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Chemistry · Chemical Bonding · Chapter notes

Chemical Bonding and Molecular Structure · Class 11 Notes

Class 11 notes on Chemical Bonding and Molecular Structure: ionic and covalent bonds, VSEPR, hybridisation, molecular orbital theory, polarity and hydrogen bonding. Built to JEE Advanced depth.

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In short

Atoms bond to reach a lower energy, not to obey a rule. From there everything follows: ionic bonding when one atom takes electrons outright, covalent when they are shared, VSEPR to predict the shape from electron pairs, hybridisation to explain it, and molecular orbital theory for the cases where the simpler pictures fail.

Contents
  1. ·How to Read This
  2. 1Why Atoms Bond the octet, and its four failures
  3. 2The Ionic Bond lattice enthalpy and Fajans
  4. 3The Covalent Bond bond parameters and resonance
  5. 4VSEPR: Predicting Shape steric number decides everything
  6. 5Hybridisation valence bond theory in practice
  7. 6Molecular Orbital Theory bond order, and why oxygen is magnetic
  8. 7Polarity and Dipole Moment
  9. 8Hydrogen Bonding the strongest of the weak forces
  10. ★Chemical Bonding · Fact Sheet
0

How to Read This

  • Chemical Bonding is Tier 1 for both exams. So this is built to JEE Advanced depth.
  • Each section carries a badge saying who needs it.
  • Blue marks a defining fact. Red marks a trap.
  • Most of the text is in points. Read the point, then study the drawing.
Think it through
The one habit that carries the chapter
  • Almost every question here is a comparison. Which bond is longer? Which molecule is more polar?
  • So do not memorise molecules one at a time.
  • Learn the rule that puts them in order, then apply it.
  • Counting the steric number answers more questions than any list.
1

Why Atoms Bond

In NEET and JEE
  • Atoms combine to reach a lower energy.
  • The octet rule works well for the first two short periods.
  • The interesting questions come from where it fails.
Four standard exceptions. Each has been an exam question in its own right.THE OCTET RULE, AND THE FOUR WAYS IT BREAKSINCOMPLETEBBF3only 6 electronsEXPANDEDPPCl510 electronsODD ELECTRONNONO11 valence electronsNOBLE GASXXeF2it exists at allThe octet is a guideline, not a law. Expect a question built on a molecule that breaks it.
Four standard exceptions. Each has been an exam question in its own right.
  • Incomplete octet: fewer than eight electrons. BF3, BeCl2, AlCl3.
  • Expanded octet: period 3 onwards can use d orbitals. PCl5, SF6, IF7.
  • Odd electron: one electron is left unpaired. NO, NO2, ClO2.
  • Noble gases form compounds at all, such as XeF2 and XeF4.
Trap alert
Formal charge is bookkeeping, not a real charge
  • Formal charge = valence electrons − non-bonding electrons − number of bonds.
  • Use it to rank possible structures. The best one has the smallest charges.
  • Any negative charge should sit on the most electronegative atom.
  • It does not tell you where the electrons really are.
2

The Ionic Bond

In NEET and JEE
  • Electrons transfer completely. The ions are then held by electrostatic attraction.
  • But no bond is purely ionic.
  • Questions almost always ask how much covalent character has crept in.
A small, highly charged cation distorts a large anion. That distortion is covalent character.FAJANS: HOW MUCH COVALENT CHARACTER AN IONIC BOND PICKS UP+−a small, highly charged cationsits beside a large anionthe cloud is pulled between them,so the pair is now sharedMore covalent when: the cation is SMALL and highly charged, the anion is LARGE,and when the cation has a pseudo noble gas shell such as Cu+, Ag+ or Zn2+LiCl > NaCl > KCl > CsClas the cation gets smallerLiI > LiBr > LiCl > LiFas the anion gets larger
A small, highly charged cation distorts a large anion. That distortion is covalent character.
  • Lattice enthalpy is the energy released when gaseous ions form one mole of solid.
  • It rises with higher charge and smaller ions, so MgO melts far above NaCl.
  • The Born-Haber cycle applies Hess's law to obtain it, since it cannot be measured directly.
  • Solubility generally falls as covalent character rises. AgCl is insoluble, NaCl is not.
3

The Covalent Bond

In NEET and JEE
  • Shared pairs. Four measurable parameters describe the result.
  • Bond length falls as bond order rises and as the atoms get smaller.
  • Bond enthalpy rises with bond order.
  • Bond order is the number of shared pairs.
  • C-C is 154 pm, C=C is 134 pm, and the triple bond is 120 pm.
Trap alert
Resonance is not oscillation
  • The molecule does not flip between the drawings.
  • It is one structure, called the resonance hybrid.
  • It is more stable than any single drawing.
  • In carbonate all three C-O bonds are identical at 129 pm, which no single Lewis structure shows.
4

VSEPR: Predicting Shape

In NEET and JEE
  • The highest-yield section in the chapter.
  • Electron pairs around the central atom repel each other.
  • They settle as far apart as they can.
  • So counting them gives you the shape, with nothing to memorise.
The five parent geometries. Everything else is one of these with lone pairs taking some positions.THE FIVE PARENT SHAPES, SET BY THE STERIC NUMBER2 LINEARsp, 180°BeCl2, CO23 TRIGONAL PLANARsp2, 120°BF3, SO34 TETRAHEDRALsp3, 109.5°CH4, NH4+5 TRIG. BIPYRAMIDALsp3d, 90 and 120PCl56 OCTAHEDRALsp3d2, 90°SF6Steric number = atoms bonded to the central atom + lone pairs on it
The five parent geometries. Everything else is one of these with lone pairs taking some positions.
The whole method in one line

Steric number = atoms bonded to the central atom + lone pairs on it

Same tetrahedral parent, three shapes, three angles, because lone pairs push harder.LONE PAIRS PUSH HARDER THAN BONDING PAIRSCHHHCH40 lone pairs109.5°NHHNH31 lone pair107°HHH2O2 lone pairs104.5°Repulsion order: lone-lone > lone-bond > bond-bond
Same tetrahedral parent, three shapes, three angles, because lone pairs push harder.
  • SN 4 with 0 lone pairs: tetrahedral, 109.5°. CH4, NH4+, SO4 2-.
  • SN 4 with 1 lone pair: trigonal pyramidal, about 107°. NH3, ClO3-, XeO3.
  • SN 4 with 2 lone pairs: bent, about 104.5°. H2O.
  • SN 5: see-saw (SF4), T-shaped (ClF3), linear (XeF2, I3-).
  • SN 6: square pyramidal (BrF5), square planar (XeF4, ICl4-).
Trap alert
In a trigonal bipyramid, lone pairs always go equatorial
  • An equatorial site has only two neighbours at 90°. An axial site has three.
  • So a lone pair feels less repulsion in the equator.
  • This is why SF4 is a see-saw and not a trigonal pyramid.
  • It is also why ClF3 is T-shaped.
Shapes not coming automatically?
Count the steric number and watch the shape appear.

The VSEPR game gives you a formula, you count, and the molecule builds itself.

Play this concept in the app
5

Hybridisation

In NEET and JEE
  • Orbitals of similar energy mix together.
  • They form new, identical orbitals called hybrids.
  • These point exactly where VSEPR said they would.
  • So the two theories agree. Hybridisation explains the why.
Steric number gives the hybridisation directly

2 → sp · 3 → sp² · 4 → sp³ · 5 → sp³d · 6 → sp³d²

  • A sigma bond is head-on overlap. It is strong and allows free rotation.
  • A pi bond is sideways overlap of unhybridised p orbitals. It is weaker and locks the geometry.
  • A single bond is one sigma. A double is one sigma and one pi. A triple is one sigma and two pi.
  • s-character: sp has 50%, sp² has 33%, sp³ has 25%.
Think it through
Why bond angle tracks s-character
  • An s orbital is spherical and holds its electrons closer to the nucleus.
  • So more s-character pulls the bonding pair in closer.
  • The other orbitals then spread wider apart.
  • That is why the angle runs 180°, then 120°, then 109.5°.
6

Molecular Orbital Theory

JEE Main and Advanced
  • Valence bond theory cannot explain why oxygen is attracted to a magnet.
  • Molecular orbital theory can.
  • It combines the atomic orbitals into new ones that spread over the whole molecule.
The MO diagram for oxygen. Two electrons sit unpaired in the antibonding pi orbitals.MOLECULAR ORBITAL DIAGRAM FOR OXYGEN2p2sO atom2p2sO atomσ*2pπ*2pσ2pπ2pσ*2sσ2sO2 moleculeTWO UNPAIRED ELECTRONSsit in the antibonding pi orbitals, so O2 is PARAMAGNETICBond order = (10 − 6) / 2 = 2Lewis structures cannot show the magnetism at all.
The MO diagram for oxygen. Two electrons sit unpaired in the antibonding pi orbitals.
Bond order and magnetism

Bond order = (bonding − antibonding) / 2

Any unpaired electron makes the species paramagnetic.

  • Filling order up to N2: σ1s, σ*1s, σ2s, σ*2s, then π2p, π2p, σ2pz.
  • From O2 onwards the σ2pz drops below the π2p pair, because s-p mixing weakens.
  • Paramagnetic: O2 and B2 with two unpaired each, NO and O2+ with one.
  • Diamagnetic: N2, C2, F2, CO and NO+.
Adding electrons to oxygen fills antibonding orbitals, so the bond order falls and the bond lengthens.ADD ELECTRONS TO OXYGEN AND THE BOND WEAKENSOOO2+bond order 2.5112 pmparamagneticOOO2bond order 2.0121 pmparamagneticOOO2-bond order 1.5133 pmparamagneticOOO2 2-bond order 1.0149 pmdiamagneticeach added electron enters an ANTIBONDING orbital, so the bond lengthens
Adding electrons to oxygen fills antibonding orbitals, so the bond order falls and the bond lengthens.
Trap alert
Removing an electron does not always weaken the bond
  • From N2 you remove a bonding electron. Bond order falls 3 to 2.5, so it weakens.
  • From O2 you remove an antibonding electron. Bond order rises 2 to 2.5.
  • So the O2+ bond is stronger and shorter than in O2.
  • Same action, opposite result. It is asked almost every year.
7

Polarity and Dipole Moment

In NEET and JEE
  • Individual bonds can be polar while the molecule is not.
  • Dipole moments are vectors, so the shape decides the answer.
  • Dipole moment μ = q × d, measured in debye.
Identical bonds, opposite outcomes. In CO2 the moments cancel; in bent water they add.BOND MOMENTS ARE VECTORS, SO SHAPE DECIDESCOOCO2 is LINEARthe two moments cancel exactlyμ = 0OHHH2O is BENTso they add to a resultantμ = 1.85 DTHE CASE THAT CATCHES PEOPLENH3 = 1.47 DNF3 = 0.24 DN-F is the more polar bond,so you would expect the reverse.In NH3 the lone pair moment ADDS.In NF3 it OPPOSES.
Identical bonds, opposite outcomes. In CO2 the moments cancel; in bent water they add.
  • μ = 0 despite polar bonds: CO2, BF3, CCl4, SF6, XeF4. All are symmetrical.
  • μ is not zero: H2O, NH3, SO2, CHCl3. Lone pairs or asymmetry break the cancellation.
  • NH3 is 1.47 D but NF3 is only 0.24 D, even though N-F is the more polar bond.
  • In NH3 the lone pair moment adds to the bond moments. In NF3 it opposes them.
8

Hydrogen Bonding

In NEET and JEE
  • Hydrogen bonding needs H attached to N, O or F.
  • It is the strongest of the weak intermolecular forces.
  • It explains why water boils far above H2S, and HF above HCl.
Whether the bond forms between molecules or inside one changes the physical properties completely.HYDROGEN BONDING: INSIDE OR BETWEEN CHANGES EVERYTHINGOHHOHhydrogen bondBETWEEN moleculesraises boiling point and solubilityONHINSIDE one moleculelowers boiling point, as in o-nitrophenolWHY IT MATTERSH2O boils far above H2SHF boils above HClo-nitrophenol is MOREvolatile than the para formwhich reverses the usual rule
Whether the bond forms between molecules or inside one changes the physical properties completely.
Trap alert
Intramolecular bonding does the opposite of what you expect
  • Normal hydrogen bonding between molecules raises boiling point and solubility.
  • But sometimes the bond forms inside one molecule, as in o-nitrophenol.
  • That molecule can then no longer bond to its neighbours or to water.
  • So o-nitrophenol is more volatile and less soluble than the para form.

★ Chemical Bonding · Fact Sheet

Every rule for revision day.

OCTET EXCEPTIONS

Incomplete: BF3, BeCl2

Expanded: PCl5, SF6, IF7
Odd electron: NO, NO2.

FAJANS

Small cation, large anion,

high charge, pseudo noble gas
= more covalent.

BOND PARAMETERS

Order up: length down

single 154, double 134,
triple 120 pm.

STERIC NUMBER

SN = bonded atoms + lone pairs

2 sp, 3 sp², 4 sp³,
5 sp³d, 6 sp³d².

LONE PAIR ANGLES

CH4 109.5°, NH3 107°, H2O 104.5°

lp-lp > lp-bp > bp-bp.

SN 5 SHAPES

1 lp see-saw (SF4)

2 lp T-shaped (ClF3)
3 lp linear (XeF2, I3-).

SN 6 SHAPES

1 lp square pyramidal (BrF5)

2 lp square planar (XeF4)
lone pairs go opposite.

BOND ORDER

BO = (Nb − Na) / 2

Unpaired electron = paramagnetic.

MO FILLING

Up to N2: π2p below σ2pz

From O2: σ2pz below π2p.

PARAMAGNETIC LIST

O2, B2, NO, O2+, O2−

Diamagnetic: N2, C2, F2, CO, NO+.

N2 vs O2 ON IONISING

N2 to N2+: 3 to 2.5, weaker

O2 to O2+: 2 to 2.5, stronger.

DIPOLE MOMENT

μ = 0: CO2, BF3, CCl4, SF6

NH3 1.47 D but NF3 0.24 D.

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