Hydrogen cyanide (HCN) Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, formal charge, hybridization
HCN is the chemical formula for hydrogen cyanide, a colorless and extremely poisonous acid. It is also sometimes known as prussic acid. This chemical compound smells like bitter almond oil and can have devastating effects on the human eyes and respiratory tract if inhaled in bulk.
If you are keen to learn about the chemical nature and properties of hydrogen cyanide (HCN), then your problem is solved. In this article, we have discussed everything about the Lewis structure of HCN, its molecular geometry or shape, electron geometry, bond angles, hybridization, formal charges, etc.
So, without any further delay, let’s start reading!
The Lewis structure of HCN consists of a central carbon (C) atom. To this C-atom, an atom of hydrogen (H) is bonded on one side via a single covalent bond while a nitrogen (N) atom is bonded to the other side of the central C atom via a triple covalent bond. There is no lone pair present on the central atom in the lewis dot structure of HCN.
Let’s see how we can draw the Lewis dot structure of HCN using the simple steps given below.
Steps for drawing the Lewis dot structure of HCN
1. Count the total valence electrons in HCN
The Lewis dot structure of a molecule is referred to as a simplified representation of all the valence electrons present in it. Therefore, the very first step while drawing the Lewis structure of HCN is to count the total valence electrons present in the concerned elemental atoms.
The valence electrons present in different elemental atoms of a molecule can be readily determined by identifying those atoms in the Periodic Table. In the HCN molecule, three different elemental atoms are present i.e., a hydrogen (H) atom, a carbon (C) atom, and an atom of nitrogen (N).
When you’ll look through the Periodic Table of elements, you will instantly identify that carbon is present in Group IV A, so it has a total of 4 valence electrons. On the other hand, nitrogen is present in Group VA, so it has a total of 5 valence electrons while hydrogen lies at the top of the Periodic Table having a single valence electron only.
∴ The HCN molecule is made up of 1 C atom, 1 N atom, and 1 H atom so the total valence electrons available for drawing the Lewis dot structure of HCN = 4 + 5 + 1 = 10 valence electrons.
2. Find the least electronegative atom and place it at the center
The second step while drawing the Lewis structure of a molecule is to identify a central atom that is most likely to share its electrons with the atoms spread around it.
Electronegativity is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond. Therefore, the least electronegative atom is more likely to share its electrons rather than attract those of others.
However, hydrogen (H) is never considered a central atom in any molecule. It is always placed as an outer atom in the Lewis structure of a molecule because it can just form a single bond with its adjacent atom, accommodating a maximum of 2 valence electrons only.
Once the fate of the H-atom is decided, the remaining two atoms in the HCN Lewis structure are carbon (C) and nitrogen (N). Carbon (E.N = 2.55) is less electronegative than nitrogen (E.N = 3.04) thus it is more prone to share its electrons with other atoms. Hence C is placed at the center of the HCN Lewis structure while the H and N atoms are placed in its surroundings, as shown below.
3. Connect outer atoms with the central atom
Now, all the outer atoms are joined to the central atom using straight lines.
As we identified already, H and N atoms are the outer atoms in HCN while C is the central atom. Thus, the H and N atoms are joined to the central C atom using straight lines.
Each straight line represents a single covalent bond i.e., a bond pair containing 2 electrons. There are a total of 2 single bonds in the structure shown above which means a total of 2(2) = 4 valence electrons are used so far, out of those initially available.
Total valence electrons available – electrons used till step 3 = 10-4 = 6 valence electrons.
This shows we still have 6 valence electrons available to be accommodated in the Lewis structure of HCN.
4. Complete the octet of outer atoms
In this step, we complete the duplet and/or the octet of the outer atoms surrounding the central C atom in the HCN Lewis structure.
H and N atoms are the outer atoms in the Lewis structure of HCN. The hydrogen (H) atom requires a total of 2 valence electrons in order to achieve a stable duplet electronic configuration.
An H-C single bond represents that 2 electrons are already available around the H atom, so its duplet is complete. Consequently, there will be no lone pair on the H atom. But the situation is a bit different for the outer N atom.
A C-N single bond represents 2 valence electrons around the nitrogen atom. It is still short of 6 electrons in order to achieve a stable octet electronic configuration.
Thus, 6 electrons are placed as 3 lone pairs around the N atom in the Lewis structure of HCN, as shown below.
5. Complete the octet of the central atom by converting lone pairs from the outer atom into covalent bonds
Electrons used till step 4 = 2 single bonds + electrons placed around the N atom, shown as dots = 2(2) + 6 = 10 valence electrons
Total valence electrons available- electrons used till step 4 = 10-10 = 0 valence electrons.
All the valence electrons available for drawing the Lewis structure of HCN are already used so there is no lone pair on the central C atom.
But the question that stays is: Is the octet of this central atom complete? There are 2 single bonds around the central C atom in the above Lewis structure which means a total of 4 valence electrons. Thus, carbon still requires 4 more electrons to achieve a stable octet configuration.
A practical solution to this problem is to convert two lone pairs present on the outer N atom into covalent bonds. This leads to a triple covalent bond between the bonded C and N atoms, as shown in the figure below.
In the above Lewis structure, the central C atom, and the outer N atom both have a complete octet while the H atom has a complete duplet. Thus, a favorable position for all the atoms involved.
As a final step, we just need to check the stability of the Lewis structure obtained in step 5 and we can do so by using the formal charge concept.
6. Check the stability of HCN Lewis structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
What are the electron and molecular geometry of HCN?
The hydrogen cyanide (HCN) molecule has an identical electron and molecular geometry or shape i.e., linear. All the bonded atoms lie symmetrically in the same plane in a linear arrangement.
Molecular geometry of HCN
The molecular geometry or shape of the HCN molecule is linear. There is a single bond and a triple bond around the central carbon (C) atom in the molecule while there is no lone pair present on this central atom.
There are no lone pair-lone pair repulsions or lone pair-bond pair repulsions present in the molecule. This makes the molecule maintain a symmetrical linear shape.
The H, C, and N atoms lie on a straight line as shown in the figure below thus the name linear is given. This linear arrangement minimizes bond pair-bond pair repulsions between the bonded electron pairs.
The molecular geometry or shape of a molecule depends on the distinction between the bond pairs and lone pairs of electrons present in a molecule.
Contrarily, the ideal electronic geometry of a molecule is determined by the total electron density regions surrounding the central atom in the molecule.
Electron geometry of HCN
The ideal electronic geometry of a molecule having two regions of electron density around the central atom in the molecule is linear. In the HCN molecule, the triple bond between the bonded C and N atoms is considered 1 region of electron density.
Similarly, the single covalent bond between the bonded H and C atoms is another separate region of electron density.
In short, there are a total of 2 regions of electron density around the central C atom in the HCN molecule. Thus, the ideal electron geometry of HCN is linear which is identical to its molecular geometry or shape.
A quick and more straightforward way of finding the electron and molecular geometry or shape of a molecule such as HCN is using the AXN method.
AXN is a simple formula to represent the atoms bonded to the central atom of a molecule and the number of lone pairs present on this central atom.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for the HCN molecule
The symbol A in the AXN formula represents the central atom present in a molecule. In the HCN molecule, carbon (C) acts as the central atom so A=C.
X denotes the atoms bonded to the central atom. As 1 nitrogen and 1 hydrogen atom are bonded to the central C atom in the HCN molecule so for HCN, X=2.
N denotes the number of lone pairs present on the central atom in the molecule. As there is no lone pair on the central C atom in the HCN molecule thus for HCN, N=0.
This shows that the AXN generic formula for the HCN molecule is AX2.
Now, have a quick look at the VSEPR chart given below and identify what electron and molecular geometries are assigned against the AX2 formula.
The VSEPR chart confirms that molecules having an AX2 generic formula have an identical electron and molecular geometry or shape i.e., linear as we already noted down for the HCN molecule.
Hybridization of HCN
The bonded C and N atoms in the HCN molecule are sp hybridized.
The electronic configuration of carbon (C) is 1s2 2s2 2p2. During chemical bonding, the two 2s electrons of carbon get unpaired. One of the two 2s electrons shifts to the empty 2p orbital. This results in an excited state electronic configuration of 1s2 2s1 2px1 2py1 2pz1.
Consequently, the 2s orbital mixes with one 2p orbital to yield two equivalent sp hybrid orbitals, each containing a single electron only. Each sp hybrid orbital has a 50% s character and a 50% p character.
The electronic configuration of nitrogen (N) is 1s2 2s2 2p3. During chemical bond formation, the 2s orbital of nitrogen hybridizes with one of the three 2p atomic orbitals to yield two sp hybrid orbitals.
One of the two sp hybrid orbitals contains paired electrons which are situated as a lone pair on the N atom in the HCN molecule. The other sp hybrid orbital contains a single electron only thus it is used to form a sigma (σ) bond with the sp hybrid orbital of carbon by sp-sp overlap.
The second sp hybrid orbital of carbon forms another sigma (σ) bond with the s orbital of H via sp-s overlap. Conversely, the unhybridized p orbitals of carbon and nitrogen atoms form two pi (π) bonds. Refer to the figure given below.
A short trick for finding the hybridization present in a molecule is memorizing the table given below. The steric number of a molecule can be used against this table to determine the required hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
There are 2 regions of electron density around the central C atom in the HCN molecule, so its steric number is 2. Thus, it has sp hybridization.
The HCN bond angle
Linear is an ideal shape and molecular geometry. There is no interference of lone pairs in the shape and geometry of the molecule. Such structures are informally called zero-dimensional.
The three bonded atoms are symmetrically arranged in a straight line in the HCN molecule, forming a mutual bond angle of 180°.
In the HCN molecule, the H-C bond length is 109 pm while the C-N bond length is 116 pm respectively.
Nitrogen (N) is more electronegative than carbon (C). There exists an electronegativity difference of 0.49 units between the bonded C (E.N =2.55) and N (E.N =3.04) atoms in the HCN molecule. Thus, the CN bond is polar with a specific dipole moment value (symbol μ).
A relatively small electronegativity difference exists between the bonded C (E.N =2.55) and H (E.N=2.20) atoms in the H-C bond. Thus, the H-C bond is only weakly polar.
Nitrogen, however, due to its high electronegativity attracts the shared electron cloud from the CH bond in addition to attracting the CN electron cloud.
Consequently, oppositely charged poles are developed in the molecule which means the hydrogen cyanide (HCN) molecule is overall polar with a net dipole moment μ=2.98 Debye (D).
The Lewis structure of HCN consists of three different atoms i.e., C, H, and N atoms. The three atoms lie in the same plane, in a linear arrangement. The carbon (C) atom is present at the center of the Lewis structure.
It is bonded via a single covalent bond to the hydrogen (H) atom on the left side. On the right side, the central C atom is bonded to an atom of nitrogen (N) via a triple covalent bond. A lone pair of electrons is also present on the outer N atom, as shown below.
How many bond pairs and lone pairs are there in the Lewis structure of HCN?
Technically there are 4 bond pairs and 1 lone pair in the Lewis structure of HCN.
Three bond pairs lie as 1 sigma and 2 pi bonds between the triple bonded C and N atoms while there is a shared bond pair between the single bonded C and H atoms.
There is no lone pair on the central C-atom while 1 lone pair of electrons is present on the N atom.
The three bond pairs between C and N atoms are considered one region of electron density while assigning shape and geometry to the HCN molecule.
What is the molecular shape and bond angle of HCN?
According to the VSEPR concept, HCN is an AX2-type molecule. Thus, it has an identical electron geometry and molecular shape i.e., linear. The H-C-N atoms lie on a straight line in a linear arrangement and form a mutual bond angle of 180°
Why is the shape of HCN linear while that of H2O angular?
Hydrogen cyanide (HCN) is a linear molecule as all the bonded atoms lie symmetrically, in the same plane and there is no lone pair on the central C atom.
Lone pair-lone pair repulsions and lone pair-bond pair repulsions disturb the symmetry of the three bonded atoms. The bond angle decreases, and the molecule adopts an asymmetric angular shape.
The total valence electrons available for drawing the HCN Lewis structure are 10.
The HCN molecule has an identical electron and molecular geometry or shape i.e., linear.
The C and N atoms present in the HCN molecule are sp hybridized.
The HCN atoms form a mutual bond angle of 180° due to the molecule’s linear shape.
The HC bond length is 109 pm while the CN bond length is 116 pm in the HCN molecule.
Due to the high electronegativity of the nitrogen (N) atom, the molecule overall has a non-uniformly distributed electron cloud thus it is polar in nature (net μ=2.98 D).
The absence of any formal charges on the bonded atoms in the HCN molecule ensures the extraordinary stability of its Lewis dot structure.
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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