polyatomic molecules:

Question 1

formal charge: definition

Answer 1

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.

Question 2

formal charge: formula

Answer 2

Formal charge = (no. of e- in valence shell of the free atom) - (no. of bonds to the atom) - (no. Of unshared e-)

Question 3

Lewis structures: definition

Answer 3

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.

Question 4

Steps to follow when Constructing a Lewis structure:

Answer 4

1. 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.
2. Divide the total number of electrons by 2 to obtain the number of electron pairs.
3. 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.
4. Place an electron pair between each pair of bonded atoms.
5. Complete the octet of each atom by placing any remaining electron pairs around the atoms.

    If there are not enough pairs – form multiple bonds 

6. Represent each bonded electron pair by a line.

Question 5

Oxidation states: definition

Answer 5

The formal number of electrons added to or removed for man atom when it forms a compound.

Question 6

What are the 2 possible situations for a covalent bond: in VBT?

Answer 6

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.

Question 7

What is the definition of a “real” structure in terms of resonance?

Answer 7

‘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.

Question 8

What does VSEPR stand for?

Answer 8

VSEPR theory = valence shell electron pair repulsion theory.

Question 9

Rules Pauling set for VSEPR theory: No single AO has:

Answer 9

Linear geometry about the central atom.

Trigonal symmetry.

Trigonal bipyramidal symmetry.

Tetrahedral symmetry.

Question 10

order of increasing repulsions - in terms of electron pairs

Answer 10

LP-LP > LP-BP > BP-BP

Question 11

Pseudostructure: definition

Answer 11

a structure that takes into account all the electron environments – including LPs.

Question 12

3 main assumptions with VSEPR theory

Answer 12

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.

Question 13

General formula = AXn - what does each part stand for?

Answer 13

A = central atom

X = surrounding atoms (all ligands)

n = amount of X

Question 14

Using VSEPR theory for tetrahedral and octahedral structures: rules:

Answer 14

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.

Question 15

What are fluxional molecules?

Answer 15

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.

Question 16

In VSEPR, what happens with single vs multiple bonds?

Answer 16

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)

Question 17

Limitations of VSEPR theory:

Answer 17

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.

Question 18

What is a transition metal?

Answer 18

Transition metal: is an element in which the valence electrons are d-electrons

Question 19

What are Coordination isomers?

Answer 19

isomers where the ligands attached to each of the 2 metals differ.

Question 20

What do linkage isomers involve?

Answer 20

involves ambidentate ligands.

Question 21

What are Polymerisation isomers?

Answer 21

isomers with the same formula, but different molar mass

Question 22

What are stability constant (formation constants)?

Answer 22

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

Question 23

potential symbols of stability constants:

Answer 23

K, Kf or β.

Question 24

What happens if Kf is large?

Answer 24

If Kf is large, then the ligand (L) binds more tightly to the metal centre than H2O.

Pushes the equilibrium towards the products.

Question 25

What happens if Kf is small?

Answer 25

If Kf is small, then the ligand binds less tightly than H2O

Question 26

What is the Chelate effect:

Answer 26

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.

Question 27

relationship between ring size and the chelate effect?

Answer 27

chelate effect decreases with increasing ring size.

Question 28

What is the Hard and Soft Acid and Base concept?

Answer 28

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)