Question

Trans-tetrazene $\left({\mathrm{N}}_4{\mathrm{H}}_4\right)$consists of a chain of four nitrogen atoms with each of the two end atoms bonded to two hydrogen atoms. Use the concepts of steric number and hybridization to predict the overall geometry of the molecule. Give the expected structure of cis-tetrazenes.

Short answer

The expected structure is:

Step-by-step solution

The Lewis structure of trans-tetrazene

In the skeleton structure, the hydrogen atom has a stable noble gas configuration. On the other hand, each nitrogen atom needs two extra electrons to complete its octet.

The electrons will be placed as lone pairs on each nitrogen atom. The hybridization of central atoms is determined by steric number.

Lewis structure of trans-tetrazene

The formula to calculate the steric number of an N atom

The formula to calculate the the steric number of N atom is as follows:

$\left(\text{SN}\right)=\left[(numberofatomsbondedtoNatom)+(numberoflonepairsonNatom)\right]$

• The number of lone pairs on N atom is 1.
• The number of atoms bonded to N atom is 3.

Substitute the values in the above formula.

The given values are substituted in equation (i).

$$\begin{gathered} \text{(SN) =(numberofatomsbondedtoNatom)+(numberoflonepairsonNatom)} \\ \text{= 3 + 1} \\ \text{= 4} \end{gathered}$$

End N atoms in ${\text{N}}_4{\text{H}}_4$has steric number 4. Therefore, end atoms have $s{p}^3$hybridization, with one lone pair. Also, $s{p}^3$hybridization with 1 lone pair has trigonal pyramidal geometry. Hence, geometry at end nitrogen atoms is trigonal pyramidal.

The formula to calculate steric number of N atom

• The number of lone pairs on N atom is 1.
• The number of atoms bonded to N atom is 2.

The formula to calculate the the steric number of N atom is as follows:

$$\begin{gathered} \text{(SN) =(numberofatomsbondedtoNatom)+(numberoflonepairsonNatom)} \\ \text{=2+1} \\ \text{=3} \end{gathered}$$

Each nitrogen atom in the centre has a steric number 3. Therefore, central atoms have$s{p}^2$ hybridization. This shows that the two nitrogen atoms are connected by a double bond.

Expected Structure

The term ‘cis’ refers to same side of the plane. Thus, hydrogen atoms bonded to nitrogen should be placed on the same side. The expected structure is:

Structure of cis-tetrazene