Orbitals and Bonding

Question 1

atomic orbital

Answer 1

area of space where an electron is

Question 2

principle quantum number

Answer 2

shell

1,2,3 etc.

Question 3

angular momentum quantum number

Answer 3

shape

s,p,d

Question 4

magnetic quantum number

Answer 4

orientation - which way it points in space

e.g. 2px, 2pz, 2py

Question 5

probability of finding an e- in 1s orbital

Answer 5

greatest at vicinity of nucleus

decreases exponentially as the distance from the nucleus increases

represented as a spherical cloud of charge

Question 6

nodal plane

Answer 6

zero probability of finding an electron

Question 7

orbitals - wave properties

Answer 7

have both amplitudes and phases (+ or -)

Question 8

what is the density of an electron in an orbital proportional to?

Answer 8

the square of the atomic orbital

probability of finding electron in particular region of space = always positive

Question 9

how many electrons can each atomic orbital occupy?

Answer 9

2 - must have different spins

Question 10

atomic core

Answer 10

complete shells

Question 11

valence shell

Answer 11

partially filled shell

Question 12

covalent bonding

Answer 12

atomic orbitals in different atoms overlap and electrons pair up with opposite spins

mutual repulsion among themselves and attraction to the nuclei of all atoms

arrange themselves to occupy MOLECULAR ORBITALS

Question 13

where is the electron density concentrated in covalent bonding?

Answer 13

the region between the 2 nuclei - where atomic orbitals overlap most

Question 14

what does the overlapping of 2 atomic orbitals produce?

Answer 14

2 molecular orbitals

bonding and antibonding (opposite phases)

antibonding MO subtracts the AO instead of adding them

Question 15

which has more energy - antibonding MO or bonding MO?

Answer 15

antibonding MO (has highest energy overall - higher than bonding MO and separated atoms)

Question 16

what are the 2 halves/lobes of the antibonding MO separated by?

Answer 16

nodal plane

Question 17

which is more stable - H2 or isolated H atoms?

Answer 17

H2 molecules - covalently bonded

Question 18

sigma bond

Answer 18

formed when 2 AOs overlap along the interatomic axis

Question 19

pi bond

Answer 19

formed when AOs overlap sideways

Question 20

which is stronger - sigma or pi bonds?

Answer 20

sigma

given interatomic distance, the overlap of carbon 2p AOs is significantly better when they approach head-to-head rather than side-to-side

Question 21

x-ray crystallography

Answer 21

enables determination of structure of crystalline material

x-rays are diffracted by interactions with the electrons rather than protons/neutrons

produces electron density map

Question 22

sum of densities of each electron in each AO =

Answer 22

total electron density of atom

Question 23

x-ray crystallography - covalent bonds

Answer 23

concentrations of electron density between atoms

Question 24

shape of molecules - long range

Answer 24

electron density has low values

shape is distorted

only see Van der Waals contact

Question 25

shape of molecules - short range

Answer 25

electron density has high values

atoms and bonds are more evident

Question 26

hybridisation

Answer 26

atoms in molecules can change shape and energies of their AOs in order to provide the best overlap and max. bonding with other atoms

Question 27

why isn’t methane planar?

Answer 27

= regular tetrahedron

repulsive interactions between pairs of electrons in the 4 bonds are minimised by maximising the angles between bonds

= valence-shell electron-pair repulsion (VESPR) theory

Question 28

sp3 orbitals

Answer 28

identical

directional - pointing towards corners of a tetrahedron

well-suited for good overlap with four 1a AOs of H atoms

Question 29

why do 4 sp3 orbital hybrids form?

Answer 29

minimises repulsion as shape is symmetrical - electrons position themselves as far away as possible

Question 30

formation of C-H bonds

Answer 30

overlap of an sp3 hybrid with a hydrogen 1s AO with pairing of electrons with opposite spins

combination of 2 orbitals on C and H atoms = 2 orbitals for each C-H bond:

bonding σ orbital
bonding σ* orbital

Question 31

energies of the bonding and antibonding orbitals

Answer 31

bonding = closer in energy to the 1s AO

antibonding = closer in energy to sp3 AO

Question 32

true molecular orbitals for methane

Answer 32

obtained by combining all 4 sp3 hybrids on C with all 4 1s AO on H

delocalised over whole molecule

4 equivalent CH bonds, each with its own bonding σ orbital = valence bond theory

Question 33

methyl cation bonding - CH3+

Answer 33

C has only 6 e- in its valence shell

CH3+ has planar geometry with 3 equivalent C-H σ bonds => trigonal arrangement (bond angle = 120)

overlap with 3 1s AOs requires 3 equivalent hybrid orbitals on C ∴ C is sp2 hybridised

leaves 1 2p AO on C unused - unoccupied AO perpendicular to plane of CH3

Question 34

σ and π bonds

Answer 34

σ comes from orbitals with lower energy

π will break before σ bond

Question 35

ethyne bonding - C2H2

Answer 35

linear geometry (bond angle = 180) with each C forming one C-H σ bond with 2 C-C π bonds

each carbon is diagonal to minimise repulsion between 2 bonds

each carbon is so hybridised