Final answer:
Group 18 in the periodic table, known as the Noble Gases, are all monatomic gases at standard temperature and pressure.
Step-by-step explanation:
The group in the periodic table that contains elements that are all monatomic gases at STP (Standard Temperature and Pressure) is Group 18. This group is also known as the Noble Gases (Helium, Neon, Argon, Krypton, Xenon, and Radon). The elements in this group are known for being nonreactive due to having filled valence shells, meaning they have a full set of electrons in their outer electron shell. As a result, they exist as single atoms (monatomic) and are all gases under standard conditions.
Learn more about Noble Gases