john 2 Flashcards
across period 2 Li Be etc, what is bond strength determined by
the bond order of each one determined by MO theory.
0 bo = weakest bond, bond doesn’t exist between the 2
highest and lowest bond strength for period 2 elements
highest bond strength = N2 (3)
lowest bond strength = Be2 (0)
what causes the zig zag like bond strength tree in period 2
first: bonding orbitals are not filled so low strength
2nd: antibonding orbitals are being filled so lower strength
what state are N O and F at room temp
diatomic gases
what are Li Be B and C in room temp
solids
diatomic period 2 strongest bond
N2
bond order of 3
(use mo diagram)
single molecular period 2 strongest bonding
C
(graphite and diamond)
based on enthalpy of atomisation ( s-> g)
period 3 follows the same trend with group 14, Si on top
what period have the greatest enthalpy of atomisation
2
3 is lower apart from their group 17 and 16
S (solid) and Cl (larger enthalpy of atomisation than F bc of weak FF bond due to the side of F)
why does carbon have the strongest enthalpy of atomisation
group 14
4 valence electrons
4n electrons for 4 bonding Mo
n= number of atoms
more or less e- = fewer bonding or more antibonding filled
group 14
state, bond length, shape, hybrid
solids at room temp
bond length increases down the group
tetrahedral 109.5 (tetravalent)
sp3 hybrids
allotrope meaning
different physical form of an element
carbon + graphite (most stable)
carbon + diamond (not most stable)
graphite carbon allotrope
most stable carbon allotrope
hexagonal layers of sp2
slanted hexagonal layers (ababab)
e- delocalised into pi orbital (double bond character)
all the carbon allotropes
diamond
graphite
graphene
fullerenes
diamond allotrope
each c is tetravalent
sp3 hybridised
not most stable allotrope
tetrahedral shape of C
109.5 *
graphite allotrope
Sp2 hybridised carbon
1e- delocalised into Pi orbital
double bond character
layers of hexagonal planes slanted
ababab shape
hexagonal layers most stable allotrope
graphene C allotrope
one single layer of graphite
1 hexagonal layer
fullerene C allotrope
a graphene layer but rolled up into a cylinder
closed sheet of graphene
sheet of hexagons and pentagons (allow bending)
sheet is wrapped around and joined up
Sn tin allotrope
white tin
beta tin
distorted octagonal array
stable at room temp
metal
more e- are delocalised than graphite
lead allotrope
metallic fcc
only forms metallic structures
closed pack structure
group 14 and bond strength
4 valence electrons make the bond strength large
can fill the MO
group 15
5 valence : 2* 3 bonding
N is gas at room temp, rest are solids
each has rhombohedral allotrope
( fused cyclohexane chairs with lone pair, tetrahedral shape, hexagonal layers) : black P, alpha As, alpha Sb, alpha Bi.
bond length increases down the group
weaker bonds (bonded to 3 not 4)
group 15, white phosphorus structure
tetrahedral shape
lone pair on each phosphorus
3 bonds per P.
anything group 15 has how many bonds
3 BONDS
1 lone pair!!!
tetrahedral shape
group 16 has how many valence electrons
6!!
can only form 2 BONDS
2 LONE PAIRS
per atom.
2 single bonds per atom
group 16 S8 crown
crown shape
2 bonds each
daisy crown of 8 S elements
group 16 helical element
single helix (like dna)
2 bonds - chain
2 lone pairs each
atom with bunny ears chain
lower atom as only 2 bonds need to break
why do group 14 have highest enthalpy of atomisation
4 bonds must be broken (all bonding e-)
why does group 15 have a lower enthalpy of atomisation than group 14
3 bonding
1 lone
easier to break 3 bonds than 4
why does group 16 have a lower enthalpy of atomisation than 14 and 15
2 bonds
2 lone pairs
per atom.
easier to break 2 bonds than 4 or 3
group 17
dimers!!
only 1 bonding electron left
single bond
3 lone pairs
F2 Cl2 Br2 etc
down group 17melting and bp
melting point and boiling point increases
increase in VDW as atomic weight increases
F2 colour
colourless
Cl2 colour
yellow green
Clover: yellow (gold) green (leprechauns)
Br2 colour
red brown liquid
Breens 🫘:red brown liquid
I2 colour
purple / black solid
Indigo (dark black purple)
group 18
noble gases
held together via VDW
low melting + bp.
m + bp increase down the group
monoatomic gases
5K liquid range between bp+mp (must keep temp very constant)
group 18 weight
heavier down the group
atomic radius increases
greater VDW attractions
group 18 has the
highest ionisation energies
but Xe and O have similar ionisation energies
u can ionise O2 into O2+ with strong oxidising agent so u can do it with Xe aswell (PtCl6 = oxi agent)
homonuclear bonds
bonds between 2 atoms of the same element
small elements have stronger bonds, why is this
smaller element = greater effective overlap = stronger bond
N-N O-O F-F are weaker than expected bc what
they’re small
repulsion causes a weaker bond
significant interaction repulsion bc of their small size
down the group what happens to bond strength
decreases
diffuse orbitals
less effective overlap
weaker bond
are sigma or pi stronger (not for N or O) but generally
sigma
overlap
point towards eachother (pz are directional)
pi = less effective overlap, non directional, weaker than sigma bond (side side interactions)
why are pi bonds better for N and O
small atoms, sigma bonds are weak due to inter atomic repulsion’s due to their small size.
pi bonds allow side side interactions allowing for pi bonds to be stronger than sigma bonds for N and O. DOUBLE BONDS ARE PREFERED
what is the favoured bond
bond with most energy
aka strongest bond
N = triple bond!! bc x3 single bond is still smaller than a triple bond
why is N a gas
it’s energetically favoured by the element
forms triple bonds bc the triple bond is the strongest (most favoured)
heteronuclear bonds
bonds between two different atoms
what is the strength of heteronuclear bonds determined by
difference in electronegativity
more polar = more electronegativity difference = more ionic character = stronger bond
pauling and his electronegativity
oxidation state definition
the charge the atom would have if a more electronegative atom bonded to it and acquired all the electrons
NF3 ( N= +3. F= -1)
valency definition
max number of univalent (1bond) atoms that can combine with an atom of another element under consideration
eg: trivalent, monovalent , connected to 3 or 1 different things.
hyper valent
main group elements (period 2 ns2 np6) that have more than 8e- in their valence shell
PF5 PCl5 etc etc!! 10e- in their valence shell not 8e- 😮😮😮😮
inert pair effect
when the valence S electrons don’t take part in bonding.
oxidation state is 2 less than group valency.
group 13 = +3 oxi state, TI group 3 = +1 oxi state
higher than expected ionisation energy of an atom means it’s bond strength when bonded to smt else is
weaker ??
lewis acid
electron pair acceptor
accepts electrons in the LUMO
acids accept
electrophile
lewis base
electron pair donor
FROM HOMO
nucleophile
brønsted-lowry acid
donates protons H+
brønsted-lowry base
accepts protons H+
brønsted-lowry lowry is based on the movement of
protons
lewis theory involves the transfer of
electron pairs.
if lewis acids accept electron pairs they must have
empty p orbitals
LUMO
group 13 have an empty p orbital
what are frustrated lewis pairs
bulky lewis acids and bases
cannot form addicts due to steric hinderance
e- can’t get close enough
they rip apart lewis acids and bases
how do double pairs affect VSEPR theory
they dont effect the shape of the molecule by changing the amount of e- pairs around the central atom. this is bc they have the same direction as a single bond. there is a sigma bond in a pi bond
they change the bond angles due to more repulsion, double bond is shorter, closer to nuc, more repulsion
in trigonal bipyramidal, how many nmr environents can there be
2, for axial and equitorial bc theyre of different energies + environments
molecular shape fluxionality
when the shape of a molecule is not rigid, atoms can move as bonds stretch or vibrate. if the energy is small enough, the molecule can change shapes rapidly (interconvert)
trigonal bipyramidal to square planar ( 4 - 1 - 1 )
berry pseudorotation
type of vibration that causes a molecule to isomerise by exchanging ligand position (axial –> equitorial,,, equitorial –> axial)
the groups move internally, they arent rotated.
happens so fast an average of both is seen by nor.
