Cyanide Ion (CN-): Lewis Structure, Molecular Geometry, Hybridization, MOT and Polarity

The cyanide ion (CN-) is the anion of the cyanide (CN) molecule. Cyanide is a fatal bioterrorist chemical that stops cellular respiration by blocking cytochrome c oxidase, causing cardiac failure, hypoxic brain damage, and immediate death.

However, even after receiving different cytochrome oxidase protection antidotes, cyanide overdose in humans can result in a delayed-onset neurological condition that includes Parkinsonism signs. Now let’s come to know the chemistry of cyanide ion (CN-).

This blog post clears your concepts about the Lewis structure of cyanide ion (CN-), molecular geometry, hybridization, and the molecular orbital theory of cyanide ion (CN-).

General Rules to Draw Lewis Structure

To draw a Lewis structure of a molecule, there are a few general steps to follow. We’ll look at each stage of drawing the Lewis structure of CN- now.

• Calculate the total number of electrons in the valence shells of the Carbon and Nitrogen atoms
• Total electron pairs exist as lone pairs and bonds
• Choosing the center atom
• Draw lone pairs
• Complete the octet of each atom
• To acquire the optimum Lewis structure, calculate the formal charge and check the stability

Before going towards the Lewis structure of CN-, let us understand some general terms used in it.

Valence Electron

Valence electrons are the total number of electrons present in the atom’s outermost shell. Only valence shell electrons involve in bond formation. These valence electrons either share or are transferred (completely or partially) for bond formation.

                                                         OR

Valance electrons are electrons in the outermost shell that are vital for bonding and forming molecules.

Formal Charge (FC)

The difference between the valence electron of each atom and the number of bonding or non-bonding electron is known as a formal charge.

The formula for determination of formal charge is

FC = V-N-B/2

Here;

V = no. of valence electrons

N = no. of nonbonded electrons

B = no. of bonded electrons

The Lewis diagram of any molecule will be more stable whenever the formal charge of an atom is lower.

Octet Rule

The Octet Rule states that every atom likes to have eight valence electrons because this is the most stable and low-energy state.

Draw Lewis Structure of Cyanide Ion (CN^–) in 6 Easy Steps

The Lewis structure merely takes into account the electrons present in the valence shell, neglecting the inner shell’s electrons. A dot represents a lone pair of electrons in a Lewis structure, whereas a straight line indicates a bond pair of electrons.

Step # 1

Determine the Total Number of Valence Electrons

Cyanide ion (CN-) is made of two elements: Carbon and Nitrogen. Carbon belongs to group IVA element with four electrons in its outermost shell while Nitrogen is in-group VA of the modern periodic table with five electrons in its outermost shell.

To calculate the total outermost electrons of a particular element, multiply the number of atoms present in a particular element by its valence shell electrons.

Now let us calculate the Valence Shell Electron of CN^–.

• For Carbon

As carbon is an IVA element of the periodic table

Therefore, the valence electrons in the outermost shell of carbon are four

Mathematical representation

Valence electron (4)* no. of atom (1) = 4

• For Nitrogen

As nitrogen is a VA element of the modern periodic table

Therefore, nitrogen has five valence electrons.

Mathematical representation

 Valence electron (5) * no. of atoms (1) =5

As CN- has a negative charge on it, we conclude one more electron in total.

Hence, the total number of valence electrons present in the CN- = 4+5+1= 10

Step # 2

Calculate the Valence Electrons Pair for the CN^–

The total electron pair of a molecule is the sum of sigma, pi, and lone pair available in the valence shell of the molecule.

Mathematical representation

Total electron pair= sigma bond+ pi bond + lone pair of valence electrons

To get the total electron pair to divide the number of valence electrons by two (2).

However, the total electron pair of cyanide ion (CN-) will be 10/2 = 5

Step # 3

Identify the Central Atom

Being the central atom in a molecule comes with its own set of requirements. The capacity of an atom to have more valence electrons is essential for it to be central.

More importantly, while determining which atom should be the centre atom, the number of atoms, as well as their electronegativity must consider.

There is no need to worry about selecting the centre atom in Cyanide ion (CN-) because there are only two atoms.

 Step # 4

Mark Lone Pair on Each Atom of CN-

After finding the central atom and Sketch the C-N, we may begin to mark lone pairs on atoms.

It’s important to remember that there is a total of five lone pairs. Out of five lone pairs, one is already marked as C-N. Therefore, we have only four electron pairs to mark as lone pairs on the C-N bond.

 Mark three lone pairs on the Nitrogen atom because the lone pair is firstly marked on the outside atom. The nitrogen atom will accept three lone pairs as outside atoms. One leftover lone pair is marked on the carbon atom.

Step#5

Complete the Octet of Each C and  N

To gain a stable Lewis structure, atoms of the molecule must fulfil their octet. (Means both C and N have eight electrons)

In the above structure, an octet of carbon is not completed. Therefore, transfer the lone pair of nitrogen into a double bond as below.

                 

As the above diagram shows, the carbon atom is not happy yet. So, to make carbon happy, transfer one more lone pair from nitrogen to carbon.                         

                               

Now, both carbon and nitrogen are happy to have 8 electrons.

As a result, a triple bond forms between carbon and nitrogen, and both have a complete octet.

Step # 6

Calculate the Formal Charge on Each Atom to Check the Stability

Let’s move towards the final but most important step. Look at how formal charge is calculated and what it signifies.

The following is the equation for calculating an atom’s formal charge:

FC (formal charge) = V (No. of valence electrons) – N (No. of non-bonded electrons) – B (No. of bonded electrons) /2

Now let’s determine the Formal charge for the Lewis structure of Cyanide ion (CN-)

Formal charge on C= (4)-(2) – (6)/2 = -1

Formal charge on N = (5) – (2) – (6)/2= 0

As we already know the smaller the formal charge, the more stable a molecule’s Lewis structure. The below structure is the best Lewis structure for the CN-since the Cyanide ion (CN-) has a -1 formal charge on it.

Why it’s Important to Know About Lewis Structures

Lewis structures are important in predicting the geometry, polarity, and reactivity of compounds (organic & inorganic).  Knowing an atom’s Lewis structure helps you to predict how it will bond, and how many bonds it will make.

This information will ultimately help us to understand the structure of molecules as well as their chemical characteristics.

Molecular Geometry & Shape of CN^–

The three-dimensional (3D) arrangement of atoms linked together to form a molecule is called molecular geometry.

The molecular geometry of any molecule is essential because it offers information on a variety of physical and chemical properties, such as polarity, phase of matter, reactivity, color, biological activities, magnetism, and so on.

To learn more about molecular geometry 

Cyanide ion (CN-) has a triple bond between the atoms of carbon, and nitrogen and one lone pair of electrons on each atom. Because it only has two atoms, it has a linear molecular geometry. When carbon and nitrogen combine, one sigma bond and two pi bonds are, create.

The Bond Angle of  CN^–

The bond angle is the angle formed by the central atom and the bounded atoms.  The number of bonded atoms determines the Bond angle.  A larger bond angle would be the result of less repulsion.

Since CN- only has two atoms, it has linear geometry. The bond angle of cyanide ion (CN-) will be 180 degrees.

 The Polarity of CN^– (Is CN^– Polar or Non-Polar)?

Cyanide ion (CN-) has a polar nature.

A polar molecule has uneven electronegativity among its constituent atoms. This causes an electric charge imbalance across the molecule, giving it a net dipole moment.

 A polar covalent bond is established. If the electronegativity difference between the atoms is 0.5–2.

The following are the factors that determine the polarity of cyanide ion (CN-).

• Electronegativity of CN-

Electronegativity describes how strongly an atom attracts electrons in a chemical bond. The electronegativity difference between two atoms determines the chemical bond.

• If the electronegativity difference is greater than 2.1, the bond will be ionic.
• When the electronegativity difference is 0.5 to 2.1, the bond will be polar covalent.
• If the electronegativity difference between two atoms is less than 0.5, the bond will be non-polar.

 The greater value of electronegativity makes the covalent bond polar. In CN_, electronegativity values for carbon and nitrogen are (2.55), and (3.04) respectively. The differences in electronegativity between both atoms (2.55-3.04 =0.49) make the C-N bond polar.

• Dipole Moment

A dipole moment is a measurement of electrical charge separation. The dipole moment is large for the bonded atom which has a more electronegativity difference.

CN- is a polar molecule because it has a net dipole moment which is 0.22D

• Molecular Geometry

The shape of any molecule is a vital factor in depicting the polarity of the molecule. Because CN only has two atoms, it has a linear molecular geometry.

Hence, we could say that CN has polar nature.

Hybridization of CN^–

 Hybridization is a process in which two atomic orbitals from the same atom that has slightly different energies combine, redistributing energy between them, and creating new orbitals with the same energies and shape. The new orbitals produced by this process are hybrid orbitals.

From the Lewis structure, we conclude that CN- contains triple bonds one is sigma, and two are pi bonds. pi bonds made by side-to-side overlap and do not play a role in the hybridization of cyanide ion (CN-)

The hybridization of any atom in a molecule can calculate by finding the hybrid orbitals for a particular atom. Carbon atom link to 1 atom with a lone pair. However, two hybrid orbitals are formed which show sp hybridization.

The electronic valence shell arrangement of the carbon and nitrogen atom is shown in the diagram below.

Now let’s summarize what you learn up till now.          

Name of anion	Cyanide ion
Formula	CN–
Valence shell electrons	10
CN- hybridization	Sp
Molecular geometry	Linear
Dipole moment	0.22D
Bond angle	180⁰
Formal charge	-1
CO Is polar or non-polar.	Polar

Molecular Orbital Diagram 

Molecular orbital theory tells you about the location of an electron within a molecule. Generally, two types of molecular orbitals are formed by combining atomic orbitals

1. Bonding molecular orbital (σ)
2. Non-bonding molecular orbital ( σ*)

In bonding molecular orbitals, electrons spend their time between the nuclei while electrons are located away from nuclei in the anti-bonding molecular orbital. Bonding orbitals are low in energy as compared to anti-bonding orbitals.

Electrons first fill lower energy orbitals (bonding) and then go towards the higher energy orbital (anti-bonding).

Are you done with the basics of molecular orbital theory?

Now move toward the molecular orbital (MO) diagram. You can draw an MO diagram of CN- simply by following the following steps.

Steps to Draw MO diagram of CN^–  

• Calculate the valence electron of both carbon and nitrogen, which are 10.
• Determine whether the molecule has hetero atomic orbital or homo atomic orbital. Surely, cyanide ion (CN^–) has a hetero-atomic molecular orbital (as it contains two different atoms).
• Fill molecular orbital by taking into account its energy (i.e.; bonding molecular orbitals are kept low than anti-bonding orbitals).
• Now, draw the MO diagram for CN^– and fill the molecular orbital with electrons.

The molecular orbital diagram helps you to determine the various properties, which include its bond order, magnetism (Para -magnetic or diamagnetic), and bond length.       

Related Videos: 

Tetrahydro Borate (BH4-); Lewis Structure, Molecular Geometry, and Hybridization, Polarity

Phosphite Ion (Po3-3); How to Draw Lewis Structure, Molecular Geometry, and Hybridization

Bromate Ion (BrO3-); How to Draw Lewis Structure, Molecular Geometry, Hybridization, And MOT Diagram

               

Bond Order of CN^–

The bond order can calculate by the following formula.

Bond order = no. of bonding electrons – no. of anti-bonding electrons /2

Bond order = 10-4/2 = 3

Hence, the bond order of CN^– ion will be 3.

Uses 

• It has an important role in medicines
• Use to increase the germination of plants
• It is also used as a food additive in the food sector
• It’s also helpful in the ornamental industry (making jewelry)

Final Verdict

Lewis structure of  (CN^–) shows that cyanide ion has a triple bond between carbon and nitrogen with a lone pair. Due to the presence of only two atoms, they form a linear combination (linear molecular geometry).

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