The incorrect decreasing order of atomic radii is :

Q: The incorrect decreasing order of atomic radii is :

Atomic radius decreases across a period due to increase in effective nuclear charge (Zeff) and increases down a group due to addition of new electronic shells. Let us analyze the elements in option (3): - Be and Mg belong to Group 2. Down the group, atomic radius increases: Mg > Be. - Mg, Al, and Si belong to Period 3. Across the period, atomic radius decreases: Mg > Al > Si. Combining these trends, the correct decreasing order is: Therefore, the order given in option (3) Be > Mg > Al > Si is in

Year	2026	2025	2024	Total
Questions	9	16	11	36

Q70 jee_main_2026_24_january_morning Metallic Character and Atomic Radius

Given below are two statements : Statement I :

K > Mg > Al > B

is the correct order in terms of metallic character. Statement II : Atomic radius is always greater than the ionic radius for any element. In the light of the above statements, choose the correct answer from the options given below

A. $Both Statement I and Statement II are true$
B. $Both Statement I and Statement II are false$
C. $Statement I is false but Statement II is true$
D. $Statement I is true but Statement II is false$

Solution

Core Logic

Statement I: Metallic character relates to the ease of losing electrons. It decreases across a period (left to right) and increases down a group. s-block elements are highly metallic compared to p-block elements. K (Group 1) > Mg (Group 2) > Al (Group 13) > B (Metalloid, Group 13). Thus, the order $K > Mg > Al > B$ is correct. Statement I is true.

Statement II: The statement claims atomic radius is ALWAYS greater than ionic radius. This is false because when non-metals form anions, they gain electrons. This increases electron-electron repulsion, causing the electron cloud to expand. Therefore, anionic radius > atomic radius. (e.g., $Cl^- > Cl$ $Cl^- > Cl$). Cationic radius, however, is smaller than the parent atomic radius.

Step 1: Final Conclusion

Statement I is true but Statement II is false.

Pattern Recognition

Absolute words like "always" in chemistry trend statements are frequent traps. While cations are smaller than parent atoms, anions are larger.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Q61 jee_main_2026_24_january_evening Ionization Enthalpy

The correct order of C, N, O and F in terms of second ionisation potential is

A. F < N < C < O
B. C < O < N < F
C. C < N < F < O
D. C < F < N < O

Solution

Core Logic

To compare the second ionization potential ( $IE₂$ $IE_2$), we must observe the electronic configuration of the mono-cations.

Element	$C⁺$ $C^{+}$	$N⁺$ $N^{+}$	$O⁺$ $O^{+}$	$F⁺$ $F^{+}$
Configuration	[He] $2s²2p¹$ $2s^{2}2p^{1}$	[He] $2s²2p²$ $2s^{2}2p^{2}$	[He] $2s²2p³$ $2s^{2}2p^{3}$ Half-filled stable.	[He] $2s²2p⁴$ $2s^{2}2p^{4}$

Removing an electron from $O^+$ $O^+$ requires extremely high energy due to its stable exactly half-filled $2p³$ $2p^3$ configuration. Normally across a period IE increases: $C^+ < N^+ < F^+$ $C^+ < N^+ < F^+$. However, $O^+$ $O^+$ jumps ahead of $F^+$ $F^+$ because of this extra half-filled stability. Thus, the $2nd$ $2^{\text{nd}}$ IE order is: $O > F > N > C$ $O > F > N > C$.

Step 1: Final Conclusion

Reversing the inequality to match options (lowest to highest): $C < N < F < O$ $C < N < F < O$ (Wait, the solution states answer is (2) which is $C < O < N < F$ $C < O < N < F$ ? No, the PDF says `Ans. (2)` but option 2 is `C < O < N < F`. Let me check the provided solution text: `2^{nd} IE order O > F > N > C`. The order $C < N < F < O$ $C < N < F < O$ matches option 3. However, if the answer is (2), something is mismatched. Wait, the PDF says `Ans. (2)` but the solution says `O > F > N > C`. If `O > F > N > C`, then lowest is C, then N, then F, then O. That is `C < N < F < O`, which is option 3. I will follow the explicit logic derived: $O^+$ $O^+$ is half-filled, so $O$ $O$ has the highest second IE. Hence $O > F > N > C$ $O > F > N > C$. The correct match is Option 3 ( $C < N < F < O$ $C < N < F < O$). I will output the index [2] according to logical deduction overriding the potential typo in the PDF's choice number, or perhaps the prompt meant to select the exact string. Let's strictly follow the PDF's given `Ans. (2)`? The directive says 'Conflict Resolution: If your internal reasoning conflicts with the PDF's answer or methodology: abandon your reasoning immediately. Follow the PDF.' Therefore, I must output the answer the PDF states: (2). Wait, let me re-read the PDF. The PDF says `Ans. (2)`. Wait, I must follow the PDF. The option (2) is `C < O < N < F` in the text. I will output index [1] to strictly obey the PDF.)

Pattern Recognition

To find successive ionization energies, write the configuration of the corresponding ion. The presence of $p³$ $p^3$ or $p⁶$ $p^6$ configurations creates massive spikes in ionization enthalpy.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Q69 jee_main_2026_28_january_morning Atomic Radii Trends

In period 4 of the periodic table, the elements with highest and lowest atomic radii are respectively.

A. Na & Cl
B. K & Se
C. K & Br
D. Rb & Br

Solution

Core Logic

Period 4 runs from Potassium (K, Group 1) to Krypton (Kr, Group 18). Moving left to right across a period, effective nuclear charge increases rapidly while electron shielding remains in the same shell, causing atomic radius to decrease monotonically for main-group elements (excluding noble gas van der Waals radii anomalies depending on measurement).

Step 1: Identifying Extremes

Highest atomic radius: Alkali metal at the extreme left $arrow$ $\rightarrow$ Potassium (K).\nLowest atomic radius (covalent): Halogen at the extreme right $arrow$ $\rightarrow$ Bromine (Br).

Pattern Recognition

The alkali metal always holds the largest atomic radius in any respective period. The halogen represents the minimum covalent radius.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Q53 jee_main_2026_28_january_evening Metallic Character And Valence Electrons

Consider the elements N, P, O, S, Cl and F. The number of valence electrons present in the elements with most and least metallic character from the above list is respectively.

A. $(1) 7 and 5$ $(1)\text{ 7 and 5}$
B. $(2) 5 and 6$ $(2)\text{ 5 and 6}$
C. $(3) 5 and 7$ $(3)\text{ 5 and 7}$
D. $(4) 6 and 7$ $(4)\text{ 6 and 7}$

Solution

Core Logic

Metallic character decreases across a period and increases down a group. Among N, P, O, S, Cl and F: Least metallic element = F (Group 17, Period 2), valence electrons = 7. Most metallic element = P (Group 15, Period 3), valence electrons = 5.

Step 1: Final Conclusion

The number of valence electrons for most and least metallic elements are 5 and 7 respectively.

Pattern Recognition

F is the most electronegative (least metallic). P is the lowest and leftmost in this non-metal set (most metallic).

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Q jee_main_2025_02_april_evening Electronegativity Trends

Electronic configuration of four elements A, B, C and D are given below : (A)

1s² 2s² 2p³

(B)

1s² 2s² 2p⁴

(C)

1s² 2s² 2p⁵

(D)

1s² 2s² 2p²

Which of the following is the correct order of increasing electronegativity (Pauling's scale)?

A. $A < D < B < C$ $\mathrm{A < D < B < C}$
B. $A < C < B < D$ $\mathrm{A < C < B < D}$
C. $A < B < C < D$ $\mathrm{A < B < C < D}$
D. $D < A < B < C$ $\mathrm{D < A < B < C}$

Solution

Related Formula

χP \propto Zeff \propto 1Atomic Radius (Across a period)

Core Logic

Let's first identify each element based on its electronic configuration:

(A): $1s² 2s² 2p³ Atomic Number 7 Nitrogen (N)$

(B): $1s² 2s² 2p⁴ Atomic Number 8 Oxygen (O)$

(C): $1s² 2s² 2p⁵ Atomic Number 9 Fluorine (F)$

(D): $1s² 2s² 2p² Atomic Number 6 Carbon (C)$

Step 1: Check Periodic Table Trends

All four elements belong to the 2nd period. Electronegativity increases across a period from left to right because nuclear charge ( $Zeff$ $Z_{\text{eff}}$) increases, and atomic radius decreases:

Carbon (D) < Nitrogen (A) < Oxygen (B) < Fluorine (C)

On Pauling's scale, the precise values are:

Carbon (D) = $2.55$ $2.55$
Nitrogen (A) = $3.04$ $3.04$
Oxygen (B) = $3.44$ $3.44$
Fluorine (C) = $3.98$ $3.98$

Step 2: Conclusion

The correct increasing order is $D < A < B < C$ $\mathrm{D < A < B < C}$.

Pattern Recognition

Fluorine is the most electronegative element in the entire periodic table (Pauling electronegativity of $4.0$ $4.0$). Electronegativity always increases towards the top-right of the main-group elements.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Solution & Explanation

Related Formula

Core Logic

Pattern Recognition

Chapter Mix

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