Chapter 24 Simple molecular substances with non-octet structures and shapes of simple molecules
24.1 Simple molecular substances with non-octet structure
Non-octet structure with less than eight outermost shell electrons
Boron trifluoride (BF3)
The boron atom contains only six (less than eight) outermost shell electrons
Boron trifluoride has a non-octet structure
Non-octet structure with more than eight outermost shell electrons
Phosphorus pentachloride (PCl5)
The phosphorus atom contains ten (more than eight) outermost shell electrons
Phosphorus pentachloride has a non-octet structure
Sulphur hexafluoride (SF6)
The sulphur atom contains twelve (more than eight) outermost shell electrons
Sulphur hexafluoride has a non-octet structure
24.2 Shapes of simple molecules
180°
120°
109.5°
107°
104.5°
Chapter 25 Polarity of bond and molecule
25.1 Polarity of bond
Electronegativity of an atom represents the power of that atom in a molecule to attract bonding
electrons
(1)
(2)
(3)
(4)
Across a period, the electronegativities of the main group elements increase from left to the right
Down a group, the electronegativities of the main group elements decrease
Fluorine is the most electronegative element with an electronegativity value of 3.98
Francium is the least electronegative element with an electronegativity value of 0.70
(5) CH bond is considered as non-polar
25.2 Polarity of molecule
Bond Polarity
Non-polar
polar
Non-polar molecules
1. Trigonal planar
2. V-shape
Molecular shape
1.
2.
3.
4.
5.
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramid
octahedral
Polar molecule
Non-polar molecule
Chapter 26 Intermolecular forces
26.1 Introducing intermolecular forces
Intermolecular forces are weak electrostatic attractions that exist between molecules
Two types of intermolecular forces:
(1) van der Waals’ forces
(2) hydrogen bonding
26.2 Van der Waals’ forces
Van der Waals’ forces exist between all molecules and are electrostatic in nature
Two types of van der Waals’ forces:
(1) dipole-dipole attractions (only exist between polar molecules)
(2) instantaneous dipole-induced dipole attractions (exist between both polar and non-polar
molecules)
Another two factors affecting the strength of van der Waals’ forces between molecules:
(1) Molecular size
(the larger the molecular size (r.m.m. , having more electrons and electron shells), the stronger
the van der Waals’ forces)
(2) Molecular shape
(the greater the area of contact between the molecules, the stronger the van der Waals’ forces)
26.3 Hydrogen bonding
Three factors for the existence of hydrogen bond:
(1) H atom is present in the molecule
(2) H atom is bonded with N/O/F atom (The 3 most electronegative atoms)
(3) Lone pair electrons are present in N/O/F atoms
(e.g. No lone pair electrons in NH4+)
Examples of hydrogen bonding:
(1) Hydrogen fluoride (HF) (one hydrogen bond for each molecule)
(2) Water (H2O) (two hydrogen bonds for each molecule)
(3) Ammonia (NH4) (one hydrogen bond for each molecule)
Effect of hydrogen bonding on properties of water
(1) A high boiling point
(2) A high surface tension
(3) A high viscosity
Effect of hydrogen bonding on properties of ehthanol
(1) A high boiling point
(2) A high viscosity
(3) Being completely miscible with water
(The hydroxyl group (-OH) of ethanol enables its molecules to form hydrogen bonds with water
molecules. Ethanol molecules interact strongly with water molcules)
Remark: (Strength of intermolecular forces)
Hydrogen bonding > Dipole-dipole attractions > Instantaneous dipole-induced dipole attractions
Chapter 27 Structures and properties of molecular crystals
27.1 Molecular crystals
Molecular crystals are crystals having an ordered arrangement of molecules.
27.2 Structure and properties of ice
Water molecules are arranged in tetrahedral shape in ice
Oxygen atoms are arranged in hexagonal shape in ice
Properties of ice:
(1) Having a open structure
(2) Less dense than liquid water
(water molecules are arranged in an orderly manner in an open structure because of the extensive
hydrogen bonding between them)
(3) Higher melting point compared with substances of similar relative molecular mass
27.3 Structure and properties of C60
C60 (buckminsterfullerene) is another allotrope of carbon, one of the commonly known fullerenes.
The 60 carbon atoms are bonded together to form a hollow sphere
Each carbon atom is bonded to three other carbon atoms
The atoms are arranged in a pattern of 20 hexagons and 12 pentagons on the surface of the sphere
One of the outermost shell electrons of each carbon atom is delocalized
Properties of C60:
(1) Lower melting point when compared with graphite and diamond
(2) Soluble in non-polar solvents like benzene C6H6 and hexane C6H14
(3) Relatively strong and hard when compared with most other molecular solids
(4) Does not conduct electricity well when pure; becomes superconducting when metal atoms (e.g.
K) are present in the structure