When two atoms form a covalently-bonded diatomic molecule, the distance between the nuclei at which the potential energy is at a minimum is called

Bond length.

Explanation:

During formation of a covalent bond between atoms the total potential energy of the new molecule is less than the sum of energy of the two atoms. This is the energy orientation for the formation of bond. This is due to gain of stability by full filling the octet of the atoms. In covalent bond sharing of electrons occur. When two atoms approach each other there is decrease in energy till the bond length of the molecule, after this if we try to minimize the distance between atoms, the repulsion forces start to act their role.

indium, and thallium have three electrons i think