molecule is determined by...

the number of electron domains

VSEPR theory states...

electron pairs in the same valence shell carry the same charge, so they repel and spread themselves out as far as possible away from each other in a three-dimensional sense

two electron domains results in...

linear shape (180°)

three electron domains with one lone pair...

bent/V-shaped (117.5°)

three electron domains results in...

trigonal planar shape (120°)

four bonded electron domains results in...

tetrahedral shape (109.5°)

four electron domains with one lone pair...

trigonal pyramid (107°)

four electron domains with two lone pairs...

bent/V-shaped (105°)

5 electron domains no lone pairs

triogonal bipyramidal 120 and 90

5 electron domains with 1 lone pair

seesaw 117.5 and 87.5

5 electron domains with 2 lone pairs

T-shape 87.5

5 electron domains with 3 lone pairs

Linear 180

6 electron domains with no lone pairs

Octahedral 90

6 electron domains with 1 lone pair

square pyramid 87.5

6 electron domains with 2 lone pairs

square planar 90

6 electron domains with 3 lone pairs

T-shape 87.5

6 electron domains with 4 lone pairs

Linear 180

a dipole molecule must satisfy these two conditions...

polar covalent bonds and asymmetrical distribution of charge

the order of strength of repulsion in electron pairs is...(from strongest to weakest)

lone-lone, bonded-lone, bonded-bonded

intramolecular bonds occur...

within the molecule

intermolecular bonds occur...

between molecules

the three types of intermolecular forces are...

London dispersion forces,dipole induced, dipole-dipole attractions and hydrogen bonds

Van de Waal's forces is a blanket term that refers to...

London dispersion forces and dipole-dipole attraction

London dispersion forces

generated by temporary or instantaneous dipoles through the random movement of electrons, present in all, most significant in non-polar

strength of London dispersion forces

weakest type of intermolecular force

London dispersion forces increase when...

the number of electrons increase (not dependant on only valence electrons)

Larger molecular size means…

More electrons → greater polarisability → stronger London Dispersion forces → more energy needed to overcome IMFs → higher boiling point

dipole-dipole attraction

attraction of two polar molecules to each other, one positive end of a molecule is attracted to the negative end of another

strength of dipole-dipole attractions

stronger than London dispersion forces, weaker than hydrogen bonds

Hydrogen bonding

combination of fluorine, oxygen or nitrogen with hydrogen (special kind of strong dipole attraction)

Solubility of covalent molecules

Intermolecular hydrogen bonds can only form if there are...

intramolecular hydrogen bonds already existent

molecules containing two atoms are described as...

diatomic

molecules containing three atoms are described as...

triatomic

bond polarity can be calculated by...

comparing the difference in the electronegativity of the two atoms, greater difference = increased polarity

net dipole moment

the overall direction that the electrons are being pulled in

Allotrope:

diff molecular arrangements of the same element in the same physical state

Diamonds structure?

• 4 covalent bonds each

• tetrahedral

• 109.5

• giant lattice

• hard: drills + glass cutters

Graphite structure:

• 3 bonds each

• graphene layers

• held together by weak Intermolecular Van der waals forces

• 120

• spare e = delocalised= conductor of electricity

• Brittle

Buckminster fullerene 60:

• 3 bonds each

• 109 and 120

• spare e delocalised

• drug delivery

Silicon structure:

• 4 bonds each sillicon

• tetrahedral

• 109.5

• giant lattice

Formal change=

valence electrons -non bonding e - ½ bonding e.

Benzene carbon hybridisation and why?

sp2 because 3 domains (1 H, 1C , 2C)

Pi system?

• 2 ring shaped clouds of electron density above and below the plane of benzene caused by the extensive sideways overlaps of P orbitals

• allows delocalised elctrons

Benzene shape and bond

hexagonal planar and 1120

Evidence for delocalisation in benzene?

• enthalpy changes of hydrogeneration

  • cyclohexene hydrogeneration = -120, so Benzene = -360 as 3 = bonds BUT actually is -208 so more stable than model

• Carbon-carbon bond lengths

  • you would think between 154 and 134 but actually all equal 140 pm so bond order is 1.5

• Saturation test

  • cyclohexene decolourises bromine water as an electrophyllic reaction takes places so it should w benzene but doesnt = no double bonds

• Infrared spec

  • benzene doesnt show peak for c=c

Paper chromatograpghy:

• blots on paper alligned with solvent level

• solvent moves up by capillary action, dissolves the substances seperating the components and moving them up

How is paper chromatograpghy optimised:

• a lid can be placed on top of the beaker to prevent solvent loss by evaporation

• paper with hydrated cellulose used- has mnay -OH groups - polar- form water layer on paper

  • can be combined with less polar iganic solvent to partition mixture components

  • seperates leaf pigments or amino acid mixtures

Thin Layer Chromatograpghy:

• Stationary phase: rectangular SiO2/ Al2O3 coated plate

• very polar surfaces- placed in non-polar solvent

• Polar substances absorob onto Silica/Alumina

• Non-polar substances move up by dissolving in the solvent

Rf value

distance by spot/ distance by solvent