The nonmetallic properties of elements are primarily caused by their tendency to gain or share electrons to achieve a stable electron configuration. Here's a breakdown:

• High Electronegativity: Nonmetals generally have high electronegativity, meaning they have a strong attraction for electrons. This is due to their atomic structure having nearly filled outer electron shells.
• Electron Affinity: Nonmetals tend to have high electron affinity, meaning they release a significant amount of energy when they gain an electron. This indicates a strong tendency to accept electrons and form negative ions.
• Ionization Energy: Nonmetals typically have high ionization energies, meaning it requires a lot of energy to remove an electron from them. This makes it difficult for them to form positive ions.
• Atomic Size: Nonmetal atoms are generally smaller than metal atoms. This allows their nucleus to exert a stronger pull on their outer electrons, increasing their electronegativity.
• Achieving a Stable Octet: Nonmetals gain or share electrons to achieve a stable octet (8 electrons) in their outermost electron shell, fulfilling the octet rule and resulting in a stable electron configuration similar to that of noble gases.

In summary, the combination of high electronegativity, high electron affinity, high ionization energy, small atomic size, and the drive to achieve a stable octet explains why nonmetals readily gain or share electrons, leading to their characteristic nonmetallic properties.