📘 Detailed Solution
🔵 Step 1: Understand the important point
Noble gases such as Ne, Ar, Kr and Xe do not normally form covalent bonds with other atoms under ordinary conditions. Therefore, their atomic size is represented by their Van der Waals radius.
On the other hand, the radius of a halogen such as F, Cl or Br is commonly considered as its covalent radius when discussing the periodic trend.
🟣 Step 2: Why is Noble Gas radius larger?
The Van der Waals radius represents the distance at which two non-bonded atoms can approach each other. It is therefore larger than the covalent radius, which represents half the distance between the nuclei of two bonded identical atoms.
Thus, although the atomic size generally decreases from left to right across a period due to increasing effective nuclear charge, the apparent radius of a noble gas is larger because a different type of radius is being used.
🟢 Step 3: Comparison
| Halogen | Noble Gas |
|---|---|
| Usually considered using covalent radius | Usually considered using Van der Waals radius |
| Atoms form covalent bonds such as Cl₂ | Atoms generally exist as isolated atoms |
| Covalent radius is smaller | Van der Waals radius is larger |
🎬 Radius Concept
✅ Final Answer
The atomic radius of noble gases is apparently larger than that of the preceding halogens because noble gases are assigned their Van der Waals radius, whereas halogens are commonly compared using their covalent radius.
The Van der Waals radius is the distance associated with non-bonded atoms and is larger than the covalent radius. Therefore, the noble gas appears to have a larger atomic radius despite the general decrease in atomic size across a period.
📝 20 MCQs with Answers
1. The atomic radius generally assigned to noble gases is:
✅ Answer
C) Van der Waals radius
2. Noble gases generally do not form:
✅ Answer
B) Ordinary covalent bonds
3. Which radius is larger?
✅ Answer
B) Van der Waals radius
4. The preceding element to a noble gas in the same period is generally a:
✅ Answer
B) Halogen
5. Chlorine is a member of:
✅ Answer
C) Group 17
6. Argon belongs to:
✅ Answer
B) Group 18
7. Van der Waals radius refers to:
✅ Answer
B) Size associated with non-bonded atoms
8. Across a period, effective nuclear charge generally:
✅ Answer
B) Increases
9. The apparent increase in noble gas radius is mainly due to:
✅ Answer
B) Use of Van der Waals radius
10. Covalent radius is related to:
✅ Answer
A) Bonded atoms
11. Which is generally greater?
✅ Answer
B) Van der Waals radius of Ar
12. Noble gases are chemically:
✅ Answer
B) Generally very unreactive
13. The Van der Waals radius is associated with:
✅ Answer
A) Non-bonded atoms
14. Which pair is correctly matched?
✅ Answer
A) Noble gases — Van der Waals radius
15. Which element follows chlorine in the same period?
✅ Answer
C) Ar
16. The radius trend across a period normally shows:
✅ Answer
A) General decrease
17. Why can noble gas radius not be directly compared with halogen covalent radius without qualification?
✅ Answer
B) Different radius definitions are being used
18. The noble gases have a complete:
✅ Answer
A) Valence shell
19. Which statement is correct?
✅ Answer
B) Van der Waals radius is generally larger than covalent radius
20. The larger noble-gas radius does NOT mean that:
✅ Answer
B) Noble gas necessarily has a physically larger bonded atom
Q. Why is the radius of noble gases larger than that of the preceding halogens?
Solution: Noble gases are assigned their Van der Waals radius, while halogens are commonly compared using their covalent radius. Since the Van der Waals radius is larger than the covalent radius, noble gases appear to have a larger atomic radius.
Q. Explain the apparent increase in atomic radius from a halogen to the noble gas in the same period.
Solution:
- Atomic size generally decreases across a period because effective nuclear charge increases.
- Halogens are commonly assigned their covalent radius.
- Noble gases are assigned their larger Van der Waals radius because they do not normally form covalent bonds.
Q. Differentiate between covalent radius and Van der Waals radius.
| Covalent Radius | Van der Waals Radius |
|---|---|
| Used for bonded atoms | Used for non-bonded atoms |
| Half the internuclear distance between identical bonded atoms | Related to the closest approach of non-bonded atoms |
| Generally smaller | Generally larger |
| Commonly used for halogens when discussing covalent bonding | Used for noble gases |
Q. Explain the periodic trend of atomic radius and the exception shown by noble gases.
Solution:
- Atomic radius generally decreases from left to right across a period.
- This is due to an increase in effective nuclear charge while electrons are added to the same principal shell.
- Halogens have their covalent radius considered because they form covalent molecules such as Cl₂.
- Noble gases generally do not form ordinary covalent molecules, so their Van der Waals radius is used.
- The Van der Waals radius is larger than the covalent radius, making the noble gas appear larger than the preceding halogen.
The apparent increase is mainly due to the different definitions of atomic radius, not a reversal of the normal periodic trend.
Q. Explain in detail why the atomic radius of noble gases is larger than that of the preceding halogens.
Detailed Solution:
Across a period, the atomic radius generally decreases from left to right. This happens because the nuclear charge increases while the added electrons enter the same principal energy level. Consequently, the effective nuclear charge increases and pulls the electron cloud closer to the nucleus.
However, an apparent exception is observed when the radius of a noble gas is compared with that of the preceding halogen. The reason is that different types of atomic radii are used.
| Halogen | Noble Gas |
|---|---|
| Forms covalent molecules | Normally exists as individual atoms |
| Covalent radius is used | Van der Waals radius is used |
| Smaller radius | Larger radius |
For example, chlorine forms the covalent molecule Cl₂. Therefore, its covalent radius can be obtained from the internuclear distance between the two bonded chlorine atoms.
Argon, however, does not normally form an Ar₂ covalent molecule under ordinary conditions. Hence, its atomic size is represented by its Van der Waals radius, based on the closest approach of non-bonded atoms.
Therefore, the noble gas appears to have a larger atomic radius than the preceding halogen even though the underlying periodic trend in atomic size across the period is a decrease.
The atomic radius of noble gases appears larger than that of the preceding halogens because noble gases are assigned their Van der Waals radius, whereas halogens are commonly assigned their covalent radius. Since the Van der Waals radius is larger than the covalent radius, the noble gas appears larger. This is therefore mainly a consequence of using different definitions of atomic radius, rather than a reversal of the normal periodic trend.
🎯 Quick Revision
Noble Gas → Van der Waals Radius
Van der Waals Radius > Covalent Radius
Reason → Different radius definitions
Across period → Atomic radius generally decreases
