The trap you need to fix: When asked to arrange elements by atomic radius across a period, students include the noble gas and wonder why the "decreasing trend" breaks. It breaks because the noble gas radius reported in data tables is a van der Waals radius — measured from non-bonded contact distances — while every other element's radius is covalent (half the internuclear distance in a bonded pair). Comparing the two is like comparing shoe sizes measured in different systems.
The core concept (NCERT Class 11 Chemistry, Chapter 3, page 12):
Atomic radius is not a sharply defined quantity. Two operational definitions matter for NEET:
Noble gases (He, Ne, Ar…) don't form conventional covalent bonds under normal conditions. Their radii are van der Waals only.
Periodic trend — same radius type only:
Ionic radii — quick rules:
Watch-out for NEET: Any question asking you to compare atomic radii across a full period — check whether a noble gas is in the list. If yes, the comparison is invalid unless they specify the same radius type.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following correctly represents the trend in covalent radius across Period 3 (Na to Cl)?
Argon's atomic radius listed in data tables is larger than chlorine's. The correct explanation is:
Among the following isoelectronic species (10 electrons each), which has the smallest ionic radius?
A cation is always smaller than its parent atom because:
Which of the following statements about van der Waals radius is correct?
In the isoelectronic pair N³⁻ and F⁻ (both with 10 electrons), which has the larger ionic radius and why?
Consider the species: Ne, Na⁺, and F⁻. All have 10 electrons. The correct order of their radii is:
An element X in Period 2 has a covalent radius of 77 pm. It forms an anion X⁴⁻. Compared to the neutral atom, the anion's radius will be:
Given
Five isoelectronic species, each with 10 electrons: - N³⁻ (Z = 7) - O²⁻ (Z = 8) - F⁻ (Z = 9) - Na⁺ (Z = 11) - Mg²⁺ (Z = 12)
Required
Arrange in order of increasing ionic radius.
Concept
In an isoelectronic series, all species have the same number of electrons. The species with the highest nuclear charge (Z) pulls the electron cloud most tightly → smallest radius. Radius decreases as Z increases.
Formula
No formula needed — this is a direct application of the isoelectronic radius rule: for constant electron count, radius ∝ 1/Z (qualitative inverse relationship).
Substitution
Order by increasing Z: N³⁻ (7) < O²⁻ (8) < F⁻ (9) < Na⁺ (11) < Mg²⁺ (12). Since radius decreases with increasing Z, the radius order is the reverse.
Calculation
Increasing radius = decreasing Z order: Mg²⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻ Published values (pm): Mg²⁺ (72) < Na⁺ (95) < F⁻ (136) < O²⁻ (140) < N³⁻ (146).
Final answer
Order of increasing ionic radius: **Mg²⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻** Note: Z values are exact integers (counting numbers) and do not affect any precision consideration.
Common trap
Students sometimes include Ne (Z = 10) in this series. But Ne's reported radius is van der Waals, not ionic — mixing radius types gives a misleading comparison (Trap: trap: atomic radius inert gas applied to the isoelectronic context).
Similar NEET-style question
"Arrange the following isoelectronic species in order of decreasing radius: Al³⁺, Na⁺, F⁻, O²⁻, N³⁻." (Same principle — order by increasing Z to get decreasing radius.) ---
Across period: atomic radius decreases (effective nuclear charge increases). Down group: atomic radius increases (additional shells). Cation < neutral atom < anion (for same element).
-- NCERT Class 11 Chemistry, Ch. 3, p. 12Energy required to ionize an electron from the n-th shell of hydrogen-like atom.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Z | nuclear charge | - |
| n | quantum number | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student includes inert-gas radius in atomic-radius trends. But inert gases use van der Waals radius (much larger than covalent), making 'monotonic decrease across period' look broken.
Atomic radius comparison includes a noble gas or trends across period 2/3.
Compare like with like: covalent radii for non-noble gases. Noble gas radii are van der Waals (no covalent bond). Don't compare noble-gas radius directly to halogen.
Category: Inorganic Exception
Student expects monotonic increase in IE across period. Anomalies: Be(s²) > B(s²p¹); N(p³ half-filled) > O(p⁴).
Compare IE values across period 2 (Li, Be, B, C, N, O, F).
Be > B (s² stable; B's p¹ easier to remove). N > O (N has p³ half-filled stability; O loses one to attain p³). Memorise these two anomalies.
Root cause: concept gap
Be>B (s² stability); N>O (N's p³ half-filled stability). Memorise these two anomalies in period 2.
Root cause: concept gap
Don't compare different radius types. Noble gases use vdW radius (much larger); halogens use covalent radius. Compare like-with-like.
3 questions from NEET 2021, 2024. Answers verified against NTA official keys.
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
swapped classes
Tempts surface-level recall.
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