How to calculate formal charges of Hydrocyanic acid (HCN) with lewis structure?
In covalently bonded molecules, formal charge is the charge assigned to an atom based on the assumption that the bonded electrons are equally shared between concerning atoms, regardless of their electronegativity.
The overall formal charge present on a molecule is a measure of its stability.
The fewer the formal charges present on the bonded atoms in a molecule (close to zero), the greater the stability of its Lewis structure.
Hydrogen Cyanide (HCN) is a colorless-to-bluish volatile liquid that rapidly evaporates into a gas and has the potential to cause immediate fatal toxicity upon inhalation.
In this article, we will calculate the formal charges present on the bonded atoms in the HCN Lewis structure and also the overall charge present on the molecule.
The formal charges can be calculated using the formula given below:
The formal charge of an atom = [valence electrons of an atom – non-bonding electrons – ½ (bonding electrons)]
The valence electrons (V.E) of an atom are the total number of electrons present in its valence shell. Valence electrons can be determined by locating the position of the elemental atom in the Periodic Table.
Non-bonding electrons(N.E) are the number of lone pairs present on the atom. (1 lone pair means 2 nonbonding electrons).
Bonding electrons (B.E) are the total electrons shared with the atom via covalent chemical bonds. (1 single bond means 2 bonding electrons).
Now let us use this formula to calculate the formal charges in the most preferred Lewis structure of Hydrogen Cyanide (HCN).
The most preferred Lewis representation of HCN is as shown below.
It consists of a total of 10 valence electrons. A Carbon (C) atom is present at the center. It is bonded to one atom of Hydrogen (H) via a single covalent bond on one side, and on the other side, the central C-atom is bonded to an atom of Nitrogen (N) via a triple covalent bond.
No lone pair of electrons is present at the central C-atom and outer H-atom, while the N-atom carries a single electron pair in the HCN Lewis structure, respectively.
It is the best possible Lewis structure of HCN because the formal charges are minimized in it, and thus, it is the most stable.
Let’s find out how we can determine the formal charges present on each atom in the HCN Lewis structure.
Formal charge on the Nitrogen atom = 5 – 2 – 6/2 = 5 – 2 – 3 = 5 – 5 = 0
∴ The formal charge on the N-atom in HCN is 0.
For Hydrogen atom
Valence electrons of Hydrogen = It is present in Group I A = 1 valence electrons
Bonding electrons around Hydrogen = 1 single bond = 2 electrons
Non-bonding electrons on Hydrogen = no lone pair = 0 electrons
Formal charge on the Hydrogen atom = 1 – 0 – 2/2 = 1 – 0 – 1 =1 – 1 =0
∴ The formal charge on the H-atom in HCN is 0.
This calculation shows zero formal charges are present on all three bonded atoms in the HCN Lewis structure, i.e., the central C-atom, the outer N-atom, and the H-atom, as shown below.
As a result, there is no overall charge on the HCN Lewis structure, and thus it is a neutral molecule.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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