Question

How many sigma and pi bonds are in the HCN molecule?

Answer 1

Hint: Hydrogen cyanide is an acid that has molecular formula $H-C\equiv N$. We know that when a pair of electrons are shared by two atoms, a covalent bond is formed. Sigma ($\sigma$) bonds and pi ($\pi$) bonds are two types of covalent bonds.

Complete step by step solution:
The strongest type of covalent chemical bond is a sigma ($\sigma$) bond. A sigma ($\sigma$) bond is formed when the atomic orbitals overlap head-on. Usually, sigma ($\sigma$) bonds are single bonds.

The sigma ($\sigma$) bonds are symmetrical and can rotate about the bond axis. Some of the most common sigma bonds are s+s, $p_z+p_z$, s+$p_z$, and $d_{z^2}+d_{z^2}$. Here, s, p, and d are atomic orbitals and z is the bond axis.
Now, the covalent chemical bonds which are formed when the atomic orbitals overlap laterally are known as pi ($\pi$) bonds. Usually, there is 1 pi ($\pi$) bond in double bonds and 2 pi ($\pi$) bonds in triple bonds.

Pi ($\pi$) bonds cannot rotate about the bond axis without breaking the bond.

Now, in HCN, we can see that there are two single bonds, H-C and C-N, hence it has two sigma ($\sigma$) bonds. Whereas it has a triple bond in $C\equiv N$ and hence has two pi ($\pi$) bonds.
So, the HCN molecule has 2 sigma ($\sigma$) bonds and 2 pi ($\pi$) bonds.

Additional Information: Some of the properties of HCN are
- It can be synthesized by combining ammonia and methane
$$2C{H}_4+2N{H}_3+3O_2\underset{\mathrm{\Delta }=1200{}^{\circ }C}{\overset{Pt}{\to }}2HCN+6H_{2}O$$
- It has an odor of bitter almond oil.
- It exists in both the gaseous as well as the liquid state.
- It has a melting point of 259.86 K and a boiling point of 299.00 K.
- It has a density of 0.69 g/l.
- It has a linear structure.
- It is extremely toxic as well as poisonous in nature and has been used as chemical weapons.
- It is colorless and is soluble and miscible in water and ethanol.

Note: It should be noted that even though pi ($\pi$) bonds are weaker than sigma ($\sigma$) bonds, but when atoms are bonded by both sigma ($\sigma$) bonds and pi ($\pi$) bonds, their strength is greater than either of the bonds alone. Hence the strength of multiple bonds (double and triple bonds) is more than that of a single bond.