Number of π-electrons in Cyclobutadienyl ion (C4H2)2-

To determine the number of π-electrons in the cyclobutadienyl ion (C4H2)2-, we need to consider the molecular structure and the valence electrons of its constituent atoms.

Molecular Structure:
The cyclobutadienyl ion consists of a cyclic structure with four carbon atoms and two hydrogen atoms. Each carbon atom is connected to one hydrogen atom, forming a double bond between them.

Valence Electrons:
Carbon has four valence electrons, and hydrogen has one valence electron.

Counting π-electrons:
In order to count the π-electrons, we need to consider the number of pi bonds present in the molecule.

Step 1: Count the total number of valence electrons in the molecule.
- Carbon contributes 4 valence electrons (4 carbon atoms x 4 valence electrons = 16 electrons).
- Hydrogen contributes 1 valence electron (2 hydrogen atoms x 1 valence electron = 2 electrons).

Total valence electrons = 16 + 2 = 18 electrons.

Step 2: Determine the number of σ-electrons.
- Each carbon atom is connected to one hydrogen atom, forming a sigma (σ) bond. This accounts for 4 sigma bonds.

Total σ-electrons = 4 x 2 = 8 electrons.

Step 3: Calculate the number of π-electrons.
- The remaining electrons after accounting for σ-bonds are attributed to π-electrons.
- In this case, there are 18 - 8 = 10 electrons left for π-electrons.

However, we need to consider the aromaticity of the cyclobutadienyl ion.

Aromaticity:
A compound is considered aromatic if it satisfies the following criteria:
1. The molecule is planar.
2. The molecule is cyclic.
3. The molecule is fully conjugated.
4. The molecule has 4n + 2 π-electrons, where n is an integer.

In the case of the cyclobutadienyl ion, it satisfies the first three criteria but does not have 4n + 2 π-electrons. Instead, it has 4n π-electrons, which makes it antiaromatic.

Antiaromaticity:
Antiaromatic compounds are less stable than non-aromatic compounds. To reduce the antiaromaticity, the cyclobutadienyl ion undergoes a process known as Huckel's rule.

According to Huckel's rule, an antiaromatic compound can become aromatic by removing two electrons from the π-system. This results in a cyclic, planar system with 4n π-electrons, making it aromatic.

In the case of the cyclobutadienyl ion, two electrons are removed from the π-system, resulting in a total of 10 - 2 = 8 π-electrons.

Answer:
Therefore, the number of π-electrons present in the cyclobutadienyl ion (C4H2)2- is 8.

It has 6 π-electrons.