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CHEM1603 Dina > Section A: structure > Flashcards

Section A: structure Flashcards

1
Q

what is the Aufbau principle?

A

orbitals of lower energies are filled before those of higher energies

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2
Q

what is the uncertainty principle?

A

momentum and position simultaneously determined

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3
Q

define ‘orbital’

A

the region in which electrons are probably located

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4
Q

what are the four quantum numbers?

A
  • the principle quantum number, n
  • the angular momentum number, l
  • the magnetic quantum number, m(l)
  • the spin quantum number, m(s)
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5
Q

what is the principle quantum number?

A

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

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6
Q

what is the angular momentum quantum number?

A 
describes the shape of the orbital; l=0...(n-1);
l=0 s-orbital
l=1 p-orbital
l=2 d-orbital
etc.
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7
Q

what is the magnetic quantum number?

A

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

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8
Q

what is the spin quantum number?

A

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

  • clockwise spin = +1/2 (↑)
  • anticlockwise spin = -1/2 (↓)
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9
Q

describe an s-orbital

A
  • same sign of wavefunction across the entire boundary surface
  • wavefunction has a different sign either side of the node
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10
Q

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

A

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

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11
Q

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

A

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

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12
Q

what is the Pauli exclusion function?

A

no two electrons can have the same four quantum numbers

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13
Q

what is Hund’s rule of maximum multiplicity?

A
  • 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
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14
Q

what is spin correlation?

A
  • parallel spins stay further away from one another
  • reduces repulsion
  • creates a lower energy arrangement
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15
Q

what is a covalent bond according to the Lewis model?

A

valence electrons are shared between two atoms

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16
Q

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

A
  • 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
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17
Q

define ‘hypervalent’

A

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

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18
Q

how was hypervalency traditionally explained?

A

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

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19
Q

what are the assumptions of VSEPR?

A
  • 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
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20
Q

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

A
  • 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
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21
Q

what does VSEPR theory stand for?

A

valence shell electron pair repulsion theory

22
Q

what is valence bond theory?

A
  • 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
23
Q

what dictates the bond strength?

A

the amount of overlap between the atomic orbitals

24
Q

what is the problem with VSEPR theory?

A
  • 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
25
Q

who developed the theory of hybridisation?

A

Linus Pauling

26
Q

what is a sigma (σ) bond?

A
  • formed by head-on overlapping between atomic orbitals

- symmetric about the bond

27
Q

what is hybridisation?

A
  • 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
28
Q

what is a pi (π) bond?

A
  • formed when orbitals overlap ‘sideways-on’

- not symmetric about the bond

29
Q

describe the bonding in ethene

A

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

30
Q

describe sp2 hybridisation

A
  • 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
31
Q

describe sp3 hybridisation

A
  • 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
32
Q

what are the dis/advantages of valence bond theory?

A

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

33
Q

what is molecular orbital theory?

A
  • 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
34
Q

how is bond order calculated?

A

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

35
Q

which atomic orbitals will combine

A

those of the same symmetry

36
Q

what does the bond order signify?

A 
the nature of the bond
0 = no bond
1 = single bond
2 = double bond
etc.
37
Q

what does a bond order of 1/2 signify?

A
  • there exists a bond with half the bond strength of a single bond
  • the existence of this bond is likely to be transient
38
Q

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

A

σ
π
σ
π

39
Q

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

A

σ
π
π
σ

40
Q

what is the disadvantage of molecular orbital theory?

A

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

41
Q

what is implied by an electronegativity greater than 2?

A

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

42
Q

what is the relationship between interaction energy and temperature?

A

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

43
Q

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

A

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

44
Q

what is the Grotthuss mechanism?

A
  • 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
45
Q

what is the lifetime of a typical hydrogen bond?

A

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

46
Q

what happens when a hydrophilic substance is dissolved in water?

A
  • there are many more ways of arranging the network of hydrogen bonds
  • the entropy increases
  • the free energy decreases
  • solvation is favourable
47
Q

what happens when a hydrophobic substance is dissolved in water?

A
  • 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
48
Q

what hybridisation occurs in alkanes?

A

sp3 hybridisation

49
Q

what hybridisation occurs in alkenes?

A

sp2 hybridisation

50
Q

what hybridisation occurs in alkynes?

A

sp hybridisation

CHEM1603 Dina (9 decks)

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