When two atoms bond, what must occur in their electron shells?

When two atoms bond, their outermost electron shells interact to form a bond, allowing for the sharing or transfer of electrons. This interaction is most effectively described by the overlapping of the outermost (valence) shells of the participating atoms. The overlapping of these shells facilitates the sharing of electrons, which is crucial in covalent bonding, or the transfer of electrons leading to ionic bonding.

In covalent bonds, for example, pairs of electrons are shared between two atoms, which occurs because their electron clouds overlap efficiently, promoting stability. In ionic bonds, while there is a complete transfer of electrons from one atom to another, the concept of electron shell interaction remains vital because the resulting positive and negative ions are held together by electrostatic forces that depend on their electron configurations.

Other options suggest misunderstandings regarding atomic behavior: protons are not exchanged during bonding, electron shells do not repel each other but rather seek stability through interaction, and filled inner shells do not directly determine the bonding capabilities of an atom. The focus is on the valence shells because they play a direct role in chemical bonding and dictate the types of bonds that can form between atoms.