Which one of the following is monatomic?

Which one of the following is monatomic?

Hydrogen
Sulphur
Phosphorus
Helium
This question was previously asked in
UPSC CDS-2 – 2019
A monatomic substance consists of individual atoms that are not bonded together to form molecules under standard conditions. Hydrogen exists as diatomic molecules (H2). Sulphur exists as polyatomic molecules, commonly S8 rings. Phosphorus commonly exists as P4 tetrahedra. Helium is a noble gas and exists as single atoms (He) under standard conditions, hence it is monatomic.
Monatomic substances are composed of single atoms; noble gases like Helium are monatomic.
All noble gases (Helium, Neon, Argon, Krypton, Xenon, Radon) are monatomic under typical conditions because they have stable electron configurations.
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