Which one of the following is monatomic?
Helium
Understanding the structure of elements is fundamental in chemistry. Elements can exist in different forms, including individual atoms or molecules made up of multiple atoms bonded together. The term monatomic is used to describe substances that exist as single, unbonded atoms.
Let's examine each option to determine which one is monatomic:
- Hydrogen: Hydrogen is a nonmetal. In its elemental form under standard conditions, hydrogen exists as diatomic molecules, meaning each molecule is composed of two hydrogen atoms bonded together. The chemical formula is \(\text{H}_2\). Therefore, hydrogen is not monatomic.
- Sulphur: Sulphur is also a nonmetal. Elemental sulphur exists in several allotropes, the most common being rhombic sulphur, which consists of molecules made of eight sulphur atoms arranged in a ring. The chemical formula for this form is \(\text{S}_8\). Thus, sulphur is not monatomic.
- Phosphorus: Phosphorus is another nonmetal with multiple allotropes. White phosphorus, a common and reactive form, exists as tetrahedral molecules composed of four phosphorus atoms. The chemical formula is \(\text{P}_4\). Therefore, phosphorus is not monatomic.
- Helium: Helium is a noble gas. Noble gases are located in Group 18 of the periodic table. A defining characteristic of noble gases is their chemical inertness, meaning they do not readily form chemical bonds with other atoms. As a result, noble gases exist as individual atoms rather than molecules. The chemical formula for helium is simply \(\text{He}\). This means helium is monatomic.
Identifying Monatomic Substances
To identify a monatomic substance, we look for elements that exist naturally as single, unbonded atoms. The noble gases (Helium, Neon, Argon, Krypton, Xenon, and Radon) are the prime examples of elements that are monatomic under standard conditions. Other elements typically form molecules (like \(\text{O}_2\), \(\text{N}_2\), \(\text{Cl}_2\), \(\text{P}_4\), \(\text{S}_8\)) or complex structures (like metals or network solids).
Comparing the Options: Monatomic vs. Polyatomic
We can summarize the forms of the given elements:
| Element | Typical Form | Number of Atoms per Molecule | Classification |
|---|---|---|---|
| Hydrogen | \(\text{H}_2\) | 2 (Diatomic) | Polyatomic |
| Sulphur | \(\text{S}_8\) | 8 (Octatomic) | Polyatomic |
| Phosphorus | \(\text{P}_4\) | 4 (Tetratomic) | Polyatomic |
| Helium | \(\text{He}\) | 1 (Monatomic) | Monatomic |
Based on this comparison, Helium is the only element among the options that exists as single atoms, making it monatomic.
Revision Table: Key Element Forms
| Element Category | Examples | Typical Form | Monatomic? |
|---|---|---|---|
| Noble Gases (Group 18) | He, Ne, Ar, Kr, Xe, Rn | Single atoms | Yes |
| Halogens (Group 17) | F, Cl, Br, I | Diatomic molecules (\(\text{X}_2\)) | No |
| Other Nonmetals (\(\text{H}\), \(\text{O}\), \(\text{N}\)) | Hydrogen, Oxygen, Nitrogen | Diatomic molecules (\(\text{H}_2\), \(\text{O}_2\), \(\text{N}_2\)) | No |
| Phosphorus | White Phosphorus | Tetratomic molecules (\(\text{P}_4\)) | No |
| Sulphur | Rhombic Sulphur | Octatomic molecules (\(\text{S}_8\)) | No |
| Metals | Fe, Cu, Au, Na | Exist as individual atoms in a lattice structure (considered monatomic in gaseous state) | Generally not considered molecular, monatomic in gas phase |
Additional Information: Monatomic vs. Polyatomic vs. Diatomic
Let's clarify these terms:
- Monatomic: Consists of single atoms. Example: Helium (\(\text{He}\)).
- Diatomic: Consists of molecules made of two atoms. Example: Oxygen (\(\text{O}_2\)).
- Polyatomic: Consists of molecules made of three or more atoms. Examples: Ozone (\(\text{O}_3\)), Phosphorus (\(\text{P}_4\)), Sulphur (\(\text{S}_8\)). Diatomic molecules are a specific type of polyatomic molecule.
Understanding these classifications helps in predicting how elements exist under normal conditions.
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