When does a covalent bond occur?

A covalent bond occurs when two atoms share electrons. This type of bond typically forms between nonmetals, which have similar electronegativities and therefore have a tendency to share rather than completely transfer their electrons. By sharing electrons, the atoms can fill their outer electron shells, achieving greater stability.

In the context of molecular formation, this sharing of electrons allows each atom to attain a noble gas configuration, which is often a stable electronic arrangement. This characteristic is fundamental in forming molecules like water (H₂O) and carbon dioxide (CO₂), where the shared electrons allow the participating atoms to fulfill the octet rule.

The other options describe different types of interactions. Losing electrons would lead to ionic bonding, where electrons are transferred rather than shared. Gaining protons doesn’t directly pertain to bond formation; instead, it relates to changes in the identity or charge of the atom. Lastly, transferring electrons between atoms typically describes ionic bonding as well, not covalent bonding. Thus, the accurate depiction of a covalent bond is the sharing of electrons, which promotes stability and is crucial for forming many biological molecules.