Final answer:
Elements 1-20 that exist as monatomic structures are He, Be, Ne, Ar, and other inert elements with complete valence shells or resistance to forming compounds.
Step-by-step explanation:
From elements 1-20, the elements that exist as a monatomic structure are helium (He), beryllium (Be), lithium (Li), and the noble gases such as neon (Ne) and argon (Ar). These elements are generally inert and do not easily form molecules with more than one atom of the element.
The other elements within the first 20, excluding hydrogen (H₂), nitrogen (N₂), oxygen (O₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂) which are diatomic molecules, can also exist in monatomic form, but it is not their most stable state.
The stability of these atomic structures can be attributed to the full valence shells of the noble gases and the individual atoms' strong resistance to bonding in inert elements like helium and beryllium.