Animated Solution for Chemistry - Chemical Bonding and Molecular Structure: Among ${\text{B}}_2{\text{H}}_6$, ${\text{B}}_3{\text{N}}_3{\text{H}}_6$, ${\text{N}}_2\text{O}$, ${\text{N}}_2{\text{O}}_4$, ${\text{H}}_2{\text{S}}_2{\text{O}}_3$ and ${\text{H}}_2{\text{S}}_2{\text{O}}_8$, the total number of molecules containing covalent bond between two atoms of the same kind is ______.

The world of chemical bonding is full of fascinating structures, and sometimes, the chemical formula alone can be quite deceptive. In this problem, we are on a mission to find molecules that contain a covalent bond between two atoms of the exact same element—what we call a homoatomic bond.

We are given six intriguing molecules: ${\text{B}}_2{\text{H}}_6$, ${\text{B}}_3{\text{N}}_3{\text{H}}_6$, ${\text{N}}_2\text{O}$, ${\text{N}}_2{\text{O}}_4$, ${\text{H}}_2{\text{S}}_2{\text{O}}_3$, and ${\text{H}}_2{\text{S}}_2{\text{O}}_8$. To solve this, we cannot just guess; we must draw their Lewis structures and analyze their connectivity.

Let's start with diborane, ${\text{B}}_2{\text{H}}_6$. Seeing the ${\text{B}}_2$ in the formula, it is incredibly tempting to assume there is a direct $\text{B-B}$ bond. However, diborane is an electron-deficient molecule.

Instead of a direct bond, the two boron atoms are held together by two bridging hydrogen atoms. These form unique $3c-2e$ (three-center two-electron) bonds, often called banana bonds. Because the borons are connected via hydrogens, there is no direct $\text{B-B}$ covalent bond.

Next up is borazine, ${\text{B}}_3{\text{N}}_3{\text{H}}_6$. This molecule is famously known as "inorganic benzene" because it forms a planar, hexagonal ring.

In this ring, the boron and nitrogen atoms strictly alternate. Each boron is bonded to two nitrogens, and each nitrogen is bonded to two borons. Despite the delocalization of $\pi$ electrons, the connectivity remains strictly $\text{B-N}$. Therefore, there are no $\text{B-B}$ or $\text{N-N}$ bonds in borazine.

Now, let's examine nitrous oxide, ${\text{N}}_2\text{O}$. A common mistake is to place oxygen in the center. However, nitrogen is less electronegative, making it the central atom.

The connectivity is linear: $\text{N}\equiv \text{N}\to \text{O}$. As we can clearly see, there is a triple bond directly connecting the two nitrogen atoms. This gives us our first molecule with a homoatomic bond!

Moving on to dinitrogen tetroxide, ${\text{N}}_2{\text{O}}_4$. This molecule is formed when two ${\text{NO}}_2$ radicals dimerize.

During dimerization, the unpaired electron on the nitrogen of each ${\text{NO}}_2$ molecule pairs up. This forms a direct $\text{N-N}$ single bond connecting the two ${\text{NO}}_2$ units. Thus, ${\text{N}}_2{\text{O}}_4$ also contains a bond between identical atoms. That is our second molecule.

Next is thiosulfuric acid, ${\text{H}}_2{\text{S}}_2{\text{O}}_3$. The nomenclature here is a massive clue. The prefix "thio-" indicates that an oxygen atom in a standard molecule (in this case, sulfuric acid, ${\text{H}}_2{\text{SO}}_4$) has been replaced by a sulfur atom.

Structurally, the central sulfur atom is double-bonded to a terminal sulfur atom ($\text{S=S}$), alongside its bonds to oxygen and hydroxyl groups. This direct sulfur-sulfur bond makes it our third valid molecule.

Finally, we look at ${\text{H}}_2{\text{S}}_2{\text{O}}_8$, known as Marshall's acid or peroxydisulfuric acid. Once again, the name reveals the structure.

The prefix "peroxy-" explicitly tells us that the molecule contains a peroxide linkage. Structurally, two sulfate-like groups are bridged by a direct oxygen-oxygen single bond ($\text{-O-O-}$). This is exactly what we are looking for, giving us our fourth molecule.

After carefully drawing and analyzing all six structures, we found that four of them contain a covalent bond between two atoms of the same kind: 1. ${\text{N}}_2\text{O}$ (contains an $\text{N-N}$ bond) 2. ${\text{N}}_2{\text{O}}_4$ (contains an $\text{N-N}$ bond) 3. ${\text{H}}_2{\text{S}}_2{\text{O}}_3$ (contains an $\text{S-S}$ bond) 4. ${\text{H}}_2{\text{S}}_2{\text{O}}_8$ (contains an $\text{O-O}$ bond)

The total number of such molecules is 4.