Question

Which of the following is a monatomic molecule?
a.) Nitrogen
b.) Hydrogen
c.) Helium
d.) Oxygen

Answer 1

Hint: Atoms don’t always exist in their elemental form. They react with many other elements present in the atmosphere and form different compounds. Sulphides, oxides and carbonates are the most common form.

Complete step by step solution:
a) Monoatomic molecule - A molecule made up of a single atom.
Example: Helium, Neon, Argon, Krypton, Radon
b) Diatomic molecule - A molecule made up of two atoms.
Example: CO or Carbon Monoxide (Carbon and Oxygen), NO or Nitric Oxide (Nitrogen and Oxygen), or Fluorine (2 Fluorine), or Hydrogen (2 Hydrogen)
b) Triatomic molecule - A molecule made up of three atoms.
Example: or Magnesium Chloride (1 Magnesium and 2 Chloride), or Water (2 hydrogen and 1 Oxygen), HCN or Hydrogen cyanide (1 Hydrogen 1 Carbon 1 Nitrogen), or Ozone (3 Oxygen)

Now let us take a look at our options:
Nitrogen: It appears in diatomic form as $$N_2$$, which is a combination of two nitrogen atoms.
Hydrogen: It appears in diatomic form as $$H_2$$, which is a combination of two hydrogen atoms.
Helium: Helium is a noble gas having very low reactivity. Therefore, it exists in its elemental form as He. It is monoatomic.
Oxygen: It appears in diatomic form as $$O_2$$, which is a combination of two oxygen atoms.
Hence, the correct option is (c).

Note: A molecule can also have even more than 3 covalently bonded atoms. These molecules are commonly termed as polyatomic molecules or radicals. For example, carbohydrates, proteins and nucleic acids.