S2.2 The covalent model HL Flashcards
What is the first step in deducing the resonance structure of molecules and ions?
Draw all valid Lewis structures, considering the distribution of electrons around atoms and looking for multiple ways to place double bonds if applicable.
What does the resonance structure of a molecule or ion represent?
It is a hybrid of all valid Lewis structures, depicting electron delocalization over the molecule or ion, which cannot be represented by a single Lewis structure alone.
What role does electron delocalization play in the formation of resonance hybrid structures?
It spreads electrons across multiple atoms or bonds, contributing to the stability of the resonance hybrid by lowering the molecule’s potential energy.
Is the resonance hybrid a real transition between structures?
No, it is an intermediate representation with bond lengths and strengths that are averages of the possible structures, not a real transition.
What physical evidence challenges the notion of alternating single and double bonds in benzene?
All carbon-carbon bond lengths in benzene are identical at 139 pm, indicating a delocalized electron system rather than distinct single and double bonds.
How does benzene’s chemical behavior differ from compounds with double bonds?
Benzene exhibits unusual stability and a reluctance to undergo addition reactions, favoring substitution reactions instead, which suggests a resonance hybrid structure.
What confirms benzene’s structure as having a ring of delocalized electrons?
X-ray crystallography confirms the resonance hybrid structure of benzene, showing a ring of delocalized electrons instead of alternating single and double bonds.
How to represent species with five electron domains around a central atom using Lewis formulas?
Calculate total valence electrons, identify the central atom, draw basic structure with single bonds, add electrons to fulfill the octet rule (forming double/triple bonds if necessary), and add extra electrons to the central atom if it’s from period 3 or below.
How to represent species with six electron domains around a central atom using Lewis formulas?
Same process as for five electron domains: Calculate valence electrons, draw structure with single bonds, adjust with double/triple bonds to meet the octet rule, and place additional electrons on a period 3 (or below) central atom.
Electron domain geometry for species with five electron domains using VSEPR.
Possible geometries include trigonal bipyramidal, see-saw, T-shaped, or linear, depending on bonding vs. non-bonding electron pairs.
Electron domain geometry for species with six electron domains using VSEPR.
Possible geometries are octahedral, square pyramidal, or square planar, determined by the number of non-bonding electron pairs.
Factors influencing molecular geometry in VSEPR model.
Geometry is influenced by the repulsion between electron pairs, with specific bond angles characteristic for each geometry type.
Formal charge
The theoretical charge of an entire molecule based on the electron distribution of the atoms within the molecule.
What is the first step in determining the preferred Lewis formula for a species with alternative Lewis formulas?
Calculate the formal charge by subtracting the number of non-bonding and bonded pairs of electrons from the original valence pairs of electrons for an atom before bonding.
What does comparing formal charges among different Lewis structures help to identify?
The structure with formal charges closest to zero for as many atoms as possible, indicating minimal charge transfer during bonding and suggesting a more stable structure.
Why are Lewis structures with formal charges closest to zero generally preferred?
They indicate a structure where the distribution of valence electrons before and after bonding remains as unchanged as possible, suggesting a more stable configuration.
What are sigma bonds?
Sigma bonds are chemical bonds formed through the direct overlap of atomic orbitals, involving electron density located precisely between the nuclei of two atoms.
How are sigma bonds formed?
They result from the axial (head-on) overlap of various types of orbitals, including s-s, s-p, or p-p orbitals, facilitating a strong, stable bond between atoms.
How are pi bonds formed?
Pi bonds are formed by the side-to-side (lateral) overlap of p-orbitals, with electron density concentrated above and below the plane of the nuclei.
How do pi bonds differ from sigma bonds in their occurrence in molecules?
Unlike sigma bonds, pi bonds do not occur in single bonds but are present in double or triple bonds alongside a sigma bond.
How can you deduce the presence of sigma and pi bonds in molecules or ions?
Single bonds contain only sigma bonds, double bonds contain one sigma and one pi bond, and triple bonds contain one sigma and two pi bonds.
How do sigma and pi bonds affect bond rotation?
Sigma bonds allow for bond rotation due to their symmetrical electron density along the bond axis, whereas pi bonds restrict rotation due to their electron density distribution.
Hybridisation
The process by which atomic orbitals within an atom mix to produce hybrid orbitals of intermediate energy.
sp hybridisation
A form of hybridisation that involves 1 s-orbital and 1 p-orbital.
sp2 hybridisation
A form of hybridisation that involves 1 s-orbital and 2 p-orbitals.
sp3 hybridisation
A form of hybridisation that involves 1 s-orbital and 3 p-orbitals.
Excitation
The process of changing an electron from a ground state to an excited state by energy absorption.
