Chem Bonding I Flashcards
Factors affecting covalent bond strength
In order of importance
- no. of bonds b/w atoms (triple>double>single) - more 2- to share b/w atoms - atoms held closer
- Extent of atomic orbital overlap (smaller - more effective overlap vs larger - more diffuse & less effective overlap)
- Polarity of bond - polar>non-polar due to extra electrostatic attraction b.w partial charges
- Presence of neighbouring lone pairs - very small atoms (P2 elements) have lone pairs in close proximity - excessive repulsion that weakens the covalent bond
Why does a covalent bond exhibit ionic character?
Diff electronegativities of the 2 atoms - unequal sharing of e- by the 2 bonded atoms - permanent dipole is formed n the bond is polar - with this separation of charge, the covalent bond is said to have some covalent character
Why is there covalent character in ionic bonds?
- Greater ratio of e- to protons - the outer e- of the anion is loosely held to the nucleus
- The cation, being +vely charged - e- deficient - attract the outer e- of the anion
- Attraction distorts the e- cloud in the anions - polarising it to some extent
- Shifts the e- density from the anions towards the cations - bonding e- may be considered partially shared b/w the ions - has some covalent character
Factors affecting degree of covalent character
- Polarising power of the cation (ability to polarise the anion) ∝ charge density ∝ charge of cation/cation radius
- Polarisibility to the anion (ease of being polarised) ∝ size
VSEPR all shapes and angles
e- pairs 0LP 1LP 2LP 3LP
2 linear 180
3 trigonal bent
planar 120 117
4 tetrahedral trigonal bent
109.5 pyramidal 105
107
5 trigonal see-saw T-shaped Linear
bipyramidal 120 & 180 90 & 180 180
120 & 90
6 octahedral sq. sq planar
90 pyramidal. 90
90
Factors affecting bond angle
LP vs BP
- LP exert greater repulsion than BP as
- LP are closer to the nucleus of an atom since they are attracted by only 1 +ve nucleus compared to bp which are attracted by 2 nuclei
- Hence, LP occupy a larger vol in space since they are not shared b/w 2 atoms - greater repulsion
2. Electronegativity (use when molecules have same no. of bp/lp) - N is > electronegative than P - bp of e- are drawn closer to N
- greater repulsion b/w bp in NH3
3. Steric repulsion (size - use when shape is the same) - size of terminal atoms increases, increase in e- clouds, increase in repulsion b/w bp
id-id - how does it work
instantaneous dipole - induced dipole attraction
- e- move constantly and randomly in an atom/molecule causing uneven distribution of e-
- the portion with more e- acquires a partial -ve charge while the other portion acquires a partial +ve charge
- this dipole induces a dipole in neighbouring molecules
- an electrostatic attraction thus occurs b/w oppositely charged molecules
- such attractions are always weak and short-lived since the unequal distribution of e- last only for a split second
- id-id interactions are forming and breaking all the time
Factors affecting id-id
- No of e-
- larger e- clouds have longer momentary distortion and are more easily polarised - stronger id-id interactions
2. surface area of contact b/w molecules - id-id are weak and short-ranged
- for molecules with comparable no. of e-, straight chair molecules have a greater SA and hence more extensive id-id interactions as compared to a near-spherical, branched molecule
pd - pd - how it works
Permanent dipole - permanent dipole attractions (in polar molecules)
- More electro-ve atoms acquire a partial -ve charge
- Less electro-ve atoms acquire a partial +ve charge
- partial +ve charge end aligns near the … -ve end of the neighbouring molecule
- electrostatic attraction b/w permanent … & … ends of the molecules is known as pd-pd
Factors affecting strength of h-bonds
*1. Extent of hydrogen bonding - more or less extensive
- u write ___(no.) no. of H-bonds per molecule - lowest no. b/w no. of H-atoms in compound & no. of LP
2. 2 scenarios: (depends on data given)
1st: Polarity of H-Y bond
H-F > H-O > H-N
cuz most to least polar, so strongest to weakest
2nd: WILLINGNESS to share LP on Y
F > O > N
most to least electro-ve
least to most willing to
What affects bond length
bond strength/no. of bonded e-
Factors affecting strength of metallic bonding
CHARGE DENSITY
- Charge - greater no. of valence e- (larger charge), the stronger the metallic bond
- Size of metal cation - for the same no. of available valence e-, the smaller metal cation will exhibit stronger metallic bonding as delocalised e- are closer to the positive nuclei, resulting in stronger attraction b/w e- and nuclei
Dimerisation eg
ICL3 - pt towards I
BH3 & NH3
AlCl3
Strength in decreasing order
Ionic > Covalent > Metallic > I-D attractions > H-bonding > Pd-pd attractions > pf-id attractions > id-id attractions
(? permanent dipole-induced dipole attraction is stronger than pd-pd & id-id)
How to compare boiling/melting pt of A & B
A has a ___ struc while B has a ___ struc. More energy is required to overcome the strong __ b/w __ as compared to the weak __ b/w ___
