polyatomic molecules: Flashcards
formal charge: definition
formal charge: the difference between the number of valence electrons in the free atom and the number of electrons assigned to that atom in a Lewis structure.
formal charge: formula
Formal charge = (no. of e- in valence shell of the free atom) - (no. of bonds to the atom) - (no. Of unshared e-)
Lewis structures: definition
a simple way of representing electrons in an atom to keep track of them when bonds form, either ionic or covalent.
Represented valence electrons as dots and arranged the dots around the symbol of the element.
Steps to follow when Constructing a Lewis structure:
- Count the number of valence electrons on each atom – for ions, just count the valence electrons and adjust the electron number to account for the charge.
- Divide the total number of electrons by 2 to obtain the number of electron pairs.
- Decide the most likely arrangement of atoms – choose which atom belongs at the centre.Usually this is the least electronegative one – however there can be exceptions to this rule.
- Place an electron pair between each pair of bonded atoms.
- Complete the octet of each atom by placing any remaining electron pairs around the atoms.
If there are not enough pairs – form multiple bonds
- Represent each bonded electron pair by a line.
Oxidation states: definition
The formal number of electrons added to or removed for man atom when it forms a compound.
What are the 2 possible situations for a covalent bond: in VBT?
The interaction between 2 orbitals each containing 1 electron.
One atom donating a lone pair of electrons in one orbital into a vacant orbital on another atom = dative bond.
What is the definition of a “real” structure in terms of resonance?
‘real’ structure = a weighted average of the different resonance forms and it is lower in energy than any single contributing resonance form - resonance forms don’t contribute equally to the molecular wavefunction.
What does VSEPR stand for?
VSEPR theory = valence shell electron pair repulsion theory.
Rules Pauling set for VSEPR theory: No single AO has:
Linear geometry about the central atom.
Trigonal symmetry.
Trigonal bipyramidal symmetry.
Tetrahedral symmetry.
order of increasing repulsions - in terms of electron pairs
LP-LP > LP-BP > BP-BP
Pseudostructure: definition
a structure that takes into account all the electron environments – including LPs.
3 main assumptions with VSEPR theory
Atoms in a molecule are held together by bonding pairs of electrons.
Atoms in a molecule that have pairs of electrons which aren’t involved in bonding are known as lone pairs of electrons.
Electron pairs are negatively charged so they repel each other – electron pairs on each atom adopt positions as far apart from each pair of electrons, to minimise repulsion.
General formula = AXn - what does each part stand for?
A = central atom
X = surrounding atoms (all ligands)
n = amount of X
Using VSEPR theory for tetrahedral and octahedral structures: rules:
1st = identify central atom.
2nd = count number of valence electrons.
3rd = Add 1 electron for each bonding atom.
4th = add or subtract electrons from the charge.
5th = divide the total of this by 2 to get the total number of electron pairs.
6th = use this number to predict the shape.
What are fluxional molecules?
Some molecules show a geometry that is an intermediate between the trigonal bipyramidal shape and the square based pyramid shape – these molecules are known as fluxional.
In VSEPR, what happens with single vs multiple bonds?
We don’t distinguish between multiple and single bonds.
A multiple bond is treated as a single region of high electron concentration.
The 2 electron pairs in a double bond act together when we need to account for their repulsion. BP(double bond) > BP(single bond)
Limitations of VSEPR theory:
VSEPR does not apply to d-block metal complexes – shapes of these are generally determined by the energies and electronic occupation of the d-orbitals.
Gives no insight into the nature of the chemical bond.
There are some cases where the assumptions made aren’t correct – VSEPR predicts the molecule TeBr62- to be pentagonal bipyramid with one site containing a lp – but crystallographic measurements show that the actual shape is octahedral - explained by the lp occupying an s orbital - this electron pair is described as stereochemically inactive = it does not influence the geometry.
What is a transition metal?
Transition metal: is an element in which the valence electrons are d-electrons
What are Coordination isomers?
isomers where the ligands attached to each of the 2 metals differ.
What do linkage isomers involve?
involves ambidentate ligands.
What are Polymerisation isomers?
isomers with the same formula, but different molar mass
What are stability constant (formation constants)?
The extent to which an aqueous cationic metal complex combines with other ligands (replacing the water molecules) is a thermodynamic problem and can thus be treated in terms of a stability constant
potential symbols of stability constants:
K, Kf or β.
What happens if Kf is large?
If Kf is large, then the ligand (L) binds more tightly to the metal centre than H2O.
Pushes the equilibrium towards the products.
What happens if Kf is small?
If Kf is small, then the ligand binds less tightly than H2O
What is the Chelate effect:
general rule is that a complex containing one (or more) 5-6 membered chelate rings is more stable (stability constant = larger) than one without a chelate ring.
relationship between ring size and the chelate effect?
chelate effect decreases with increasing ring size.
What is the Hard and Soft Acid and Base concept?
Cations (Lewis acids) and ligands (Lewis bases) are classified as “hard” (less polarisable) or “soft” (more polarisable).
Hard cations form more stable complexes with hard ligands.
Soft cations form more stable complexes with soft ligands.
Hard acids prefer to complex with hard bases.
Soft acids prefer to complex with soft bases.
Hard-Hard interactions can be considered to have a greater electrostatic (ionic) contribution.
Soft-Soft interactions are more orbital based (covalent) interactions (overlap)
