The bond will be perfectly covalent when the electronegativity difference of bonded atoms is:

Explanation

A covalent bond is formed when two atoms share electrons to achieve a more stable electron configuration. In a perfectly covalent bond, the electronegativity difference between the two bonded atoms is zero. Electronegativity is a measure of an element's ability to attract electrons in a chemical bond. When the electronegativity difference is zero, it means that the two atoms have the same electronegativity, and they share electrons equally, resulting in a purely covalent bond. For example, in a diatomic molecule like hydrogen (H2) or chlorine (Cl2), the two atoms have the same electronegativity, and the bond between them is perfectly covalent because they share electrons equally. As you move across the periodic table, electronegativity differences between elements increase, leading to more polar covalent and ionic bonds. So, a perfectly covalent bond occurs when there is no electronegativity difference between the bonded atoms.