| What is the structure of XeF4 and ICl4-? | square planar (90 degrees)
because of 2 lone pairs on the central element
4 bonds
square planar because of 3 lone pairs and one from the ion |
| What is the structure of ClF3? | T-shaped because of 2 lone pairs and 3 bonds |
| What is a sigma bond? (4) | head on overlap of atomic orbitals so that the electron density is between the nuclei of the bonding atoms
S-S
P-P
S-P |
| What is a pi bond? (7) | sideways overlap of atomic orbitals so electron density is above and below the nuclei of the bonding atoms
can only happen with p orbitals
does not happen with single bonds only with double, triple
there is always a sigma bond with a pi bond
double bond = 1 pi bond
triple bond = 2 pi bonds |
| What is formal charge? | atomic charge of a molecule, if all the atoms had the same electronegativity
used to determine which lewis structure is preferred by whichever is closest to 0 |
| What is the formula for formal charge? | Number of total valence electrons - Number of non-bonding electrons - number of bonding electrons / 2 |
| If both lewis structure's FC is 0? (2) | whichever individual atoms are closest to 0 is preferred
If the negative charge is on the more electronegative atom (F, O, N) |
| What are delocalized pi electrons? | electrons that are shared between more than 2 nuclei = not fixed between a pair of atoms
exist in all molecules or ions where there is more than one lewis structure possible (Resonance structures)
represented by a dashed line |
| Why do electrons delocalize in resonance structures of a molecule? | because the bond lengths and strengths are equal as there are equal contributions from both of the resonance structures
makes a resonance hybrid (mixture of both the resonance forms) |
| What is delocalization? | sharing of a pair of electrons bw 3 or more atoms |
| How do we know if something has delocalized pi electrons? | if more than one resonance structure is possible and the only difference is the position of a double bond |
| An example of a molecule with delocalised electrons? (7) Ben | Benzene C6H6 has 2 resonance structures where the double and single bonds are in different positions
The bonds are intermediate = resonance hybrid structure
Unhybridised P orbital == overlap to form a delocalised pi system
shows with a dashed line
6 pi electrons - one form each unhybridised orbital
circle shows delocalised pi electrons |
| What are resonance structures? | 2 or more lewis structures to represent a molecule or ion that can't be described with just one lewis structure
difference in position of double bonds |
| Give another example of delocalised electrons in a molecule (ethan) | ethanoate ion CH3COO-
difference between the two resoncance structures is a different position for the double bond between O-C |
| What is VSEPR theory? | valence shell electron pair repulsion theory is used to predict the geometry of molecules
electron pairs repel each other and spread as far apart as possible |
| What is electron domain? | bonds / lone pairs of electrons around an atom in a molecule
single doube and triple bonds all count as one electron domain |
| Which atoms can have expanded octets? | atoms in period 3 and onwards such as (Cl, Br, S and P) can use their d orbitals in bonding |
| What is hybridisation? | it is the mixing of atomic orbitals to produce hybrid orbitals in bonding
3 types are = sp3, sp2, and sp |
| Describe sp3 hybridisation | eg: carbon atom 2s2 and 2p2
an electron from the 2s sub level is promoted to an empty orbital in the 2p sub level = excited state (1 e in 2s and 3 in 2p)
The 2s and 2p orbitals mix to form sp3 hybrid orbitals = degenerate same energy
explains why carbon can form 4 bonds |
| Describe sp2 hybridisation | an electron is promoted to the 2p sub level = excited state
mix to form 3 sp2 orbitals with one unhybridised p orbital
unhybridised p orbital forms a pi bond (double bond) |
| Describe sp hybridisation | electron is promoted = excited
1 2s and 1 2p orbital mixes to form sp while 2 unhybridised p orbitals remain
2 unhybridised orbitals form 2 pi bonds |
| molecular geometry of sp, sp2 and sp3 | linear (2 domains) 180 degrees eg: co2
trigonal planar (3 domains)) 120 degrees BF3
Tetrahedral (4 domains) 109.5 H2O can be tetra, bent or trigonal pyramidal |
| What are free radicals? | highly reactive atoms with unpaired electrons (an O2 atom with 2 unpaired electrons |
