Diatomic Molecules

Question 1

Bond length?

Answer 1

Mean distance between centres of two atoms in a molecule

Question 2

Bond dissociation enthalpy?

Answer 2

Measure of bond strength, defined as standard enthalpy change for the reaction in which the bond in questions is broken

Question 3

Paramagnetic?

Answer 3

Attracted to a magnet caused by unpaired electrons, electrons spins able to align with the applied magnetic field

Question 4

Diamagnetic?

Answer 4

Repelled by a magnetic caused by spin paired electrons, much weaker than paramagnetic effects, found in all materials but only observed in absence of other types of magnetism

Question 5

Electronegativity?

Answer 5

Power of atom in molecules to attract electrons to itself

Question 6

Pauling electronegativity?

Answer 6

Average of homonuclear bond dissociation enthalpies measure of ionic character of XY bond.

Question 7

Mulliken electronegativity?

Answer 7

Mulliken scale simply considers the electronegativity of an element as the average of the ionization potential and the electron affinity of that element.

Question 8

Electronegativity trends?

Answer 8

Increases across a period and decreases down a group - when an atom has two ore more possible oxidation states the electronegativity increases with increasing oxidation state

Question 9

Successes of Lewis model?

Answer 9

Simple way of describing bonding
Easily extended to give predictions of molecular shape
Introduced concept of bond order

Question 10

Failures of Lewis model?

Answer 10

Doesn’t explain how bonding occurs
Does not always work - many stable molecules whose atoms do not have full octets and many stable molecules that have expanded octets
Fails to explain the bonding in many transition metal compounds
On closer inspection failure to describe bonding in simple molecules such as oxygen - doesn’t expect oxygen to be paramagnetic

Question 11

Process of Pauling electronegativity?

Answer 11

Noticed bond dissociation enthalpies for heteronuclear diatomic of general formula XY always greater than average bond dissociation enthalpy of the homonuclear diatomic
Difference between experimental value of bond dissociation enthalpy and that calculated by average of homonuclear bond dissociation enthalpy was a measure of ionic character of XY bond - difference in electronegativity between X and Y:
[Xp(X) - Xp(Y)] = (deltaD)^1/2
delta D is the difference between observed value of bond dissociation enthalpy of heteronuclear diatomic XY and average bond dissociation enthalpies our X2 and Y2:
deltaD = D(X-Y) - (D(X-X)+D(Y-Y))/2
So electronegativity:
[Xp(X) - Xp(Y)] = 0.102(deltaD)^1/2

Question 12

Born Oppenheimer approximation in VB and Mo theory?

Answer 12

Since nuclei are much heavier than electrons they move more slowly so the approximation allows the nuclei to be regarded as stationer with only the electrons moving - allowing the Schrödinger equation to be solved just for the wave functions of the electrons

Question 13

VB theory?

Answer 13

Bond arises from the interaction between the atomic orbitals on the two atoms to give a bonding orbital that is located between the two atoms the bonding orbitals contains two electrons

Question 14

MO theory?

Answer 14

Atomics orbitals of the atoms in the molecules combine to form a set of molecular orbitals that are spread over the whole molecule - said to be delocalised. MOs are property of a molecule as a whole and the electrons in the molecules are distributed within these orbitals

Question 15

Orbital Approximation in MO theory?

Answer 15

Allows the overall wave function for an atom containing N electrons to be written as a product of N single electron wave functions - useful as single electron wave functions can be calculated form the Schrödinger equation whereas wave function for more than on electron cannot. Assumes that each electron moves in an average potential that incorporates the interaction with the nuclei and the other electrons in the molecules

Question 16

Linear Combination of Atomic Orbitals approximation in MO theory?

Answer 16

As atomic orbitals are single electron wave functions they have wave character and so the atomic orbitals on one atom interfere with those on another atom which can occur constructively or destructively- constructive interference occurs from in phase combination of wave functions (adding wave functions together) destructive interference results from out of phase combination of wave functions (subtracting wave functions from each other). LCAO allows MOs to be formed from addition of subtraction of AOs