Diatomic Molecules Flashcards
Bond length?
Mean distance between centres of two atoms in a molecule
Bond dissociation enthalpy?
Measure of bond strength, defined as standard enthalpy change for the reaction in which the bond in questions is broken
Paramagnetic?
Attracted to a magnet caused by unpaired electrons, electrons spins able to align with the applied magnetic field
Diamagnetic?
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
Electronegativity?
Power of atom in molecules to attract electrons to itself
Pauling electronegativity?
Average of homonuclear bond dissociation enthalpies measure of ionic character of XY bond.
Mulliken electronegativity?
Mulliken scale simply considers the electronegativity of an element as the average of the ionization potential and the electron affinity of that element.
Electronegativity trends?
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
Successes of Lewis model?
Simple way of describing bonding
Easily extended to give predictions of molecular shape
Introduced concept of bond order
Failures of Lewis model?
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
Process of Pauling electronegativity?
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
Born Oppenheimer approximation in VB and Mo theory?
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
VB theory?
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
MO theory?
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
Orbital Approximation in MO theory?
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
