Bonding
Why might the e⁻s not be shared equally betw 2 atoms that are different? (2)
(1) 1 atom may be bigger than other (↑atomic radius) (2) 1 atom may have greater nuclear charge (no. p⁺)
When show HB, (3)
(1) 3 dashed lines (2) 180⁰ across HB (3) show any lone pair(s)
Requirements for HB (2)
(1) H covalently bonded to NOF (highly elec-ve) (2) Lone pair on 2nd NOF
Why are HBs so much stronger?
(1) NOF atoms have lone pairs of e-s (2) H atoms are highly e- deficient → NOF is v. elec-ve → attracts e-s toward itself → H +ve charge and v. small → v. strong elec field (bc small)
3 types of IMFs
(1) Van der Waals Forces (2) Dipole-dipole forces (3) Hydrogen bonding
Effect of lone pairs on bond angle (2)
(1) lone pairs repel more than bond pairs (1) bond angle will be lower
Perfume is a mixture of fragrant compounds dissolved in a volatile solvent. When applied to the skin, the solvent evaporates, causing the skin to cool for a short time. After a while, the fragrance may be detected some distance away. Explain these observations. (4)
(1) solvent has low Bpt/weak IMFs (2) solvent needs E to evaporate (overcome IMFs) (3) E taken from skin (so cools) (4) fragrance slowly spreads (5) by diffusion
Because VdW Fs caused by changing position of e⁻ cloud (3)
(1)↑e⁻ = ↑inst. dipole (2) VdWF size α no e⁻ present (3) VdWF size α Ar/Mr
Dative covalent bond
(2 e-s can come from just one of the atoms) Shared pair of e⁻s (lone pair) on X donated to Y
Water - liquid - solid
(Liquid) HBs break and reform easily as molec move (Solid) water molecules not free to move - HBs held in fixed positions - molecule packed less closely - ↓ρ — form on top of ponds
evidence of spacing - s, l, g
- (s) not easily compressed - (l) not easily compressed - (g) easily compressed
Strength of HB
- 10% of covalent - weaker so can break/make in conditions where covalent unaffected
In terms of e- pairs, explain why the bond angles in NH₄⁺ are all 109⁰ (4)
- 4 bonding pairs of e⁻ - repel equally - as far apart as possible - tetrahedron
diamond
- C - macromolecular - 4 covalent bonds/ea C atom - giant 3D lattice of strong covalent bonds = v. hard = v. high Mpt = doesn't conduct elec (no free charged particles)
graphite
- C - strong covalent and weaker VdWFs - ea C forms 3 cov bonds (= 'trigonal planar') - leave spare delocalised e- in p orbital = conduct elec (deloc e-)(along planes of hexagons) = soft (layers can slide over each other (held by weak VdWF) = v. high Mpt (cov bonds)
Dipole-dipole Fs - HCl
- Cl more electronegative than H - e⁻ pulled towards Cl atom - → dipole
Explain, in terms of the IMFs present in each compound, why HF has a higher Bpt than HCl (3)
- HF = Hydrogen bonding - HCl = permanent dipole-dipole bonding - HBs strongest IMF
turning a (s) to (l) - fusion
- T doesn't change - heat E provided is absorbed as the forces betw particles are weakened
How do VdW Fs work? (5)
- at any instant, the e- cloud may be distorted - 1 side (fewer e-) = δ⁺ (& converse) - instantaneous dipole - this dipole induces polarity on another atom
2 biological examples of dative covalent bonding
- chlorophyll - haemoglobin
spacing - s, l, g
- close - close - far apart
evidence of 3 states of matter arrangement - (s)
- crystal shapes have straight edges - solids have definite shapes
evidence of movement - s, l, g
- diffusion: v. slow - expand on heating - diffusion: slow - evaporate - diffusion: rapid - exert pressure
Why is ice being less dense important?
- forms on top of ponds - insulate pond - fish live through winter - help life continue
4 basic crystal types
- ionic - metallic - molecular - macromolecular
Why do lone pairs affect bond angle? (2)
- lone pairs repel more than bonding pairs
Why are lone pairs of e- opposite each other?
- lone pairs repel more than bonding pairs - so are as far apart as possible
elec conductivity - metals - alloys - ionic - covalent
- metals & alloys: (s) and (l) -- deloc e- that hold metal ions together - (l) or (dissolved in water) -- ions - gen. don't -- no charged particles
Noble gases - BPts (4)
- molar mass increases down the group - so no. e⁻ ↑ - VdW Fs get stronger - → Bpt ↑
molecular crystals - low Mpt/enthalpies of melting
- molec held in a reg array by IMF (not covalent (w/in)) - VdWFs much weaker than covalent/ionic/metallic
macromolecular - high Mpt
- must break strong covalent bonds (- not weak IMF) e.g. graphite, diamond, SiO2
evidence of 3 states of matter arrangement - (l)
- none direct but a (l) changes shape to fill bottom of container
what happens when heat gas?
- particles gain more KE - move faster - get much further apart - expand a lot
Uses for HB (3)
- proteins (C=O, N-H) - ironing (heat breaks HBs in creased mat) - DNA (HB hold 2 strands together - templates for dna replication)
arrangement of particles - s - l - g
- regular - random - random
What does δ⁺ mean?
- slight positive charge - e- deficient
what happens when heat solid?
- supply E to particles - makes them vibrate more about a fixed posn - slightly increase avg d betw particles - solid expands
movement - s, l, g
- vibrate about a point - rapid 'jostling' - rapid
How to draw molecules with lone pairs
1. Draw dot and cross diagram (+/- e- to main middle atom for charge as needed)2. Count no. bonding and lone pairs of e- 3. See basic shape 4. Reduce angle per lone pair (2 or 2.5⁰)
dative covalent bond Qs - 2 NBs
1. must be specific with what element donates (& receives) 2. be specific with whether atom/molecule/ion (chlorine or chloride)
Dimer
2 molecules joined together
covalent bond between
2 non metals
Reduce angle how many degrees per lone pair?
2 or 2.5⁰
A bond might be covalent with a _____ of ionic character A bond might be ionic with a _____ of covalent character
A bond might be covalent with degree of ionic character A bond might be ionic with degree of covalent character
When partial charges are separated like C-Cl... what forms?
A dipole forms
What is electronegativity?
Ability of an atom to attract e⁻s/e⁻ density in a covalent bond to itself
silver ion
Ag⁺
There are weak electrostatic attractions between all atoms and molecules (VdWFs) because
All atoms made up of +ve and -ve charges (even though neutral overall)
Van der Waals forces act between
All atoms/molecules at all times
Aluminium chloride has Mr = 267 - formula?
Al₂Cl₆
what is a lone pair of e-?
An unshared electron pair in a covalent bond
VdW Fs are additional to
Any other IMFs
HB - why does NOF need lone pair of e-
Attached to δ⁺ H in another molecule → HB
2 molecules which both have dipoles will... - why?
Attract one another - no matter start position (repel → 'flip' → attract)
barium ion
Ba²⁺
Bpts - H₂O, HF, NH₃ - higher than those of the hydrides of the other elements in their group
Bc HBs betw - ↑IMF of attraction - req more E to separate
Boiling points of noble gases - gradual increase in Bpt
Bc only IMF = VdW - ↑w/ no. e- present
Shape - e⁻ pairs 2 bonding pairs 2 lone pairs
Bent 104.5⁰
Shape - e⁻ pairs 2 bonding pairs 1 lone pair
Bent 118⁰
carbonate ion
CO₃²⁻
VdW Fs caused by (1)
Changing position of e⁻ cloud
Electronegativity C-Cl bond
Cl more electronegative than C → gains partial -ve charge →→ δ⁻ C gains partial +ve charge →δ⁺
Dative covalent bond aka
Coordinate bond
Allotropes
Different forms of the same element
Dipole moment sums up
Effect of all the bonds in the molecule - in molec w/ >1 polar bond, the effects of each bond may (1) cancel → molec with no dipole moment (2) add up → reinforce each other
AlCl₃ to Al₂Cl₆
Exists as a dimer - when sublime to (g)
iron (II) ion
Fe²⁺
iron (III) ion
Fe³⁺
Element with highest electronegativity value
Fluorine
How to work out shape of molecule? e.g. SO₄²⁻
Gives shape but not double bonds
What molecules have all 3 IMFs?
H covalently bonded NOF
where do hydroxonium ions come from? e.g. HCl
HCl → H⁺(aq) + Cl⁻(aq) H⁺ ion quickly bonds to H₂O → H₃O⁺
Why does water have higher Bpt than NH₃ & HF
H₂O can form 4 HBs NH₃ - 2
HCs: Bpts
Increase as chain length increase - molar mass increases down the group - so no. e⁻ ↑ - VdW Fs get stronger - → Bpt ↑
Elements become more/less electronegative as you go down a group
Less electronegative
Electronegativity Polar bonding
Lies between the 2 extremes of ionic and covalent bonding electronegativity diff = 0.5-2.0 A bond might be only slightly polar
Shape - e⁻ pairs 2
Linear 180⁰
What scale is often used to show relative electronegativity values?
Linus Pauling's scale
What is a polar molecule?
Molecule with both a slight +ve bole and a -ve pole
Dipole-dipole Fs act between
Molecules with permanent dipoles
Elements become more/less electronegative as you go from L to R across a period
More electronegative
property that best shows giant or simple molecular
Mpt (/Bpt)
ammonium ion
NH₄⁺
sodium hydrogen sulphate
NaHSO₄
What difference does a charge have on the shape of a molecule?
None - but when drawing e-, pay attention
How does atomic radius affect electronegativity?
Nucl of smaller atom closer to shared pair of e⁻s → greater FoA betw smaller-atom nucleus & shared e⁻ pair
O-H - polar
O is more electronegative than H → pulls the shared pair of e⁻s towards itself
Shape - e⁻ pairs 6
Octahedral 90⁰
VdW Original [atom] has ___ ___ in the _____ which will be _____
Original [atom] has induced dipoles in the nearby atoms which will be attracted to original [atom]
What does pulling a shared pair of e⁻s towards itself cause? (O-H)
Partial -ve (δ⁻) charge on O atom Partial +ve (δ⁺) charge on H atom
Lead ion
Pb²⁺
How does nuclear charge affect electronegativity?
RH atom has greater nuclear charge than LH atom → greater FoA betw nucl of RH (7+) atom & shared pair of e⁻s (than LH (5+) atom and shared pair of e⁻s)
Forming bond - exo/endo
Release E - Endo
Shape - e⁻ pairs 4 bonding pairs 1 lone pair
See saw 90⁰, 120⁰
Dipole moment depends on
Shape of molecule
Covalent bond is
Share e-s - attraction between +ve nuclei and their shared e- forms bond
Shape - e⁻ pairs 4 bonding pairs 2 lone pairs
Square planar 90⁰
Shape - e⁻ pairs 5 bonding pairs 1 lone pair
Square pyramid (umbrella) 90⁰
HB - why does H atom need to be bonded to a v. elec-ve atom?
Strong partial +ve charge (δ⁺) on H atom
Shape - e⁻ pairs 4
Tetrahedral 109.5⁰
Covalent bond between 2 atoms that are the same e.g. H₂ - e⁻s?
The e⁻s are shared equally betw the atoms - identical FoA betw nucleus of ea atom & shared pair of e⁻s
Covalent bond between 2 atoms that are different - e⁻s?
The e⁻s may not be shared equally betw the atoms
What is attracted to what - H₂O molecules
The lone pair of e- on [O] of another [H₂O] molecule is strongly attracted to e- deficient H atom
Electronegativity Ionic bonding occurs when
There are big differences in electronegativity - usually betw metal and non-metal
Electronegativity Covalent bonding occurs when
There are small differences in electronegativity, - usually between 2 non-metals
Effect of double bonds on molecule shape
Treat as single bonds - behave exactly as single for repulsion purposes - shape same
Shape - e⁻ pairs 5
Trigonal bipyramid 90⁰, 120⁰
Shape - e⁻ pairs 3
Trigonal planar 120⁰
Shape - e⁻ pairs 3 bonding pairs 2 lone pairs
Trigonal planar 120⁰
Shape - e⁻ pairs 3 bonding pairs 1 lone pair
Trigonal pyramidal 107⁰
Strength of imfs - order
VdW < Dipole-dipole < HBs
electron deficient
atoms that can have less than an octet - Be, B, Al can form covalent molecules don't achieve a full outer shell
-ate on anion means e.g. sulphate
cont. O → SO₄²⁻
ionic - how to identify compound formula
crossover rule
Why is HB (N-H-O) linear (180⁰)?
e- pair in N-H covalent bond repels e-s in HB between N and H
where does the extra e come from in a hydroxide ion?
e.g. KOH → K⁺ + OH⁻ - e from K → K⁺ donated to OH → OH⁻
hypervalent elements
e.g. S, P, Si, Cl
property that best shows bonding type
electrical conductivity
E needed to melt aka
enthalpy change of melting/fusion
E needed to vaporise liquid aka
enthalpy change of vaporisation
enthalpy vs T
heat energy change measured under constant pressure T depends on avg KE of particles and so related to speed - greater E = faster
Strongest IMF
hydrogen bonding (weaker than covalent)
metallic crystals/bonding
lattice of positive ions embedded in a delocalised sea of e- - attraction of +ve to -ve (! count no. e- drawn to ion)
ionic bonding involves (3)
many strong electrostatic attraction between oppositely-charged ions in a giant ionic lattice
ionic bond is between
metal and non-metal
why do molecular crystals not conduct electricity?
no charged particles to carry charge
evidence of 3 states of matter arrangement - (g)
none direct but a gas will fill its container
-ide on anion means e.g. sulphide
only one element → S⁻
What is an intermolecular force?
the force of attraction between atoms/molecules
The greater the difference between the electronegativity values...
the greater the polarity of the covalent bond
since bond forming is exothermic, electron-deficient compounds (Be, B, Al) are still...
thermodynamically stable - E is released when the elements bond to form a molecule
ionic - electricity
will conduct if molten/dissolved
