The two elements with atomic numbers differing by only 1 that are most chemically different are fluorine (atomic number 9) and neon (atomic number 10). Fluorine is a highly reactive nonmetal, while neon is a noble gas known for its inertness. This difference in chemical behavior arises from their electron configurations.
Fluorine has one fewer electron than neon, and its outer electron shell is not fully filled. This makes fluorine highly reactive, seeking to gain one electron to achieve a stable electron configuration. It readily forms compounds by accepting electrons from other elements. Fluorine is the most electronegative element and is known for its ability to form strong bonds, particularly with hydrogen.
On the other hand, neon has a completely filled outer electron shell, making it very stable and chemically unreactive. Neon does not readily form compounds or participate in chemical reactions due to its full complement of electrons. It is commonly used in lighting applications because of its bright, inert glow.
Therefore, fluorine and neon exhibit contrasting chemical properties, making them the two elements with atomic numbers differing by only 1 that are most chemically different.