Sigma vs Pi bond: The Identifications and Main Differences

Sigma bond is the first bond between two atoms whereas pi bonds constitute all of the remaining bonds present in a compound. A single bond always contains one sigma bond and a double bond contains one sigma and one pi bond. Similarly, a triple bond contains one sigma and two pi bonds.

Prerequisites
Valence Bond Theory (VBT)
Molecular Orbital Theory (MOT)
Bonding: A phenomenon of joining
Sigma and Pi bonding

Difference between sigma and pi bond

Sigma bond Pi bond
It is the head-on overlapping result of two atomic orbitals It is the parallel overlapping result of two unhybridized orbitals
The linear and co-axial bond formed between two atomic orbitals is termed a sigma bond The sidewise or lateral bond formed between two parallel orbitals is termed a pi bond
A stronger covalent bond formed due to transfer of approaching orbitals with heads facing each other A relatively weaker bond formed due to transfer of approaching orbitals with lateral sides facing each other's
A sigma bond always occurs as a single bond A pi bond is formed in a compound after a sigma bond has already formed
For example, The only bonds between alkanes, etc For example, The bonds between alkenes and alkynes except sigma bond, etc

Sigma bond corresponds to the linear or co-axial bond formed between anti-parallel orbitals of two atoms whereas a pi bond is a bond formed between parallel orbitals of two atoms when these orbitals are perpendicular to the sigma bond. The orbital overlap in the sigma bond is referred to as head to head overlap but for pi bonds, it is called lateral overlap.

Both sigma and pi bonds are actually covalent bonds formed between two atoms.

  • Sigma bond is the strongest covalent bond formed by head-on overlapping of atomic orbitals. It is denoted by ‘σ’.
  • A covalent bond formed by the lateral or side-by-side overlap of half-filled atomic orbitals of atoms is called a pi bond. It is denoted by ‘π’.

Stability

The stability of sigma and pi bonds depends upon the extent of

  1. Overlapping of orbital

The greater the overlapping is, the stronger the bonding will be. This phenomenon is due to the increase in electron density when approaching orbitals overlap.

A sigma bond is stronger than the pi bond due to a greater and stronger overlap of orbitals.

  1. Bond length

The strength of the bond depends upon the bond length as well. The greater the bond length, the weaker the bond is.

A sigma bond is stronger than the pi bond due to the shorter bond length.

Rules to identify sigma and pi bonds

A single bond always contains one sigma bond. A double bond contains one sigma and one pi bond. A triple bond will contain one sigma and two pi bonds.

  1. Single bond = 1 σ bond

For example, all single (covalent) bonds are sigma bonds, like H-P, Na-O, Mg-O, Al-F, etc.

  1. Double bond = 1 σ bond and 1 π bond

For example, all double bonds like C=C, O=O, etc.

  1. Triple bond = 1 σ bond and 2 π bond

For example, all triple bonds like N≡N, C≡C, C≡N, etc.

Sigma vs Pi bond by VBT (Valence Bond Theory)

According to VBT:

  • Atoms with unpaired electrons tend to combine with other atoms which also have unpaired electrons.
  • The unpaired electrons pair up and the atoms attain a stable configuration like a nearest noble gas.
  • The number of bonds formed by an atom is the same as the number of unpaired electrons in the ground state.
  • The bond is formed by the pairing of electrons so the spin of two electrons must be opposite (Pauli’s Exclusion principle).
  • The two types of overlapping are possible:
  1. Axial overlapping (head-on)
  2. Parallel overlapping (sidewise/lateral)
  • The extent of overlapping decides the strength of bonds.

For example

Hydrogen molecule

The two hydrogen atoms HA and HB each have one electron overlap to form a diatomic molecule H2. There exists just a head-on approach, making this bond a sigma bond. there is no possibility of a pi bond in hydrogen molecules due to limited orbitals.

H = 1s1

H = 1s1

Valence bond theory for Sigma bonds

Oxygen molecule

According to VBT, The two oxygen atoms combine to form the oxygen molecule when they are sp2 hybridized.

The ground state electronic configuration of each atom is following:

O = 1s2, 2s2, 2px2, 2py1, 2pz1

O = 1s2, 2s2, 2px2, 2py1, 2pz1

The new hybridized electronic configuration of oxygen atoms is as;

Valence bond theory for Sigma bonds

The hybridized state of oxygen atoms contains two lone pairs, one (half) bond pair which makes a sigma bond. An electron is present in the 2pz orbital, which corresponds to the pi bond.

The VBT can explain the bonding of some simple molecules but in some cases like Be, B, and C, etc, it may not predict the correct results.

Sigma vs Pi bond by MOT (Molecular Orbital Theory)

In valence bond theory (VBT), we deal with the overlapping of atomic orbitals so the electrons are localized between the two atoms. In molecular orbital theory (MOT), the molecular orbitals are formed by the linear combination of atomic orbitals, and the electrons are delocalized over the whole molecule.

According to MOT

  • The important point of MOT is that atomic orbitals of different atoms combine to form molecular orbitals. This combination is known as the linear combination of atomic orbitals (LCAO).
  • There are two types of molecular orbitals:
  1. Bonding molecular orbitals
  2. Antibonding molecular orbitals
  • The bonding molecular orbitals have lower energy and the antibonding molecular orbitals are having high energy.
  • A node or nodal plane is a region where the probability of finding electrons is zero.
  • The number of molecular orbitals produced must always be equal to the number of atomic orbitals combined.
  • The interaction between atomic orbitals, for example, 1s orbitals of the hydrogen molecule forming bonding and antibonding molecular orbitals can be represented as the energy level diagram.
  • After the construction of the energy level diagram, the filling of molecular orbital with electrons will be according to:
  1. Pauli exclusion principle
  2. Aufbau principle
  3. Hund’s rule

According to the above principles, the lowest orbitals (LUMO) will fill first. Each orbital has a maximum of two electrons with the opposite spin. 

For example,

Formation of hydrogen (H2) molecule

There are two electrons in the hydrogen molecule. These two electrons occupy lower energy bonding molecular orbital making hydrogen limited to sigma bonds only.

 

H = 1s1

H= 1s1

Hydrogen molecule molecular prbital theory

Formation of Oxygen (O2) molecule

The electronic configuration of the oxygen atom is

1s2, 2s2, 2px2, 2py1, 2pz1

The electronic configuration of the O2 molecule is

2s)2 (σ*2s)2 2px)2 (π2py ,π2pz)4 (π*2py ,π*2pz)2

The energy level diagram of the O2 molecule predicts the above-mentioned properties. Moreover, it also proves some unpaired electrons in pi orbitals making oxygen able to make sigma as well as pi bonds.

Energy level diagram for oxygen molecule

The bond order of the O2 molecule is 2. One of the bonds is σ-bond and the other is π-bond.

The paramagnetic behavior of the O2 molecule is also proved by MO-Theory.

Types of overlapping

The two types of overlapping are possible:

  1. Axial overlapping (head-on overlapping)
  2. Parallel overlapping (sidewise overlapping)

Axial overlapping (Head-on overlapping)

This type of overlapping formed a sigma bond.

  • s-s overlapping (formation of H2 molecule)
  • s-p overlapping (formation of HF molecule)
  • p-p overlapping (formation of F2 molecule)
  • -pπ overlapping (formation of SO2 molecule)

Parallel overlapping (sidewise overlapping)

Side by side and parallel overlap of unhybridized molecular orbitals create pi bonds.

Bond Order

The difference between the bonding and antibonding electrons divided by two gives the bond order.

Bond order 1 → represents a single bond (sigma bond).

Bond order 2 → represents a double bond (sigma and pi bond).

Key Takeaway(s)

Sigma vs Pi bond: The identifications and main differences

Related Resources:

ConceptS berg

What is the number of sigma and pi bonds in a benzene molecule?

Benzene is an aromatic compound having a molecular formula C6H6. The number of σ (sigma) and π (pi) bonds in benzene are 12 and 3 respectively. [C-C = 6], [C-H = 6], and [C=C = 3].

What are the possible combinations of orbitals in sigma bonds?

There are the following combinations of orbitals that are possible in sigma bonds:

  • S-S overlapping

In the S-S type of overlapping s-orbital of one atom overlaps with the s-orbital of another atom. S-orbital can never form pi-bond. For example, H-H

  • S-P overlapping

In this type of overlapping s-orbital of one atom overlaps with the p-orbital of another atom resulting in the formation of a sigma bond. For example, H-F, H-Cl, etc. 

  • P-P overlapping

In this type, the p-orbital of one atom overlaps axially (head-on) with the p-orbital of another atom resulting in the formation of a sigma bond. For example, F-F, Cl-Cl, O-O, etc.

  • There also exists an opportunity for pπ-pπ bonding.

How many pi bonds exist in double and triple bonds?

The first bond between two atoms is always a sigma bond. The remaining bonds will be pi bonds. A double bond contains one and a triple bond contains two pi bonds along with a sigma bond.

What is a sigma bond example?

A sigma bond is formed by the axial (head-on) overlapping between two atomic orbitals. It is the strongest type of bond due to a greater extent of overlapping. It is formed by the combination of s-s, s-p, and p-p orbitals.

Is the sigma bond stronger than pi?

The stability of a bond depends upon the extent of overlapping. The greater the overlapping there is, the stronger the bond will be. The reason is an increase in the electron density.

The sigma bond is stronger than the pi bond due to greater overlapping and shorter bond length.

How many pi bonds are in azidothymidine?

There are 28 single and 5 double bonds in azidothymidine. A double bond contains one sigma and one pi bond, which is why azidothymidine contains 5 pi bonds.

How many sigma and pi bonds are there in CH4?

There are four sigma and zero pi bonds in the methane molecule. It has a tetrahedral shape and all carbons are sp3 hybridized.

Do ionic compounds form sigma and pi bonds?

Sigma and pi bonds are formed due to the overlapping of atomic orbitals. It means that sharing of electrons occurs between two atoms. So, in fact, covalent bonds are formed. Ionic compounds are formed due to the complete transfer of electrons, which is why ionic compounds apparently do not form sigma and pi bonds.

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