Question

Why does the structure H-C=C-H fail to make sense chemically?

Short answer

The H-C=C-H does not make sense chemically because each carbon atom has three covalent bonds in this structure instead of the required four.

Step-by-step solution

Understanding the electron structure of carbon and hydrogen

The atomic number of carbon is six, and its electronic configuration is 1s², 2s², 2p⁴. Thus, it has four electrons in its valence shell and requires four more electrons to complete its octet.

On the other hand, the atomic number of hydrogen is one, and its electronic configuration is 1s¹. This means that it has one valence shell electron and requires one more to complete octet.

Formation of covalent bond

The bridge or bond formed between two atoms through the mutual sharing of electrons is known as a covalent bond. Covalent bonding is the attraction or repulsion between two atoms when they share an electron or bonding pair.

Understanding the structure 

There are four valence electrons in carbon. As a result, the element carbon requires four additional shared electrons from an atom to remain stable.

The carbon in H-C=C-H is chemically connected with one other carbon, forming two bonds and one with hydrogen, forming just three bonds. Thus, both carbons have just three bonds, leaving one valence electron available.

Thus, the middle bond should be triple, not double in this structure, in order to be chemically stable.