How did Gilbert N. Lewis describe the difference between ionic and covalent bonding?

American chemist Gilbert N. Lewis distinguished between ionic and covalent bonding based on the behavior of valence electrons in relation to the octet rule, which posits that atoms tend to bond until they achieve the stable electron configuration of a noble gas.1

• Ionic bonds: Lewis proposed that these bonds form through the complete transfer of electrons from the valence shell of one atom to another. This transfer results in the formation of oppositely charged ions (cations and anions) that are held together by electrostatic attraction.1
• Covalent bonds: In contrast, Lewis described a covalent bond as the sharing of a pair of electrons between two atoms. Rather than one atom donating electrons to another, the shared pair occupies the space between the two nuclei, allowing both atoms to effectively complete their valence shells.12

While Lewis introduced these foundational concepts, he did not have the benefit of quantum mechanics to explain the underlying physical reasons for electron pairing, a task later addressed by Linus Pauling.31