Hybridization of Molecules – Types, Examples, and Exceptions

Hybridization is a fundamental concept in chemistry that explains how atoms form bonds in molecules by mixing their atomic orbitals to create new, equivalent hybrid orbitals. This theory was introduced by Linus Pauling to explain the shapes of molecules that could not be accounted for by simple valence bond theory.

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What is Hybridization?

Definition:
Hybridization is the process in which atomic orbitals of similar energy mix to form new, equivalent orbitals called hybrid orbitals. These orbitals have specific geometries that explain the shape and bond angles of molecules.

Key Points:

• It occurs during bond formation.

• Hybrid orbitals have identical energy and shape.

• The type of hybridization determines the geometry of the molecule.

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Why Hybridization is Important?

Hybridization helps explain:

• The bond angle in methane (109.5°) instead of the expected 90°.

• The planar structure of ethene (C₂H₄).

• The linear shape of CO₂ despite carbon having 2 double bonds.

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Types of Hybridization

1. sp Hybridization

• Mixing: 1 s orbital + 1 p orbital.

• Geometry: Linear (180° bond angle).

• Example: BeCl₂, CO₂, C₂H₂ (acetylene).

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2. sp² Hybridization

• Mixing: 1 s orbital + 2 p orbitals.

• Geometry: Trigonal planar (120° bond angle).

• Example: BF₃, C₂H₄ (ethene).

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3. sp³ Hybridization

• Mixing: 1 s orbital + 3 p orbitals.

• Geometry: Tetrahedral (109.5° bond angle).

• Example: CH₄ (methane), NH₃ (ammonia), H₂O (water).

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4. sp³d Hybridization

• Mixing: 1 s orbital + 3 p orbitals + 1 d orbital.

• Geometry: Trigonal bipyramidal (90° & 120° bond angles).

• Example: PCl₅, PF₅.

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5. sp³d² Hybridization

• Mixing: 1 s orbital + 3 p orbitals + 2 d orbitals.

• Geometry: Octahedral (90° bond angle).

• Example: SF₆.

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Example – Hybridization in Methane (CH₄)

• Carbon's electronic configuration: 1s² 2s² 2p².

• In the excited state, one 2s electron is promoted to the empty 2p orbital.

• The 2s and three 2p orbitals hybridize to form four sp³ hybrid orbitals.

• These orbitals arrange tetrahedrally and form σ bonds with four hydrogen atoms.

Result: CH₄ has a tetrahedral geometry with 109.5° bond angles.

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🌟 Hybridization on the Basis of Bond Pairs and Lone Pairs of Electrons

Hybridization is the mixing of atomic orbitals (s, p, d) to form new equivalent orbitals called hybrid orbitals. The arrangement of these hybrid orbitals depends not only on bond pairs (shared electron pairs) but also on lone pairs (non-bonded electron pairs) around the central atom.

This concept is best explained using VSEPR theory (Valence Shell Electron Pair Repulsion theory). According to it:

• Bond pairs (BP) and Lone pairs (LP) both occupy space around the central atom.

• Lone pairs exert greater repulsion than bond pairs, which slightly distorts bond angles.

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✨ Key Rule:

$$\text{Hybridisation type depends on (Bond pairs + Lone pairs) around the central atom.}$$

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🧪 Examples

1. CH₄ (Methane)

• Central atom: C

• Electron pairs: 4 bond pairs, 0 lone pairs

• Total = 4 → sp³ hybridization

• Geometry: Tetrahedral, Bond angle = 109.5°

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2. NH₃ (Ammonia)

• Central atom: N

• Electron pairs: 3 bond pairs + 1 lone pair = 4

• Total = 4 → sp³ hybridization

• Lone pair compresses bond angle → Pyramidal shape

• Bond angle ≈ 107° (less than 109.5° due to lone pair repulsion)

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3. H₂O (Water)

• Central atom: O

• Electron pairs: 2 bond pairs + 2 lone pairs = 4

• Total = 4 → sp³ hybridization

• Shape: Bent / V-shaped

• Bond angle ≈ 104.5° (further reduced due to 2 lone pairs)

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4. BeCl₂ (Beryllium chloride)

• Central atom: Be

• Electron pairs: 2 bond pairs, 0 lone pairs

• Total = 2 → sp hybridization

• Shape: Linear, Bond angle = 180°

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5. BF₃ (Boron trifluoride)

• Central atom: B

• Electron pairs: 3 bond pairs, 0 lone pairs

• Total = 3 → sp² hybridization

• Shape: Trigonal planar, Bond angle = 120°

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🔑 Takeaway:

• Only the regions of electron density (bond pairs + lone pairs) decide hybridization.

• Lone pairs reduce bond angles but do not change the hybridization type.

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Quick Summary Table

Hybridization	Orbitals Mixed	Geometry	Bond Angle	Example
sp	1 s + 1 p	Linear	180°	CO₂, BeCl₂
sp²	1 s + 2 p	Trigonal planar	120°	BF₃, C₂H₄
sp³	1 s + 3 p	Tetrahedral	109.5°	CH₄, NH₃, H₂O
sp³d	1 s + 3 p + 1 d	Trigonal bipyramidal	90°, 120°	PCl₅
sp³d²	1 s + 3 p + 2 d	Octahedral	90°	SF₆

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Key Takeaways

• Hybridization explains the shapes and bond angles of molecules.

• The type of hybridization depends on the number of electron domains around the central atom.

• Exceptions occur in molecules with lone pairs, resonance, or unusual bonding

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•    Exceptional Cases in Hybridization              1. NH₃ (Ammonia)

  • Prediction: sp³ → Tetrahedral → 109.5°

  • Reality: 3 bond pairs + 1 lone pair → Trigonal pyramidal, angle 107°

  • Reason: Lone pair–bond pair repulsion compresses angle.

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  2. H₂O (Water)

  • Prediction: sp³ → Tetrahedral → 109.5°

  • Reality: 2 bond pairs + 2 lone pairs → Bent (V-shape), angle 104.5°

  • Reason: Two lone pairs exert stronger repulsion.

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  3. PCl₅ (Phosphorus pentachloride)

  • Prediction: sp³d → Trigonal bipyramidal (90° & 120°)

  • In gas phase: PCl₅ is stable.

  • In solid state: it exists as [PCl₄]⁺ [PCl₆]⁻ (not simple sp³d).

  • Reason: d-orbitals involvement is debated; some chemists argue it’s better explained by molecular orbital theory.

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  4. SF₆ (Sulfur hexafluoride)

  • Prediction: sp³d² → Octahedral → 90°

  • Works well, but Sulfur exceeds octet (12 e⁻) → violates octet rule.

  • Exception accepted only with expanded octet elements (3rd period and beyond).

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  5. ClF₃ (Chlorine trifluoride)

  • Prediction: sp³d → Trigonal bipyramidal

  • Reality: 3 bond pairs + 2 lone pairs → T-shaped

  • Reason: Lone pairs occupy equatorial positions, distorting shape.

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  6. XeF₂ (Xenon difluoride)

  • Prediction: sp³d → Trigonal bipyramidal

  • Reality: 2 bond pairs + 3 lone pairs → Linear

  • Reason: Lone pairs occupy equatorial sites (more stable), leaving a straight line.

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  🔑 Summary Rule for Exceptions

  • Lone pairs reduce bond angles.

  • Heavier elements (like P, S, Xe) can show expanded octet → unusual hybridizations.

  • Some modern theories (Molecular Orbital Theory) suggest that d-orbital participation in sp³d and sp³d² is not always accurate.

  • Exceptional Cases in Hybridization

    1. No Hybridization: Some molecules, like O₂ and F₂, follow pure orbital overlap without hybridization.

    2. Distorted Geometry: Lone pairs can reduce bond angles (e.g., NH₃ – 107°, H₂O – 104.5°).

    3. d-Orbital Participation Debate: In hypervalent molecules like SF₆ and PCl₅, modern quantum theory suggests more complex bonding than simple d-orbital hybridization.

    4. Resonance Structures: In benzene (C₆H₆), each carbon is sp² hybridized, but π electrons are delocalized over the ring.

Cannizzaro Reaction

                              Cannizzaro Reaction

Cannizzaro Reaction – Mechanism, Examples, and Exceptions

The Cannizzaro Reaction is a fascinating transformation in organic chemistry that showcases a unique redox process involving aldehydes. Discovered by Stanislao Cannizzaro in 1853, this reaction remains a classic example of how subtle changes in molecular structure can dictate completely different chemical behaviors.

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What is the Cannizzaro Reaction?

The Cannizzaro reaction is a base-induced disproportionation of an aldehyde without α-hydrogen into a primary alcohol and a carboxylate salt.

Simply put:

One molecule gets oxidized to a carboxylate.

Another molecule gets reduced to a primary alcohol.

General Reaction:

2 RCHO \xrightarrow{\text{Conc. NaOH}} RCH_2OH + RCOONa

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Why Only Non-Enolizable Aldehydes?

For the Cannizzaro reaction to occur, the aldehyde must lack α-hydrogens (hydrogens attached to the carbon next to the carbonyl group).

If α-hydrogens are present, the aldehyde prefers the aldol condensation pathway instead, as enolate formation is easier under basic conditions.

Examples of suitable aldehydes:

Formaldehyde (HCHO)

Benzaldehyde (C₆H₅CHO)

p-Nitrobenzaldehyde

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Mechanism of the Cannizzaro Reaction

1. Nucleophilic attack

Hydroxide ion (OH⁻) attacks the carbonyl carbon, forming a tetrahedral alkoxide intermediate.

2. Hydride transfer

This intermediate donates a hydride ion (H⁻) to another molecule of aldehyde.

3. Product formation

The hydride donor becomes a carboxylate anion.

The hydride acceptor becomes a primary alcohol.

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Examples

Example 1 – Formaldehyde Reaction

2 HCHO + NaOH \longrightarrow CH_3OH + HCOONa

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Example 2 – Benzaldehyde Reaction

2 C_6H_5CHO + NaOH \longrightarrow C_6H_5CH_2OH + C_6H_5COONa

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Exceptional Cases

1. Aldehydes with α-Hydrogen

Prefer aldol condensation instead of Cannizzaro (e.g., acetaldehyde).

2. Cross Cannizzaro Reaction

Two different non-enolizable aldehydes react together—often, formaldehyde acts as the reducing agent due to its high reactivity.

3. Effect of Electron-Withdrawing Groups

Aldehydes with –NO₂, –CF₃, or similar groups react faster because these groups increase the electrophilicity of the carbonyl carbon.

4. Intramolecular Cannizzaro

In dialdehydes without α-hydrogen, both aldehyde groups can react internally to form hydroxy acids.

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Key Takeaways

Cannizzaro Reaction = Base + Non-enolizable aldehyde → Alcohol + Carboxylate salt.

Works only with aldehydes without α-hydrogen.

Can occur in intermolecular (between two molecules) or intramolecular (within the same molecule) forms.

Widely used in organic synthesis to convert aldehydes directly to alcohols and acids without external oxidizing or reducing agents.

💡 Did you know? The Cross Cannizzaro Reaction is often used in labs to prepare pure alcohols or acids when two different aldehydes are available—formaldehyde usually serves as the reducing partner.

Shape of orbitals....

According to wave mechanical concept  " The maximum probability of the existence of the electron is at a given distance from the nucleus " .  According to Bohr's calculation and according to wave mechanics the value of that distance have same value.

Wave mechanics  says that  "The space  around  the nucleus where probability of finding of electron is very high " is called  atomic orbitals.

1. Shape of S- orbitals--
              
--For S-orbitals  l = 0 therefore m = 0 but the value of  n  can vary.
 --All the s orbitals has spherical  shape , maximum electron find in this region.
 -- S orbitals can hold maximum 2 electrons, each with opposite spin.

1.                                                                                                               

                                                                                                                        

2. Shape of P- orbitals -

            

 -  All  orbitals  have l (Azimuthal q. number) = 1 and 3 possible values of m (-1 , 0, +1)

 -All the p orbitals (Px, Py, Pz)  are dumbell shape ,the region where the probability of finding of electron is very high.

3. Shapes of d- orbitals-

  -There are total 5 possible values for orbitals

dz2,

dxz

,dyz

,  dxy

,    dx2−y2

-Above picture shows that first four d-orbital are double dumbell shape while last dz orbital is dumble shape with a collar in xy plane.It means maximum electron can be find in that given orientation or in that planes.

-For d orbitals n = 3 onwards.
   maximum numbers of electron in d- orbitals = 10 .

4. Shape of f- orbitals-
 - Shape of f orbitals are very complex then  d- orbitals ..
-There are total 7 f- orbitals .

-Maximum number of electron found in f - orbitals = 14.

Place of  s, p, d, f  orbitals in periodic table--

Filling of electron in s, p, d, f orbitals follow the given order--

      1s->2s->2p-> 3s-> 3p-> 4s-> 3d-> 4p.....so on

Energy of orbitals in increase in following order--

      1s< 2s<2p< 3s< 3p< 4s <3d< 4p ...





=> Value of energy and radius of orbitals increases with increase in number of orbitals  or  with increase in  Principal quantum number  n (shell).l

8. Quantum numbers..

We know that any atom is composed of many shells named  K, L, M, N..etc. All the shells contains subshells i.e. s, p, d, f, ...etc, and each subshell composed of various orbitals..i.e. 

S, px , py , pz

dz2,

dxz

,dyz

,dxy

,dx2−y2

--Simply this is a  process of naming of these shells, subshells ,  orbitals and their spin orientation by a numbers and these numbers are called Quantum number.
-- It is found by the solution of Schrodinger wave equation for hydrogen atom.

There are four types of Quantum number

1. principal q.no. (n)-  which reprasent main energy level or shells in which the electron is present.        
                    n= 1, 2, 3, 4, ....
2. Orbital angular momentum quantum number or Azimuthal  q. no. (l)- It described the orbital motion of the electron around the nucleus which is described by the orbital angular momentum of the electron.thus it is called orbital angular momentum quantum number.     
                                 l = n-1
 3. magnetic q. no.  (m)- Since the magnetic or electrical field generated by the angular momentum of the electron interact with external magnetic or electric field.The electron orient or revolve themselves in a specific regions of space around the nucleus called orbitals.The number of  orbitals in a given sub energy level (l) within a principal energy level (n) is given by the number represented by m ,called magnetic quantum number.
Possible values of m is
                                     m = 2l +1
  
4. Spin q. no.  (s)-- It arises by the spnning of electron around the nucleus as well as around its own axis. We will discuss it below. 
    

Shells	K, L, M, N..	Represented as   n
Subshells	S, p, d, f…	Represented as   l
Orbitals	S, px , py , pz dz2, dxz   ,dyz ,dxy ,dx2−y2 ,fz3 ,fxz2 ,fyz2 ,fxyz ,fz(x2−y2) ,fx(x2−3y2) ,fy(3x2−y2. …	Represented as  m
Spin	Up  or Down	Represented as  s

Example:-- for electronic configuration  given below  the value of n , l, and no. of electron  is 1, 0, 2 respectively.
    

--Possible values for n, l, m, s                             

n	1, 2, 3, 4…
l	0 to n-1
m	+ ι , 0, - ι
S	+1/2, or   -1/2

-- Quantum number presents the position of electron and it also indicates the distance of electron from the nucleus..
If n = 1
    l = 0   
then the position of electron = 1s
If  n = 2
    l = 1
then position of electron = 2p
 If  n =3
    l = 2
then position of electron = 3d
-- Picture given below shows the distance of orbitals from nucleus and the energy as well..                                     

 --If we know the principal quantum number we can determine the radius , velocity , and energy of the electron .

Spin quantum number-
                      Fourth q. no. doesn't follow from wave mechanical treatment. It arises from the spectral evidence that electron in its motion around the nucleus also rotates or spin about its own axis. Because of this rotation the electron has magnetic moment called spin magnetic moment which can be either up or down spin .
    
-Spin angular momentum is characterised by a Q . number S
                       S = 1/2,   either +1/2 or -1/2.
 Hydrogen spectra fine structure is observed as a doublet corresponding to two possibilities for the Z-component of the angular momentum , where for  any given direction Z the value of Spin  angular momentum S is ..
                   

                                               Sz =   ± 1/2 ћ

--Its solution give to possible  Z- component for electron  spin up and spin down

--When atom have even no. of electron in each orbitals , orientation of one electron will be opposite to other , or each electron will be opposite in orientation to that of its immediate neighbour.

-In the early year of Quantum mechanics atomic spectra external field can't be predicted with just n, l, m.

-Unlenbeck , Goudsmit and Kronig introduce an idea of self rotation of the electron , which would naturally give rise to an angular momentum vector in addition to the one associate with orbital rotation (l , m).

-- Electron Spin magnetic moment   µs = -( e/2m) g s

                where e= charge  and      g  =  Lande - g - factor.

Thus , these four quantum numbers play an important role to determine the value of radius , energy level (n), sub energy level (l), the orientation of  orbital(m) and  the direction of spin.

In other word " Quantum numbers serve as an address for an electron.

7. Electronic configuration of the Element..

According to Rutherford model of an atom, atom is made of outer electron cloud   (-vely charged electron) and inner nucleus (neutral neutron and +vely charged proton).Niels Bohr agreed with this concept , he also agreed that electron revolve round the nucleus like a planet moves round sun.He applied the Plank's quantum theory to the electron revolving round the nucleus
         The orbits , there fore called as energy levels or energy shells. Bohr gave numbers 1, 2, 3, ...etc to these energy levels, there are they now called as Principal quantum numbers.these energy levels are also designated by  k, l, m, n, etc...

The energy levels which are more far from the nucleus associated with greater amount of energy                                                        -The energy of outermost electron is greater than innermost.

-These outermost electron helps atom to react with other because they can pullout easily  from the orbital .

-Each shell made of many sub shells which are themselves composed of atomic orbitals.

-First shell K  have 1 sub shell = s

-Second shell L  = 2 sub shells = s , p

-Third shell M = 3 sub shells =s , p, d

-fourth shell N = 4 sub shells = s , p, ,d, f

No.	Sub shell	Name
1	s	Sharp
2	p	Principal
3	D	Diffused
4	f	fundamental

Shells	Sub shells	No. of electron	Type of sub shells found in shell	Distribution electron in sub shells
K	s	2	only s	1s
L	P	6	s ,p,	2s , 2p,
M	d	10	s, p, d,	3s , 3p, ,3d
N	f	14	s, p, ,d, f , d,	4s , 4p , 4d , 4f
…	g and so on	18…	s , p, d, f, g,…	Alphabetically  so on

-The reactivity of the element is highly dependent upon its electronic configuration.

- The distribution of electron in various orbital (s, p,d ,f ) is known as electronic configuration.

-Mathematically it is described by Slater rules.

-In representing position of electron in various shells and sub shells ,the following rules are observed..

=> Major  energy shells written first then sub shells and after that the no. of electron in particular sub shell

for example :-     

1s²,   where ,   1 represents the shell i.e. K

                        s represent the sub shell
                    and  2 indicates the no of electron present in sub shell.

-According to Afbau principal--

               The electrons enter in the various orbitals in the order of increasing energy.

- According to pauli exclusion principal--

                An orbital can contain a maximum of 2 electron and these two electrons must be in opposite spin.                                              

                                                                       

↑

↓

                                                            

-According to Hund's rule--

           It states that electron pairing in orbital of the same energy level will not take place unless all available orbitals of subshell contain one electron  each with parallel spin.   

Order of filling of electron in various orbitals is given below :--

Above picture shows normal periodic table of element, but from the picture given below we can find the outermost orbital of the element and electronic configuration.

There are some special cases of electronic configuration.
From periodic table we can see that the electronic configuration of the elements  are going in normal way till Argon(18) last electron goes in 3p orbital. But in case of Potassium(19) last electron should fill in 3d orbital but it is not so, it goes in 4s orbital instead of 3d orbital because 4s has low energy in comparison to 3d .Thus electron prefer to go in lower energy orbital then higher energy level.

-The next ten elements from ( Z=21 to 30) Sc to Zn are called Transition elements.
In these elements addition of electron take place in inner 3d orbitals while outer 4s orbital remains fully occupied.