Carbon’s reactivity is driven by the quest for a filled outer shell. However, forming C⁴⁻ or C⁴⁺ ions is unfavorable due to the challenges associated with holding or removing four electrons, making such ion formation energetically impractical for carbon.

Carbon addresses the electron gaining or losing challenge by sharing valence electrons with other atoms, a strategy also adopted by various elements to form molecules through the sharing of electrons.

Hydrogen, with one electron in its K shell, shares electrons with another hydrogen atom to form H₂. This sharing allows both hydrogen atoms to attain the electronic configuration of the nearest noble gas, helium.

A single covalent bond is formed when two atoms share a pair of valence electrons. This shared pair constitutes the bond, enabling both atoms to achieve a stable noble gas configuration in their outermost shells.

The electronic configuration of chlorine is 2, 8, 7. Its valency is 1, as chlorine requires one more electron to complete its outer shell.

In the electron dot structure of Cl₂, two chlorine atoms share one pair of electrons, forming a single covalent bond to achieve stable electronic configurations.

Nitrogen, with an atomic number of 7, forms a diatomic molecule (N₂) with a triple bond. Each nitrogen atom contributes three electrons, leading to three shared pairs, forming a triple bond in the electron dot structure of N₂.

Different areas of association in the fore-brain interpret sensory information by combining inputs from various receptors and stored information, ultimately leading to the formation of decisions and responses.

The fore-brain serves as the main thinking part of the brain, specializing in receiving and interpreting sensory impulses from various receptors for hearing, smell, sight, and other senses.

Muscle cells change their shape and shorten at the cellular level by utilizing special proteins that respond to nervous electrical impulses, leading to new arrangements of proteins that result in the muscle cells adopting a shorter form.