Identifying Polyatomic Non-metals

Let's understand what a polyatomic molecule is and then examine the given options to determine which non-metal is polyatomic.

A molecule is formed when two or more atoms are chemically bonded together. Based on the number of atoms they contain, molecules can be classified as:

• Monatomic: Consists of a single atom (e.g., noble gases like Helium, Neon, Argon).
• Diatomic: Consists of two atoms (e.g., $\text{O}_2$, $\text{H}_2$, $\text{N}_2$).
• Triatomic: Consists of three atoms (e.g., $\text{O}_3$, $\text{H}_2\text{O}$).
• Polyatomic: Consists of more than one atom. This term is broadly used for diatomic, triatomic, and molecules with many atoms. However, it is often used specifically to refer to molecules containing more than two or three atoms.

Now let's look at the provided non-metal options:

Analyzing the Non-metal Options

Option 1: Sulphur

Elemental Sulphur commonly exists as octatomic molecules with the chemical formula $\text{S}_8$. These molecules have a ring structure containing 8 Sulphur atoms bonded together.

Since the $\text{S}_8$ molecule contains 8 atoms, it clearly consists of more than one atom, making it a polyatomic molecule. Specifically, it is an octatomic molecule, which is a type of polyatomic molecule.

Option 2: Argon

Argon (Ar) is a noble gas. Noble gases exist as individual atoms and do not readily form molecules under standard conditions. Therefore, Argon is a monatomic element.

Option 3: Oxygen

Oxygen (O) is a non-metal. Elemental Oxygen typically exists as diatomic molecules with the chemical formula $\text{O}_2$ (containing 2 oxygen atoms). Another allotrope of Oxygen is Ozone, which exists as triatomic molecules with the formula $\text{O}_3$ (containing 3 oxygen atoms).

Both $\text{O}_2$ and $\text{O}_3$ are considered polyatomic as they contain more than one atom. However, compared to Sulphur's $\text{S}_8$ structure, $\text{O}_2$ and $\text{O}_3$ contain fewer atoms.

Option 4: Helium

Helium (He) is also a noble gas, similar to Argon. It exists as individual atoms and is therefore a monatomic element.

Comparing Options and Conclusion

Based on the molecular structure of these non-metals, we can classify them:

Among the given options, Sulphur exists commonly as $\text{S}_8$ molecules, which are clearly polyatomic, containing many atoms. While Oxygen ($\text{O}_2$, $\text{O}_3$) is also polyatomic, Sulphur's $\text{S}_8$ form represents a substance with a significantly larger number of atoms per molecule compared to diatomic or triatomic molecules. Noble gases like Argon and Helium are monatomic.

Therefore, Sulphur is the most appropriate answer as a polyatomic non-metal among the given choices.

Revision Table: Non-metal Atomicity

Additional Information on Polyatomic Elements

Besides Sulphur ($\text{S}_8$), another common example of a polyatomic non-metal element is Phosphorus. White Phosphorus exists as tetrahedral $\text{P}_4$ molecules, containing four phosphorus atoms.

The form a non-metal takes depends on temperature, pressure, and allotropic form. For instance, Oxygen can be $\text{O}_2$ or $\text{O}_3$. However, at room temperature and pressure, Sulphur typically exists as rhombic sulphur, which is made of $\text{S}_8$ rings.

Understanding the common molecular structures of elements is important for classifying them as monatomic, diatomic, or polyatomic.