Difference Between Bond Pair and Lone Pair

Main Difference – Bond Pair vs Lone Pair

Every elements have electrons in their atoms. These electrons are in shells that are located outside the nucleus. One shell can have one or more orbitals. The orbitals that are closest to the nucleus are s, p and d orbital. An orbital can be divided into several sub-orbitals. One sub-orbital can hold a maximum of two electrons. When there are no electrons, it is called an empty orbital. When there is one electron in a sub-orbital, it is called an unpaired electron. When the sub-orbital is filled with a maximum of two electrons, it is called an electron pair. The electron pairs can be found in two types as bond pair and lone pair. The main difference between bond pair and lone pair is that bond pair is composed of two electrons that are in a bond whereas lone pair is composed of two electrons that are not in a bond.  

Key Areas Covered

1. What is a Bond Pair
      – Definition, Identification, Examples
2. What is a Lone Pair
      – Definition, Identification, Examples
3. What is the Difference Between Bond Pair and Lone Pair
      – Comparison of Key Differences

Key Terms: Bond Pair, Covalent Bond, Double Bond, Lone Pair, Non-bonding Electron Pair, Orbital, pi Bond, Sigma Bond, Single Bond, Unpaired Electrons, Valence Electrons

What is a Bond Pair

A bond pair is a pair of electrons that are in a bond. A single bond is always composed of two electrons that are paired with each other. These two electrons together are called the bond pair. Bond pairs can be seen in covalent compounds and coordination compounds. In covalent compounds, the covalent bond is composed of a bond pair. In coordination compounds, the coordination bond is composed of a bond pair.

In coordination compounds, the ligands donate their lone electron pairs to a central metal atom. Although they were lone pairs they form coordination bonds that are similar to covalent bond after the donation; hence they are considered as a bond pair. This is because the two electrons are being shared between two atoms.

In covalent compounds, two atoms share their unpaired electrons to make them paired. This pair of electrons is called the bond pair. When there are double or triple bonds, there are bond pairs per each bond. For example, if there is a double bond, there are two bond pairs. Since a covalent bond is formed through the hybridization of orbitals of two atoms, a bond pair resides in hybridized orbitals. These hybridized orbitals can form either sigma bonds or pi bonds. Therefore bond pairs can be observed in either sigma bonds or pi bonds.  

Figure 1: The coordination bond between NH3 and BF3

In the above example, the electron pair on the N atom of NH3 molecule is donated to the B atom of BF3 molecule. Thereafter, the coordination bond looks like a covalent bond. Therefore, the electron pair is now a bond pair.  

What is a Lone Pair

Lone pair is a pair of electrons that are not in a bond. The electrons of the lone pair belong to the same atom. Therefore, a lone pair is also called a non-bonding electron pair. Although electrons in the innermost shells are also coupled and do not participate in the bonding, they are not considered as lone pairs. The valence electrons of an atom that are coupled with each other are considered as lone pairs.

Sometimes these lone pairs can be donated to another atom which has empty orbitals. Then it forms a coordination bond. Thereafter, it is not considered as a lone pair since it becomes a bond pair. Some elements have only one lone pair. Some other elements have more than one lone pair. For example, Nitrogen (N) can form a maximum of three covalent bonds. But the number of valence electrons it has is 5. Therefore, three electrons are shared with other atoms to form bonds whereas other two electrons remain as a lone pair. But halogens have 7 electrons in their outermost orbital. Therefore, they have 3 lone pairs along with one unpaired electron. Therefore, halogens can have one covalent bond by sharing this one unpaired electron.

Lone pairs change the angle of bonds in a molecule. For example, consider a linear molecule composed of a central atom having two bonds. If there are no lone pairs, the molecule will remain as a linear molecule. But if there are one or more lone pairs on the central atom, the molecule would no longer be linear. Due to the repulsion caused by lone pairs, the bond pairs are repelled. Then the molecule become angular instead of linear.

As shown in the above image, ammonia has one lone pair, water molecule has 2 lone pairs and HCl has 3 lone pairs.

If an atom has empty orbitals, the lone pairs can be split into unpaired electrons through hybridization of orbitals and can participate in bonding. But if there are no empty orbitals, lone pairs will remain as a pair of electrons and not participate in bonding.

For example, nitrogen (N) is composed of 5 electrons in the outermost orbital. Two electrons in 2s orbital and other three are in three p orbitals. Since nitrogen has no empty orbitals, the electron pair in 2s orbital will remain as a lone pair.

Figure 3: The orbital diagram of nitrogen (N)

But when considering phosphorous (P), it also has 5 electrons in the outermost orbital: 2 electrons in 3s orbital and other 3 electrons in three p orbitals. But, phosphorus can form maximum of 5 bonds. That is because it has empty 3d orbitals.

Figure 4: The orbital diagram for phosphorous and the possible hybridization

Phosphorous can have five bonds by including the 5 electrons in sp³d¹ hybridized orbitals. Then, there are no lone pairs on phosphorous.

Difference Between Bond Pair and Lone Pair

Definition

Bond Pair: Bond pair is a pair of electrons that are in a bond.

Lone Pair: Lone pair is a pair of electrons that are not in a bond.

Bonding

Bond Pair: Bond pairs are always in bonds.

Lone Pair: Lone pairs are not in bonds but can form bonds by donating the lone pair (coordination bonds).

Atoms

Bond Pair: The two electrons belong to two atoms in bond pairs.

Lone Pair: The two electrons belongs to the same atom in lone pairs.

Origin

Bond Pair: A bond pair is created due to sharing of electrons by two atoms.

Lone Pair: A lone pair is created due to absence of empty orbitals.

Conclusion

Bond pair and lone pair are two terms used to describe coupled electrons. These electron pairs cause the reactivity, polarity, physical state and chemical properties of compounds. Ionic compounds may or may not have bond pairs and lone pairs. Covalent compounds and coordination compounds essentially have bond pairs. They may or may not have lone pairs. The difference between bond pair and lone pair is that a bond pair is composed of two electrons that are in a bond whereas a lone pair is composed of two electrons that are not in a bond. 

References:

1.”Lone pair.” Wikipedia. Wikimedia Foundation, 09 July 2017. Web. Available here.  27 July 2017. 
2.”Definition of bonding pair – Chemistry Dictionary.” Chemistry-Dictionary.com. N.p., n.d. Web. Available here. 27 July 2017. 

Image Courtesy:

1. “NH3-BF3-adduct-bond-lengthening-2D-no-charges” By (สาธารณสมบัติ) via Commons Wikimedia
2. “ParSolitario” By V8rik at en.wikipedia – Transferred from en.wikipedia (Public Domain) via Commons Wikimedia