Section A: structure

Question 1

what is the Aufbau principle?

Answer 1

orbitals of lower energies are filled before those of higher energies

Question 2

what is the uncertainty principle?

Answer 2

momentum and position simultaneously determined

Question 3

define ‘orbital’

Answer 3

the region in which electrons are probably located

Question 4

what are the four quantum numbers?

Answer 4

• the principle quantum number, n
• the angular momentum number, l
• the magnetic quantum number, m(l)
• the spin quantum number, m(s)

Question 5

what is the principle quantum number?

Answer 5

describes the size of the orbital and its distance from the nucleus; n=1,2,3,etc.

Question 6

what is the angular momentum quantum number?

Answer 6

describes the shape of the orbital; l=0...(n-1);
l=0 s-orbital
l=1 p-orbital
l=2 d-orbital
etc.

Question 7

what is the magnetic quantum number?

Answer 7

describes the orientation of the orbital; m(l)= -1,0,+1 for p-orbitals; m(l)= -2,-1,0,+1,+2 for d-orbitals

Question 8

what is the spin quantum number?

Answer 8

describes the spin of the electron; m(s)= -1/2 or +1/2;

• clockwise spin = +1/2 (↑)
• anticlockwise spin = -1/2 (↓)

Question 9

describe an s-orbital

Answer 9

• same sign of wavefunction across the entire boundary surface
• wavefunction has a different sign either side of the node

Question 10

what is described by the radial distribution function of an orbital?

Answer 10

how the probability of finding an electron varies with distance from the nucleus

Question 11

where is there a zero probability chance of finding an electron?

Answer 11

at the nucleus; at a node (where radial distribution function is equal to zero)

Question 12

what is the Pauli exclusion function?

Answer 12

no two electrons can have the same four quantum numbers

Question 13

what is Hund’s rule of maximum multiplicity?

Answer 13

• electrons will spread out to occupy the maximum possible number of orbitals of the same energy to maximise the number of parallel spins
• different orbitals occupy different regions of space and so reduce electrostatic repulsion by spreading out
• lower energy arrangement

Question 14

what is spin correlation?

Answer 14

• parallel spins stay further away from one another
• reduces repulsion
• creates a lower energy arrangement

Question 15

what is a covalent bond according to the Lewis model?

Answer 15

valence electrons are shared between two atoms

Question 16

what is the problem with the Lewis model and the octet rule?

Answer 16

• period 3 elements begin to show some deviations from the octet rule eg. SF6, PCl5
• it cannot explain hypervalency very satisfactorily
• it cannot explain the observation that oxygen is paramagnetic and must therefore have unpaired electrons

Question 17

define ‘hypervalent’

Answer 17

requires more than an octet of electrons to draw a Lewis structure

Question 18

how was hypervalency traditionally explained?

Answer 18

by the availability of low-lying d-orbitals to accommodate the extra electrons

Question 19

what are the assumptions of VSEPR?

Answer 19

• electrons in bonds and lone pairs are considered as ‘charge clouds’ that repel one another
• there is a lowest energy arrangement when these clouds are as far apart as possible
• lone pairs repel more than bonding pairs
• a multiple bond is treated as a single electron pair so there is no need to consider resonance structures

Question 20

if a lone pair has a choice between an axial and an equatorial position, which will it ‘choose’ and why?

Answer 20

• it will choose the equatorial position
• the lone pair is less repelled by the two axial bonding pairs than it would be by the three equatorial bonding pairs if it occupied the axial position

Question 21

what does VSEPR theory stand for?

Answer 21

valence shell electron pair repulsion theory

Question 22

what is valence bond theory?

Answer 22

• half-filled atomic orbitals on two atoms overlap to form a bonding orbital
• bonding electrons are most likely located in the region between the atoms, giving rise to a bond

Question 23

what dictates the bond strength?

Answer 23

the amount of overlap between the atomic orbitals

Question 24

what is the problem with VSEPR theory?

Answer 24

• it does not explain how s, p and d atomic orbitals contribute to covalent bonding
• it does not explain how electrons from s and p orbitls form equivalent covalent bonds, as in CH4

Question 25

who developed the theory of hybridisation?

Answer 25

Linus Pauling

Question 26

what is a sigma (σ) bond?

Answer 26

- formed by head-on overlapping between atomic orbitals | - symmetric about the bond

Question 27

what is hybridisation?

Answer 27

- the concept of mixing atomic orbitals into new hybrid orbitals - each orbital can be described as a wavefunction - wavefunctions can be combined both constructively and destructively

Question 28

what is a pi (π) bond?

Answer 28

- formed when orbitals overlap 'sideways-on' | - not symmetric about the bond

Question 29

describe the bonding in ethene

Answer 29

the two carbon atoms in ethene form bonds in two ways: 1) an sp2 orbital from each can overlap to form a σ bond 2) the unhybridised p(z) orbital of each carbon atom can overlap 'sideways-on' to form a π orbital

Question 30

describe sp2 hybridisation

Answer 30

- the s, p(x) and p(y) orbitals combine to form three equivalent hybrid orbitals - the p(z) orbital remained unhybridised - the valence electrons are distributed between the three sp2 orbitals and the p(z) orbital - the total energy of the three sp2 orbitals is equal to the energy of the s and two of the p atomic orbitals

Question 31

describe sp3 hybridisation

Answer 31

- the s, p(x), p(y) and p(z) orbitals combine to form four equivalent hybrid orbitals - the valence electrons are distributes between the four sp3 orbitals - the total energy of the three sp3 orbitals is equal to the energy of the s and p atomic orbitals

Question 32

what are the dis/advantages of valence bond theory?

Answer 32

advantage: gives more insight than the Lewis model into the nature and properties of a bond disadvantage: cannot explain the paramagnetism of oxygen

Question 33

what is molecular orbital theory?

Answer 33

- combinations of waves can either be constructive or destructive - orbitals combine constructively to increase electron density between the atoms, forming a bonding molecular orbital that contributes to bonding - orbitals combine destructively to decrease electron density betweent he atoms, forming an anti-bonding molecular orbital that negates bonding

Question 34

how is bond order calculated?

Answer 34

bond order = [number of bonding electrons - number of anti-bonding electrons] / 2

Question 35

which atomic orbitals will combine

Answer 35

those of the same symmetry

Question 36

what does the bond order signify?

Answer 36

``` the nature of the bond 0 = no bond 1 = single bond 2 = double bond etc. ```

Question 37

what does a bond order of 1/2 signify?

Answer 37

- there exists a bond with half the bond strength of a single bond - the existence of this bond is likely to be transient

Question 38

what is the order of molecular orbitals in B,C,N?

Answer 38

σ π σ π

Question 39

what is the order of molecular orbitals in F,O,Ne?

Answer 39

σ π π σ

Question 40

what is the disadvantage of molecular orbital theory?

Answer 40

it is difficult to predict the order in energy of orbitals except for the simplest of molecules

Question 41

what is implied by an electronegativity greater than 2?

Answer 41

the bond is so polarised that is can be considered ionic ie. a complete transfer of electrons from one atom to another

Question 42

what is the relationship between interaction energy and temperature?

Answer 42

interaction energy is inversely dependent on temperature, so the interaction becomes weaker as the temperature increases

Question 43

which non-covalent interactions have the same relationship between interaction energy and the distance between the atoms and what is this relationship?

Answer 43

the interaction energy is dependent on 1/r^6 for: dipole-dipole dipole-induced dipole dispersion

Question 44

what is the Grotthuss mechanism?

Answer 44

- conduction of H+ through water is an order of magnitude faster than that of similarly charged ions - hydrogen bonds can become covalent bonds and vice versa - in this way, charge can be transferred from one location to another in aqueous solution without the diffusion of any atoms

Question 45

what is the lifetime of a typical hydrogen bond?

Answer 45

~ 1 - 10 ps (~ 10^-12 - 10^-11 s)

Question 46

what happens when a hydrophilic substance is dissolved in water?

Answer 46

- there are many more ways of arranging the network of hydrogen bonds - the entropy increases - the free energy decreases - solvation is favourable

Question 47

what happens when a hydrophobic substance is dissolved in water?

Answer 47

- there are fewer ways of arranging the network of hydrogen bonds - the entropy decreases - the free energy increases - solvation is unfavourable and the molecules seek to separate themselves from the water to minimise the contact area; known as the hydrophobic effect

Question 48

what hybridisation occurs in alkanes?

Answer 48

sp3 hybridisation

Question 49

what hybridisation occurs in alkenes?

Answer 49

sp2 hybridisation

Question 50

what hybridisation occurs in alkynes?

Answer 50

sp hybridisation