How are strong bonds formed between atoms?

Strong bonds between atoms are formed primarily through significant overlap of their electrons, a principle fundamental to both covalent and metallic bonding. In the case of covalent bonds, atoms share one or more pairs of electrons, and this sharing occurs due to the close proximity of their atomic orbitals. When these orbitals overlap significantly, the shared electrons hold the two nuclei together, providing the bond strength.

In metallic bonding, the scenario involves a 'sea of electrons' that are delocalized over many atomic nuclei, leading to a bond that is both strong and flexible. This extensive overlap of electron clouds contributes to the characteristics of metals, such as electrical conductivity and malleability.

The other choices do not adequately describe the mechanism of bond formation. Weak interactions between outer shells would not result in strong bonds; rather, they might describe weaker forces like Van der Waals interactions. Complete electron transfer, while characteristic of ionic bonding, does not involve overlapping orbitals and typically leads to electrostatic attractions rather than the strong covalent or metallic bonds indicated in the question. Moreover, the distance between atomic nuclei is inversely related to bond strength; closer proximity allows for stronger interactions, whereas greater distances tend to weaken any potential interactions between atoms.