IB Chapter 14 AHL Notes

Lewis Structures

• Covalent bonds result from overlapping atomic orbitals
• Lewis structures: a notation of dots and cross to represent the outer energy level electrons (valence shell electrons) of all the atoms in the molecule
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• Pie de imagen: Carbonate Ion Lewis Structure, Source: YCIS Kognity

Resonance Structures

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• Pie de imagen: Resonance form of the carbonate ion, Source: YCIS Kognity
• Resonance Structures (discussed in Chapter 4)
• The actual ion is called a resonance hybrid, because it is a combination of all of the shown resonance structures
• The bond strengths are all equal, it is stronger than a single bond but weaker than a double bond
• Dotted lines along with all the single bonds can be used to represent the whole resonance hybrid (overall charge should also be shown)
• The resonance hybrid is more stable than the possible resonance structures
• Curly arrows can be used to show the movement of electron pairs in the ion (for a resonance structure)
• The carbonate ion is an equivalent resonance structure (all resonances are of equal energy and make equal contributions to the resonance hybrid)

Formal Charge (FC)

• Not all resonance structures are equivalent resonance structures. If the central atom can expand its octet (true for elements of period 3 and beyond), formal charge can be used to determine what would be the most stable structure.
• FC = V - ½B - LOR
• FC = V - (½B + L)
• V=Valence Electrons
• B=number of bonding electrons
• L=number of lone pair electrons
• The structure with most elements that have an FC closest to 0 will be the most stable, and therefore the preferred structure
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• Pie de imagen: Formal Charge for SO2, Source: YCIS Kognity

Octets

• Some atoms don't have sufficient number of valence electrons and don't satisfy the octet rule, e.g. BeCL2, BF3, and are called electron deficient
• Elements in and beyond period 3 can expand their octet because they have empty d orbitals, which allows for the unpairing of electrons to make more bonds, e.g. sulphur
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• Pie de imagen: Expansion of the octet in Phosphoris, Source; YCIS Kognity

Molecular Shape

• Because certain atoms can expand their octect, there are further possible structures beyond 4 domains
• 5 domains: Triangular Bipyramid
• 6 Domains: Octahedron
• Electron Domain Geometry is determined by the positions of all the electron domains, but molecular geometry depends only on the positions of bonded atoms that make up the molecule
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• Pie de imagen: Molecular Structures, Source: YCIS Kognity

Molecular Orbitals

• The overlap and merging of atomic orbitals, each containing an unpaired electron on two different atoms forms a covalent bond.
• The strength of a bond directly correlates with the degree of the overlap of the two atomic orbitals and the match in their en energy levels
• Sigma Bonds: the electron density occupies the space between and around the two nuclei, formed by the overlap of atomic orbitals (e.g. overlap of two s orbitals, s orbital and p orbital, 2 p orbitals, etc.)
• Pi Bond: Two p orbitals aligned parallel to each other can undergo sideways overlap producing a pi bond
• A double bond is formed of a pi bond and a sigma bond, and a triple bond consists of a sigma and two pi bonds
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• Pie de imagen: Source: YCIS Kognity

Ozone Depletion

• Ozone=O3, gaseous allotrope of oxygen, protects the Earth from UV Radiation's effects
• The Catalytic Cycle involving chlorine free radicals (ch.6) generated from CFCs results in ozone depletion, Nitrous Oxides can also cause ozone depletion
• The breakdown of Oxygen requires UV radiation of a shorter wavelength than that for the breakdown of ozone. This is because the strong double bond in oxygen is stronger and shorter, so a shorter wavelength is required to fit into the bonds and break them, compared to Ozone's weaker and longer bonds.
• Ozone also maintains the temperature of the atmosphere. The production of ozone is exothermic, and therefore the stratosphere's temperature rises with increased altitude where this reaction takes place
• Ozone is a resonance hybrid, the bond angles are around 117 degrees, ozone is sp2 hybrised
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• Pie de imagen: The Catalytic Cycle that Causes Ozone Depletion, Source: YCIS Kognity

Hybridization

• The orbitals of the same atom can merge (hybridization) and then create a set of hybrid atomic orbitals
• The three most common types of hybridization are sp, sp2 and sp3
• The four sp3 orbitals are formed from one s orbital and 3 p orbitals
• Hybrid orbitals can form stronger bonds than unhybridized atomic orbitals because they have an electron density that is concentrated to one side of the nucleus (one lobe of the orbital is larger than the other) and so can bring about more effective overlap
• sp3: Diamond, CH4, C2H6, etc.
• sp2: Graphite, Fullerene, etc.
• sp: CH=CH
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• Pie de imagen: Formation of four sp3 hybrid orbitals on Carbon, Source: YCIS Kognity

Hybridization and Molecular Shape

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• Pie de imagen: Source: YCIS Kognity