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Cheat sheet of p1 and p2 for chem a level, 03. Chemical Bonding. CAIE
Definitions
Electronegativity is the ability of an atom to attract a pair of electrons towards itself in a covalent bond |
Ionic bonding is the electrostatic attraction between oppositely charged ions (positively charged cations and negatively charged anions |
Metallic bonding is the electrostatic attraction between positive metal ions and delocalised electrons |
Covalent bonding is electrostatic attraction between the nuclei of two atoms and a shared pair of electrons |
Bond energy is the energy required to break one mole of a particular covalent bond in the gaseous state |
Bond length is the internuclear distance of two covalently bonded atoms |
Sigma bond is the covalent bond formed by ‘head on’ overlap of atomic orbitals |
Pi Bond is the covalent bond formed by sideways overlap of atomic orbitals |
Dipole-Dipole Forces are intermolecular attractions between molecules which are permanently polarised |
Hydrogen Bond is the electrostatic attraction between a hydrogen atom which is bonded to a very electronegative atom and an electronegative atom of a neighbouring molecule |
Van Der Waals Forces are intermolecular forces of attraction which arise from temporary dipoles in molecules |
Properties of water
High melting & boiling points |
Water has high melting and boiling points which is caused by the strong intermolecular forces of hydrogen bonding between the molecules |
High surface tension |
Surface tension is the ability of a liquid surface to resist any external forces. The water molecules at the surface of liquid are bonded to other water molecules through hydrogen bonds These molecules pull downwards on the surface molecules causing the surface them to become compressed and more tightly together at the surface |
Density |
In ice the water molecules are packed in a 3D hydrogen-bonded network in a rigid lattice. Each oxygen atom is surrounded by hydrogen atoms. This way of packing the molecules in a solid and the relatively long bond lengths of the hydrogen bonds means that the water molecules are slightly further apart than in the liquid form |
Hydrogen bonding in water, causes it to have anomalous properties such as high melting and boiling points, high surface tension and anomalous density of ice compared to water
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Factors influencing electronegativity
Nuclear charge |
Increase in nuclear charge increases electronegativity |
Atomic radius |
Increase in atomic radius decreases electronegativity |
Shielding by inner shells |
Shielding causes decrease in electronegativity |
Paulings electronegativity to predict bonds
Paulings electronegativity |
Bond |
< 1.0 |
Covalent |
1.0 - 2.0 |
Polar covalent |
> 2.0 |
Ionic |
Metallic bonding
In a metal, atoms are packed together in a lattice
Electrostatic attraction increases with:
Increase in positive charge
Decrease in size of metal ions
Increase in number of mobile electrons
σ and π bonds
A pi bond is weaker than a sigma bond because the overlapping of charge clouds is less than in a sigma bond |
H₂ has 1 σ bond |
C₂H₆ has only σ bonds |
C₂H₄ has 1 σ and 1 π bond |
HCN has 1 σ and 2 π bonds |
N₂ has 1 σ and 2 π bonds |
VSEPR Theory and Molecular Shapes
Van der Waals' Forces & Dipoles
Id - id forces increase with |
Increasing number of electrons (and atomic number) in the molecule |
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Increasing the places where the molecules come close together |
For small molecules with the same number of electrons, pd - pd forces are stronger than id - id |
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Electronegativity relations
Ionic bonding
Ionic bonds are non-directional, each cation will attract any neighbouring anion and vice versa to form a huge ionic lattice
Covalent bonding dot and cross
Special octets
If the central atom is from Period 2 of the Periodic Table, the total number of electrons surrounding it cannot exceed eight ( but can less than eight ) .
If the central atom is from Period 3 and beyond , the total number of electrons surrounding it can exceed eight
Hybridisation: sp, sp², sp³
sp |
two different bonds(one may be triple bond) |
sp2 |
three different bonds(one may be double bond) |
sp3 |
four different bonds |
Hydrogen bonding
Hydrogen bonding is the strongest form of intermolecular bonding |
For hydrogen bonding to take place the following is needed: |
A species which has an O or N (very electronegative) atom with an available lone pair of electrons |
A species with an -OH or -NH group |
For hydrogen bonding to take place, the angle between the -OH/-NH and the hydrogen bond is 180o |
Bond Energy and Bond Length: Reactivity
Shorter bond length generally means higher bond energy, making molecules less reactive |
Triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the two atoms |
Triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the two atoms |
The reactivity of a covalent bond is greatly influenced by: The bond polarity, the bond strength, the bond type (σ/π) |
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