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Chemistry > Shapes & Structures Of Molecules Part 2 > Flashcards

Shapes & Structures Of Molecules Part 2 Flashcards

1
Q

How many electrons in an orbital?

A

2

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2
Q

What are degenerate orbitals?

A

These with the same energy, such as the three 2p orbitals.

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3
Q

How many quantum numbers are used to describe the movement and trajectories of each electron in an atom?

A

4- n, l, ml and ms.

Any electron in an atom has a unique set of four quantum numbers.

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4
Q

What is the principal quantum number, n?

A

Describes the energy of an electron and its most probable distance from the nucleus- which SHELL it is in.

Integers n=1,2,3,4…

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5
Q

What is the angular momentum (azimuthal) quantum number, l?

A

Determines the SHAPE of an orbital (sub shell) as well as the orbital angular momentum of an electron (momentum as it moves about in the orbital).

L takes integer values from 0 to n-1.
L=0(s), 1(p), 2(d), 3(f)…

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6
Q

How is orbital angular momentum calculated?

A

ħ*sqrt(l(l+1))

Where ħ is reduced Plancks constant= h/2pi

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7
Q

What is the magnetic quantum number, ml?

A

Distinguishes the available orbitals in a sub-shell and defines their orientation (specifically the component of the angular momentum on a particular axis).

For l=0, ml=0
For l=1, ml=-1,0,+1
etc.

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8
Q

Which quantum numbers define an electron’s spin angular momentum?

A 
s= 1/2 (magnitude)
ms= +1/2,-1/2 (orientation
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9
Q

What is a wave function?

A

A mathematical function describing the properties of a particle such as an electron.
Psi^2 gives the probability of finding the electron at a given set of coordinates (prob. density).

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10
Q

Is an orbital a wave function?

A

Yes, a one-electron wavefunction.
Each orbital has a different wavefunction (so electron has different properties) defined by the quantum numbers n, l, and ml.

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11
Q

How can the wavefunctions for one-electron systems be calculated?

A

Solving Schrödingers equation. The solutions are the atomic orbitals.

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12
Q

How to calculate the energy associated with each wavefunction for a one-electron system?

A

En= - Rh x (z/n)^2

Where Rh is the Rydberg constant, with units of energy.

Thus for a given nucleus, the energy of an orbital depends on n only (I.e., for one-electron systems, 2s and 2p are degenerate).

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13
Q

Why are the energies of orbitals negative?

A

As n gets larger, En tends towards zero.
At zero energy, n is so large that the electron and nucleus are separate.
Thus Rh corresponds to the ionisation energy for the atom.

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14
Q

What is the advantage of writing the wavefunction in polar coordinate form instead of Cartesian form?

A

It can then be written as the product of two functions.
R(r) x Y(theta, phi)

Where R is the radial part, defined by n and l.
Where Y is the angular part, defined by l and ml.

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15
Q

Why are s orbitals said to have spherical symmetry?

A

The wavefunction is independent of theta and phi and depends only on r

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16
Q

What is the Radial Distribution Function, RDF?

A

Shows the electron density at a distance r from the nucleus, summed over all angles. It may be thought of as the sum of the electron density in a thin shell at a radius r from the nucleus.
RDF= [R(r)]^2 x 4pi*r^2

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17
Q

The radial part of the wavefunction for the 2s orbital equals 0 (/RDF=0) at a radius of about 2 Bohr radii. What does this mean?

A

There is a radial node here.

The electron will not be found here.

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18
Q

Why are the radial parts of the wavefunctions for all 3 2p orbitals all the same?

A

They do not depend on the value of ml.

None have radial nodes (but at r=0, psi=0).

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19
Q

Why are the angular parts of the wavefunctions of the 2p orbitals different?

A

They do depend on ml; hence they have different orientations (ie. along each axis) and one angular node each (e.g. 2px orbital has angular node in yz plane).

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20
Q

How many radial nodes does the 3s orbital have ?

A

2 (change of sign/phase twice).

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21
Q

How many nodes do the 3p orbitals have?

A

2- one radial and one angular.

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22
Q

How many nodes do the 3d orbitals have?

A

2 (both angular nodes)

There define planes for the xy, xz, yz and x2-y2 orbitals, and nodal cones for the z2 orbitals (semi-angles theta= 54.7 and 125.3 degrees).

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23
Q

Give formulae for the total number of nodes possessed by an orbital.

A

Total no. of nodes = n-1
No. of angular nodes = l
No. of radial nodes = n-1-l

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24
Q

Why is it impossible to solve the Schrödinger equation for multi-electron systems?

A

In addition to attraction between the nucleus and electrons there is also electron-electron repulsion, which complicates things.

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25
Q

What is the orbital approximation?

A

An approximation made for the energies of the electrons in atoms. It assumes the effects of all other electrons can be averaged out and the atom thought of as a hydrogen-like system (one electron) with a modified nuclear charge. The modified potential is spherically symmetric and centred on the nucleus.

26
Q

Why is the effective nuclear charge, Zeff, different for different electrons?

A

How well electrons screen one another depends on which orbital they’re in.
Electrons in the same orbital shield each other by about 30% of one proton’s charge.
Electrons in the 2s orbital have little effect on those in the 1s orbital.
2s orbital electrons experience more of the nuclear charge than those in the 2p orbital since it penetrates the 1s orbital more (they are therefore lower in energy explaining why in multi-electron systems the orbitals are no longer degenerate).

27
Q

How does the Zeff of a valence electron change across a period?

A

Increases across a period (additional protons).

In turn the energy of an orbital decreases.

28
Q

Why do core electrons take little part in reactions?

A

They are too low in energy

29
Q

What is a molecular orbital?

A

Orbitals arising from the linear combination of atomic orbitals of appropriate symmetry.

This combination can be in-phase or out-of-phase (when two waves interact, the do so in both constructive and destructive manners), and this leads to a bonding and an anti-bonding MO.

30
Q

Why is the bonding MO (in-phase combination) lower in energy than the atomic orbitals, whilst the anti-bonding MO (out-of-phase combination) higher in energy?

A

The bonding MO arises out of constructive overlap of the AOs so when it is occupied, there is increased electron density between the nuclei.
Also the electron becomes more delocalised leading to a lower kinetic energy of the system.

Occupation of the anti-bonding MO leads to less electron density in the internuclear region, pulling the nuclei apart. It contains a node when the AOs completely cancel one another out which results in the electron having a greater KE.

31
Q

Why is H2 more stable than H2+?

A

H2+ has one electron in its bonding MO; H2 has two. Therefore twice the energy is required to split the molecule.

32
Q

With one electron in the bonding and one in the antibonding MOs, why is the total energy always positive?

A

Antibonding MOs are generally raised slightly more in energy relative to the component AOs. Thus bonding + antibonding energies is positive for all bond lengths. Consequently there is no minimum energy and such a molecule is unstable (will minimise its energy by falling apart).

33
Q

Bond order calculation?

A

1/2 x [no. of electrons in bonding MOs - no. of electrons in antibonding MOs]

34
Q

When is an MO given a pi label?

A

If the inter nuclear axis contains a modal plane (I.e. when two p orbitals combine side on).

35
Q

When may MOs be labelled g or u?

A

Firstly the molecule must possess a centre of inversion.

g= even, wavefunction does not change sign passing through centre of inversion. 
u= odd, change of sign.
36
Q

Antibonding orbitals May be labelled with an asterisk.

A

.

37
Q

What makes a molecule/atom paramagnetic?

A

The presence of unpaired electron(s) in orbitals.

38
Q

What determines how well AOs combine?

A

1) AOs must have a suitable symmetry to interact (consider overlap integral- multiply values at a particular point).
2) AOs must be close enough in energy for significant bonding/anti-bonding interactions to occur.
3) The interaction between two large orbitals is less than that between two small orbitals.

39
Q

What is different about the energy level diagram showing heteronuclear diatomics?

A

Bonding MO lowered less / antibonding MO raised less.

Unequal contributions from AOs of different atoms (ionic character).

40
Q

What are non-bonding MOs?

A

Orbitals that are the same energy as the original AOs because they are unable to interact with the other AOs.

41
Q

In N2, why are the (2p) sigma g&u* raised in energy, and the (2s) sigma g&u* lowered in energy compared to in O2?

A

Due to s-p mixing.

Interactions do not solely occur between orbitals of the same kind; MOs are made up of interactions between a number of orbitals. To generate these more accurate MOs, the MOs that initially form from just two AOs can then be allowed to interact, or mix, further, depending on symmetry and how close in energy they are.
The closest in energy are the (2p)sigma g and (2s) sigma g (so they are raised/lowered in energy respectively).
The (2s) sigma u* and (2p) sigma u* can similarly interact.

42
Q

How are s-p mixed MOs labelled?

A

By their symmetry.

i.e the first Mo of sigma g symmetry is labelled ‘1 sigma g’, ‘2 sigma g’, etc.

43
Q

For which homonuclear diatomics will s-p mixing take place?

A

All first row elements up to and including nitrogen

44
Q

What is hybridisation?

A

The concept of mixing AOs to form new hybrid AOs in order to accommodate for the geometry of a molecule which often does not align with the direction AOs point in.
e.g.) 2s and three 2p orbitals can combine to give four equivalent sp3 HAOs.

45
Q

What are the advantages of hybridisation?

A
  • Simplifies the bonding scheme
  • Gives more directional HAOs that point towards the bonding atoms or direction of lone pairs
  • Gives MOs in which electrons are not delocalised over many atoms but localised in between atoms
  • More useful when trying to draw mechanisms
46
Q

What’s are the disadvantages of hybridisation?

A
  • Does not give the best picture of the different energy levels within the molecule
  • Encourages a localised view of electrons whereas in reality the electrons are spread over many atoms.
47
Q

How is a bond angle of 104.5 such as that in water accommodated for by hybridisation?

A

HAOs that point at 109.5 have 1/4 s character and 3/4 p character.
In water the bond angle is less than this (i.e closer to 90 degrees) so therefore the HAOs have increased p character (0.19:0.81).

48
Q

Which orbitals hybridise in an octahedral transition metal complex?

A

s, three p and dz2 and d(x2-y2)

49
Q

Which orbital hybridise in an square planar transition metal complex?

A

s, px, P.O. and d(x2-y2)

pz cannot be used as it lies in the nodal plane of the d orbital

50
Q

Which orbitals hybridise to form a trigonometry bipyramidal structure?

A

sp3d or spd3

51
Q

What is used to predict the form of the MOs resulting from combining out-of-plane p orbitals?

A

A rule based on sine waves.

MOs have increasing number of nodes. Height under sine wave represent contribution each p AO makes.

52
Q

How to predict the shape of a molecule from scratch?

A

Assume a starting geometry, work out it’s energy, and alter the geometry slightly until the energy is at a minimum.

53
Q

Reaction between lithium hydride and borane?

A

LiH + BH3 —> LiBH4

54
Q

What is the HOMO in LiH?

A

That which results from interaction between the lithium 2s and hydrogen 1s.

55
Q

What is the LUMO of BH3?

A

The boron 2pz AO

[2s, 2px and 2py all overlap with hydrogen 1s AOs].

56
Q

Why is there a nucleiphilic addition reaction between a hydride ion and the carbonyl group of methanal?

A

Carbonyl carbon bares a partial positive charge so there is an initial electrostatic attraction.
The highest energy electrons are those in the hydride HOMO (in contrast to methanal’s HOMO, the oxygen lone pairs.
The closest in energy LUMO is methanals pi*.
The H 1s also interacts with the carbonyl pi MO, so three new MOs form: a C-H bonding MO, a C-H antibonding MO, and a nonbonding MO (O lone pair). Overall the energy of the electrons is lowered during the reaction.

57
Q

Why does the hydride attack methanal at an angle of approx 107?

A

If it attacks from above or in the same plane as the carbonyl there will be no net interaction.

58
Q

Why don’t nucleophiles usually react with alkenes by attacking into the C-C pi*?

A

A C=C bond has no dipole moment (no initial electrostatic attraction).
Also it is too high in energy to match with the nucleophile HOMO.

59
Q

What is the HOMO and LUMO in a nucleophilic substitution reaction?

A

HOMO- nonbonding lone pairs in nucelophile

LUMO- C-X sigma*

60
Q

What is the difference between the SN1 and SN2 reaction mechanisms of nucleophilic substitution?

A

SN1- C-X bond breaks forming carbocations intermediate.
SN2- transition structure forms as C-X and breaks and C-Nu forms simultaneously. It explains why the carbon centre undergoes an inversion.

Chemistry (7 decks)

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