Chemical Bonding and Molecular Structure — Revision Notes

⚡ 30-Second Revision

Ionic Bond — Electron transfer (metal + non-metal).
Covalent Bond — Electron sharing (non-metal + non-metal).
VSEPR — Electron pairs repel, minimize repulsion. LP-LP > LP-BP > BP-BP.
Hybridization — Mixing atomic orbitals for new hybrid orbitals.

- Steric Number 2: $sp$, Linear - Steric Number 3: $sp^2$, Trigonal Planar - Steric Number 4: $sp^3$, Tetrahedral - Steric Number 5: $sp^3d$, Trigonal Bipyramidal - Steric Number 6: $sp^3d^2$, Octahedral

MOT — Atomic orbitals $ightarrow$ Molecular orbitals (bonding/antibonding).

- Bond Order = $\frac{1}{2} (N_b - N_a)$. - Paramagnetic: Unpaired electrons. Diamagnetic: All paired electrons.

H-Bonding — H bonded to F, O, or N. Strong dipole-dipole interaction. Affects BP, solubility.

2-Minute Revision

Chemical bonding explains how atoms achieve stability. Ionic bonds involve electron transfer, forming ions (e.g., NaCl). Covalent bonds involve electron sharing, forming molecules (e.g., $H_2O$). Molecular structure, the 3D arrangement of atoms, is predicted by VSEPR theory, which minimizes electron pair repulsions (lone pairs repel more than bond pairs).

This leads to geometries like linear, trigonal planar, tetrahedral, trigonal pyramidal, and bent. Hybridization, explained by Valence Bond Theory, describes the mixing of atomic orbitals to form new hybrid orbitals ($sp, sp^2, sp^3$, etc.

) that dictate these geometries. Molecular Orbital Theory (MOT) provides a more advanced view, combining atomic orbitals into molecular orbitals. It helps calculate bond order (a measure of bond strength) and predict magnetic properties (paramagnetic if unpaired electrons, diamagnetic if all paired).

Remember the different MO energy orders for $le 14$ and $> 14$ electrons. Hydrogen bonding, a special intermolecular force involving H with F, O, or N, significantly impacts physical properties like boiling point and solubility.

Always consider both bond polarity and molecular geometry to determine overall molecular polarity.

5-Minute Revision

Start with the fundamental drive for bonding: achieving stability, often via the octet rule. Distinguish between ionic bonds (complete electron transfer, metal-nonmetal, high melting points, conductive in molten/aqueous state) and covalent bonds (electron sharing, nonmetal-nonmetal, lower melting points, generally non-conductive). Coordinate bonds are a special case of covalent where one atom donates both electrons.

For molecular structure, VSEPR theory is your primary tool. Count electron pairs (bond pairs + lone pairs) around the central atom. Remember the repulsion order: LP-LP > LP-BP > BP-BP. This dictates the electron geometry (e.g., 4 pairs = tetrahedral) and then the molecular geometry (e.g., $CH_4$ is tetrahedral, $NH_3$ is trigonal pyramidal, $H_2O$ is bent, all from tetrahedral electron geometry). Practice examples like $SF_4$ (seesaw) and $XeF_2$ (linear).

Hybridization (from VBT) explains the orbital picture. Determine the steric number (sigma bonds + lone pairs) to find hybridization: 2 $ightarrow sp$, 3 $ightarrow sp^2$, 4 $ightarrow sp^3$, 5 $ightarrow sp^3d$, 6 $ightarrow sp^3d^2$. For example, in $C_2H_4$, each carbon is $sp^2$ hybridized (3 sigma bonds, 0 lone pairs). Remember $sigma$ bonds are axial overlap, $pi$ bonds are lateral overlap of unhybridized p-orbitals.

Molecular Orbital Theory (MOT) is crucial for bond order and magnetic properties. Calculate total electrons. Use the correct MO energy diagram: for $le 14$ electrons, $pi 2p$ is below $sigma 2p_z$; for $> 14$ electrons, $sigma 2p_z$ is below $pi 2p$. Fill electrons according to Hund's rule. Bond Order = $rac{1}{2}(N_b - N_a)$. If unpaired electrons exist, it's paramagnetic (e.g., $O_2$); if all are paired, it's diamagnetic (e.g., $N_2$).

Finally, Hydrogen Bonding occurs when H is bonded to F, O, or N. It's a strong intermolecular force. Intermolecular H-bonding increases boiling point, viscosity, and solubility in water (e.g., $H_2O, NH_3$). Intramolecular H-bonding (within the same molecule) can *decrease* boiling point by reducing intermolecular interactions (e.g., o-nitrophenol). Always consider both bond polarity and molecular geometry to determine if a molecule is overall polar or non-polar.

Prelims Revision Notes

1
Octet Rule — Atoms bond to achieve 8 valence electrons (2 for H). Exceptions: electron deficient ($BF_3$), expanded octet ($PCl_5, SF_6$), odd electron ($NO$).
2
Lewis Structures — Represent valence electrons and bonds. Formal Charge = (Valence e-) - (Non-bonding e-) - $rac{1}{2}$(Bonding e-).
3
Ionic Bond — Metal + Non-metal. Electron transfer. High MP/BP, conductive in molten/aq. state. Factors: low IE, high EGE, high Lattice Enthalpy.
4
Covalent Bond — Non-metal + Non-metal. Electron sharing. Lower MP/BP, generally non-conductive. Polar vs. Non-polar covalent based on electronegativity difference.
5
VSEPR Theory — Predicts molecular geometry. Electron pairs (BP + LP) repel. Order of repulsion: LP-LP > LP-BP > BP-BP.

* 2 EP: Linear ($180^circ$) * 3 EP: Trigonal Planar ($120^circ$) * 4 EP: Tetrahedral ($109.5^circ$) * 5 EP: Trigonal Bipyramidal * 6 EP: Octahedral * Lone pairs distort ideal geometries (e.g., $NH_3$ trigonal pyramidal from tetrahedral, $H_2O$ bent from tetrahedral).

1
Hybridization (VBT) — Mixing of atomic orbitals to form new hybrid orbitals. Steric Number (SN) = $sigma$ bonds + LP.

* SN 2: $sp$, Linear * SN 3: $sp^2$, Trigonal Planar * SN 4: $sp^3$, Tetrahedral * SN 5: $sp^3d$, Trigonal Bipyramidal * SN 6: $sp^3d^2$, Octahedral * Single bond: $1sigma$. Double bond: $1sigma, 1pi$. Triple bond: $1sigma, 2pi$.

1
Molecular Orbital Theory (MOT) — LCAO method. Bonding MOs (lower energy), Antibonding MOs (higher energy).

* MO Energy Order: * For $le 14$ e- ($H_2$ to $N_2$): $sigma 1s < sigma^*1s < sigma 2s < sigma^*2s < pi 2p_x = pi 2p_y < sigma 2p_z < pi^*2p_x = pi^*2p_y < sigma^*2p_z$ * For $> 14$ e- ($O_2, F_2$): $sigma 1s < sigma^*1s < sigma 2s < sigma^*2s < sigma 2p_z < pi 2p_x = pi 2p_y < pi^*2p_x = pi^*2p_y < sigma^*2p_z$ * Bond Order (BO) = $rac{1}{2} (N_b - N_a)$.

Higher BO $ightarrow$ shorter bond, stronger bond, more stable molecule. * Magnetic Properties: Paramagnetic if unpaired electrons (e.g., $O_2$). Diamagnetic if all electrons paired (e.g., $N_2$).

1
Hydrogen Bonding — Special dipole-dipole interaction. H bonded to F, O, or N.

* Intermolecular H-bonding: Between molecules. Increases BP, MP, viscosity, solubility in water (e.g., $H_2O, NH_3$). * Intramolecular H-bonding: Within same molecule. Decreases BP (e.g., o-nitrophenol).

1
Molecular Polarity — Depends on bond polarity and molecular geometry. Symmetrical molecules with polar bonds can be non-polar (e.g., $CO_2, CCl_4$).

Vyyuha Quick Recall

For VSEPR geometries and lone pair effects, remember: 'Lone Pairs Larger Push' (LP-LP repulsion is strongest). For hybridization, count the 'SN' (Steric Number = Sigma bonds + Lone Pairs) and map it: 'Some People Prefer Pasta Dishes Deliciously Done' for $sp, sp^2, sp^3, sp^3d, sp^3d^2, sp^3d^3$ corresponding to SN 2, 3, 4, 5, 6, 7.