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Chemical Bonding and Molecular Structure: one-page cheat sheet

NEET Chemistry · Unit 3 · Inorganic Chemistry

ScoreChemby CSJ Sir
Weightage: 4–5 questions · 16–20 marks

1 · Octet rule & formal charge

F.C. = V - L - \frac{1}{2} S

_{3}

central O

+ 1

; double-bonded O

0

; single-bonded O

- 1

IncompleteBeH

_{2}

, BF

_{3}

, AlCl

_{3}

(central atom has fewer than 8 electrons)

Odd electronNO, NO

_{2}

ExpandedPF

_{5}

(10), SF

_{6}

(12): Period 3 onwards, empty

d

-orbitals

The octet rule cannot explain shapes or noble-gas compounds such as XeF

_{2}

, XeF

_{4}

, KrF

_{2}

2 · Ionic bond & Fajans' rules

Favoured bylow ionisation enthalpy, high electron gain enthalpy, high lattice enthalpy

Lattice enthalpyenergy to separate 1 mol of crystal into gaseous ions (NaCl:

+ 788 kJ mol^{- 1}

)

More covalent when	Order
small cation	LiCl $>$ NaCl $>$ KCl
large anion	LiI $>$ LiBr $>$ LiCl $>$ LiF
high charges	AlCl $_{3}$ $>$ MgCl $_{2}$ $>$ NaCl
18-electron cation	Cu $^{+}$ , Ag $^{+}$ $>$ Na $^{+}$ , K $^{+}$

3 · Bond parameters & dipole moment

Bond order

BO \propto bond enthalpy \propto 1/ bond length

Resonancehybrid is more stable; CO

_{3}^{2 -}

C–O bond order

1.33

; O

_{3}

O–O

128

Dipole

μ = Q \times r

;

1 D = 3.336 \times 1 0^{- 30} C m

μ = 0

_{2}

, BF

_{3}

, BeF

_{2}

, CH

_{4}

, CCl

_{4}

, SF

_{6}

, XeF

_{4}

, p-dichlorobenzene

_{3}

(

1.47

D): lone-pair dipole adds to N–H dipoles. NF

_{3}

(

0.23

D): it opposes them.

4 · VSEPR shapes & hybridisation

Repulsion: lp–lp $>$ lp–bp $>$ bp–bp. Steric number = $σ$ bonds + lone pairs on the central atom: 2 $s p$ , 3 $s p^{2}$ , 4 $s p^{3}$ , 5 $s p^{3} d$ , 6 $s p^{3} d^{2}$ .

Type	Hyb.	Shape	Examples
AB $_{2}$	$s p$	linear	BeCl $_{2}$ , CO $_{2}$
AB $_{3}$	$s p^{2}$	trigonal planar	BF $_{3}$ , SO $_{3}$
AB $_{2}$ E	$s p^{2}$	bent	SO $_{2}$ , O $_{3}$
AB $_{4}$	$s p^{3}$	tetrahedral	CH $_{4}$ , NH $_{4}^{+}$
AB $_{3}$ E	$s p^{3}$	pyramidal	NH $_{3}$ ( $10 7^{\circ}$ ), PCl $_{3}$
AB $_{2}$ E $_{2}$	$s p^{3}$	bent	H $_{2}$ O ( $104. 5^{\circ}$ )
AB $_{5}$	$s p^{3} d$	trigonal bipyramidal	PCl $_{5}$
AB $_{4}$ E	$s p^{3} d$	see-saw	SF $_{4}$
AB $_{3}$ E $_{2}$	$s p^{3} d$	T-shaped	ClF $_{3}$ , BrF $_{3}$
AB $_{2}$ E $_{3}$	$s p^{3} d$	linear	XeF $_{2}$ , I $_{3}^{-}$
AB $_{6}$	$s p^{3} d^{2}$	octahedral	SF $_{6}$
AB $_{5}$ E	$s p^{3} d^{2}$	square pyramidal	BrF $_{5}$ , IF $_{5}$
AB $_{4}$ E $_{2}$	$s p^{3} d^{2}$	square planar	XeF $_{4}$

s p^{3} d

shapes the lone pairs sit in equatorial positions.

5 · $σ$ and $π$ bonds

σ

head-on overlap along the axis; stronger

π

sideways overlap of parallel

p

-orbitals

C–Cethane

1 σ

; ethene

1 σ + 1 π

; ethyne

1 σ + 2 π

; C

_{2}

molecule: two $π$ bonds and no

σ

bond

d s p^{2}

square planar, e.g. [Ni(CN)

_{4}

]

^{2 -}

;

s p^{3} d^{2}

and

d^{2} s p^{3}

give octahedral

6 · Molecular orbital theory

BO = \frac{N _{b} - N _{a}}{2}

\leq 14 e^{-}

_{2}

, C

_{2}

, N

_{2}

π 2 p < σ 2 p_{z}

(2s–2p mixing)

> 14 e^{-}

_{2}

, F

_{2}

σ 2 p_{z} < π 2 p

Species	$e^{-}$	BO	Magnetism
N $_{2}$	14	3	diamagnetic
O $_{2}^{+}$	15	2.5	paramagnetic
O $_{2}$	16	2	paramagnetic
O $_{2}^{-}$	17	1.5	paramagnetic
O $_{2}^{2 -}$	18	1	diamagnetic

= 0

(He

_{2}

, Be

_{2}

) means the molecule does not exist. A higher BO gives a shorter bond.

7 · Hydrogen bonding

StrengthF–H···F

>

O–H···O

>

N–H···N

IntermolecularHF, H

_{2}

O, alcohols, p-nitrophenol: raises boiling point and viscosity

Intramolecularo-nitrophenol, salicylaldehyde: lowers boiling point, steam volatile

8 · Quick orders

Bond angleCH

_{4}

(

109. 5^{\circ}

)

>

_{3}

(

10 7^{\circ}

)

>

_{2}

O (

104. 5^{\circ}

)

Group 15 hydridesNH

_{3}

>

_{3}

>

AsH

_{3}

>

SbH

_{3}

Bond lengthC–C

>

C=C

>

\equiv

Bond enthalpyC–C

<

C=C

<

\equiv

Seen in NEET 2022

Which has the maximum lone pair–lone pair repulsion?

SF_{4} : 1 IF_{5} : 1 ClF_{3} : 2 XeF_{2} : 3

Count lone pairs on the central atom: XeF $_{2}$ .

Six traps to remember

Octet rule cannot explain shapes or noble-gas compounds.
Smaller cation, larger anion, higher charge: more covalent character.
Lone pairs in $s p^{3} d$ shapes go equatorial; XeF $_{2}$ is linear.
$μ = 0$ for symmetrical molecules; NH $_{3}$ has more dipole than NF $_{3}$ .
O $_{2}^{+}$ is paramagnetic (BO 2.5); BO 0 means no molecule.
Intramolecular H-bonding lowers boiling point; intermolecular raises it.

Chemical Bonding and Molecular Structure: free one-page cheat sheet