Final answer:
The atom whose outermost shell structure most closely resembles that of neon (Ne, atomic number 10) is the noble gas argon (Ar, atomic number 18), which also has a full outer shell making it very stable.
Step-by-step explanation:
The atom whose outermost shell structure most closely resembles that of neon (atomic number 10) would be another noble gas with a completed valence shell. Neon has ten electrons with a full outer shell configuration of 2s²2p⁶. This full shell is characteristic of noble gases, and the next noble gas after neon is argon (Ar), with an atomic number of 18. Argon's outermost shell structure also represents a completed octet, similar to neon's configuration. Therefore, the atomic number that most closely resembles the outermost shell structure of neon is 18.
A neon atom is very stable due to its filled outermost energy level. A neon atom has two electrons in its first energy level and eight electrons in its second energy level, making it chemically inert.