The number of valence electrons present in the metal among Cr, Co, Fe and Ni which has the lowest enthalpy of…

Q: The number of valence electrons present in the metal among Cr, Co, Fe and Ni which has the lowest enthalpy of atomisation is:

Let's look at the enthalpy of atomisation values for the given 3d transition metals: - **Chromium (Cr)**: 397 kJ mol-1 - **Iron (Fe)**: 416 kJ mol-1 - **Cobalt (Co)**: 425 kJ mol-1 - **Nickel (Ni)**: 430 kJ mol-1 Among the choices, **Chromium (Cr)** has the lowest enthalpy of atomisation (397 kJ mol-1), due to a highly stable half-filled d-subshell configuration which leads to weaker metallic bonding relative to the other metals listed. The valence electronic configuration of Cr is: Total valenc

Year	2026	2025	2024	Total
Questions	10	17	17	44

Q jee_main_2025_29_jan_morning Melting Points of Transition Elements

The correct option with order of melting points of the pairs (Mn, Fe), (Tc, Ru) and (Re, Os) is :

A. $Fe < Mn$ $\mathrm{Fe} < \mathrm{Mn}$ , $Ru < Tc$ $\mathrm{Ru} < \mathrm{Tc}$ and $Re < Os$ $\mathrm{Re} < \mathrm{Os}$
B. $Mn < Fe, Tc < Ru$ and $Re < Os$ $\mathrm{Re} < \mathrm{Os}$
C. $Mn < Fe, Tc < Ru$ and $Os < Re$ $\mathrm{Os} < \mathrm{Re}$
D. $Fe < Mn$ $\mathrm{Fe} < \mathrm{Mn}$ , $Ru < Tc$ $\mathrm{Ru} < \mathrm{Tc}$ and $Os < Re$ $\mathrm{Os} < \mathrm{Re}$

Solution

Formulas Used

Melting point trends in $3d$ $3d$, $4d$ $4d$, and $5d$ $5d$ series transition metals depend on the extent of metallic bonding and $d$ $d$-electron participation.

Core Logic

According to NCERT transition element periodic trends:

$3d$ $3d$ Series ( $Mn$ $\mathrm{Mn}$ vs $Fe$ $\mathrm{Fe}$): Manganese ( $Mn$ $\mathrm{Mn}$, $3d⁵ 4s²$ $3d^5 4s^2$) has an abnormally low melting point compared to Iron ( $Fe$ $\mathrm{Fe}$, $3d⁶ 4s²$ $3d^6 4s^2$) because its stable, half-filled $d⁵$ $d^5$ configuration holds $d$ $d$-electrons more tightly, reducing their participation in metallic bonding $arrow Mn < Fe$ .
$4d$ $4d$ Series ( $Tc$ $\mathrm{Tc}$ vs $Ru$ $\mathrm{Ru}$): Technetium ( $Tc$ $\mathrm{Tc}$, $4d⁵ 5s²$ $4d^5 5s^2$) similarly shows a dip in melting point compared to Ruthenium ( $Ru$ $\mathrm{Ru}$, $4d⁷ 5s¹$ $4d^7 5s^1$) due to the stable $4d⁵$ $4d^5$ configuration $arrow Tc < Ru$ .
$5d$ $5d$ Series ( $Re$ $\mathrm{Re}$ vs $Os$ $\mathrm{Os}$): Rhenium ( $Re$ $\mathrm{Re}$, $5d⁵ 6s²$ $5d^5 6s^2$) has optimal interatomic interaction and a higher melting point than Osmium ( $Os$ $\mathrm{Os}$, $5d⁶ 6s²$ $5d^6 6s^2$) $arrow Os < Re$ .

Combining these trends yields: $Mn < Fe$ $\mathrm{Mn} < \mathrm{Fe}$, $Tc < Ru$ $\mathrm{Tc} < \mathrm{Ru}$, and $Os < Re$ $\mathrm{Os} < \mathrm{Re}$

Pattern Recognition

Stable half-filled $d⁵$ $d^5$ configurations in $3d$ $3d$ ( $Mn$ $\mathrm{Mn}$) and $4d$ $4d$ ( $Tc$ $\mathrm{Tc}$) restrict $d$ $d$-electron delocalization, creating characteristic dips in melting point curves compared to adjacent metals.

Correct Option: (C)

Q jee_main_2025_29_jan_morning Preparation and Properties of Potassium Dichromate

The molar mass of the water insoluble product formed from the fusion of chromite ore

(FeCr₂O₄)

with

Na₂CO₃

in presence of

O₂

is ________

g mol⁻¹

.

Numerical Answer. Answer: 160 to 160

Solution

Related Formula

Balanced fusion reaction process description

Core Logic

Write the balanced chemical equation for the industrial preparation stage of chromate salts:

4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ arrow 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂

Evaluating the solubilities of the products:

$Na₂CrO₄$ $\mathrm{Na_2CrO_4}$ is highly soluble in water.
$Fe₂O₃$ $\mathrm{Fe_2O_3}$ (Iron(III) oxide) is water-insoluble.

Molar Mass of $Fe₂O₃$ $\mathrm{Fe_2O_3}$:

M = (2 \cdot 55.85) + (3 \cdot 16.0) (2 \cdot 56) + (3 \cdot 16) = 112 + 48 = 160 ~g/mol

Pattern Recognition

Transition metal oxides in high oxidation states with minimal ionic breakdown parameters reliably act as insoluble precipitates in water.

Chapter Mix

Class 12 Chemistry: The d-and f-Block Elements

Q jee_main_2024_01_february_morning Oxidising Properties

In acidic medium,

K₂Cr₂O₇

shows oxidising action as represented in the half reaction

Cr₂O₇²⁻ + XH^+ + Ye^- arrow 2A + ZH₂O

X, Y, Z and A are respectively are:

A. $8, 6, 4 and Cr₂O₃$ $8, 6, 4 \text{ and } Cr_2O_3$
B. $14, 7, 6 and Cr³⁺$ $14, 7, 6 \text{ and } Cr^{3+}$
C. $8, 4, 6 and Cr₂O₃$ $8, 4, 6 \text{ and } Cr_2O_3$
D. $14, 6, 7 and Cr³⁺$ $14, 6, 7 \text{ and } Cr^{3+}$

Solution

Core Logic

The balanced half-reaction for the dichromate ion acting as an oxidising agent in an acidic medium is:

Cr₂O₇²⁻ + 14H^+ + 6e^- arrow 2Cr³⁺ + 7H₂O

Step 1: Compare with Given Equation

Comparing this with the given equation $Cr₂O₇²⁻ + XH^+ + Ye^- arrow 2A + ZH₂O$ :

$X = 14$ $X = 14$ $Y = 6$ $Y = 6$ $Z = 7$ $Z = 7$ $A = Cr³⁺$ $A = Cr^{3+}$

Pattern Recognition

In acidic medium, dichromate ( $Cr₂O₇²⁻$ $Cr_2O_7^{2-}$) always requires $14H^+$ $14H^+$ to balance $7O$ $7O$ atoms, forming $7H₂O$ $7H_2O$. Chromium reduces from +6 to +3 state, taking $6e^-$ $6e^-$ overall.

Chapter Mix

Class 12 Chemistry: The d-and f-Block Elements Class 11 Chemistry: Redox Reactions

Q73 jee_main_2024_29_january_evening Lanthanoid Oxidation States

Which of the following acts as a strong reducing agent? (Atomic number : Ce = 58, Eu = 63, Gd = 64, Lu = 71)

A. $Lu³⁺$ $\mathrm{Lu}^{3+}$
B. $Gd³⁺$ $\mathrm{Gd}^{3+}$
C. $Eu²⁺$ $\mathrm{Eu}^{2+}$
D. $Ce⁴⁺$ $\mathrm{Ce}^{4+}$

Solution

Related Formula

Electronic configuration of Eu = [Xe] 4f⁷ 6s²

Core Logic

The most common and stable oxidation state for lanthanoids is $+3$ $+3$. In the case of Europium:

Eu²⁺ = [Xe] 4f⁷

This configuration possesses a highly stable half-filled $f$ $f$-subshell. However, because the $+3$ $+3$ state is universally favored by thermodynamics in solution, $Eu²⁺$ $\text{Eu}^{2+}$ readily undergoes oxidation to lose one more electron:

Eu²⁺ arrow Eu³⁺ + 1e^-

By releasing an electron to stabilize into the $+3$ $+3$ state, it behaves as a potent reducing agent.

Step 1: Evaluation

Conversely, $Ce⁴⁺$ $\text{Ce}^{4+}$ acts as a powerful oxidizing agent to return to $+3$ $+3$, while $Lu³⁺$ $\text{Lu}^{3+}$ and $Gd³⁺$ $\text{Gd}^{3+}$ are already perfectly configured at their native stable limits.

Pattern Recognition

Europium( $II$ $II$) has a stable half-filled $f⁷$ $f^7$ configuration, yet easily loses an electron to attain the highly stable $+3$ $+3$ state typical of lanthanoids, making it a strong reducing agent.

Chapter Mix

Class 12 Chemistry: d and f Block Elements

Q75 jee_main_2024_29_january_evening Properties of Zinc, Cadmium and Mercury

Which of the following statements are correct about Zn, Cd and

Hg

? A. They exhibit high enthalpy of atomization as the d-subshell is full. B. Zn and Cd do not show variable oxidation state while Hg shows

+I

and

+II

. C. Compounds of Zn, Cd and Hg are paramagnetic in nature. D. Zn, Cd and Hg are called soft metals. Choose the most appropriate from the options given below:

A. B, D only
B. B, C only
C. A, D only
D. C, D only

Solution

Related Formula

General configuration of Group 12: (n-1)d¹⁰ ns²

Core Logic

Analyzing each statement based on inorganic chemistry principles:

Statement A is false: Because their $d$ $d$-subshell is completely full ( $d¹⁰$ $d^{10}$), these elements do not form strong metallic bonds. As a result, they exhibit the lowest enthalpy of atomization in their respective periods.
Statement B is true: $Zn$ $\text{Zn}$ and $Cd$ $\text{Cd}$ show only a stable $+2$ $+2$ oxidation state, whereas $Hg$ $\text{Hg}$ exhibits variable states forming both $+1$ $+1$ (as $Hg₂²⁺$ $\text{Hg}_2^{2+}$) and $+2$ $+2$.
Statement C is false: With a fully paired $d¹⁰$ $d^{10}$ subshell, their compounds lack unpaired electrons and are explicitly diamagnetic.
Statement D is true: Due to weak metallic bonds, these elements have low melting points and are classified as soft metals.

Step 1: Selection Verification

Statements B and D are true, matching choice (1).

Pattern Recognition

Group 12 metals have a full $d¹⁰$ $d^{10}$ subshell, leading to exceptionally weak metallic bonding, low enthalpies of atomization, and diamagnetic characteristics.

Chapter Mix

Class 12 Chemistry: d and f Block Elements

The number of valence electrons present in the metal among Cr$\mathrm{Cr}$, Co$\mathrm{Co}$, Fe$\mathrm{Fe}$ and Ni$\mathrm{Ni}$ which has the lowest enthalpy of atomisation is:

Solution & Explanation

Core Logic

Pattern Recognition

Chapter Mix

Reference Study Guides

More d- and f-Block Elements Previous-Year Questions — Page 6

Solution

Formulas Used

Core Logic

Pattern Recognition

Solution

Related Formula

Core Logic

Pattern Recognition

Chapter Mix

Solution

Core Logic

Step 1: Compare with Given Equation

Pattern Recognition

Chapter Mix

Solution

Related Formula

Core Logic

Step 1: Evaluation

Pattern Recognition

Chapter Mix

Solution

Related Formula

Core Logic

Step 1: Selection Verification

Pattern Recognition

Chapter Mix

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The number of valence electrons present in the metal among $Cr$ $\mathrm{Cr}$, $Co$ $\mathrm{Co}$, $Fe$ $\mathrm{Fe}$ and $Ni$ $\mathrm{Ni}$ which has the lowest enthalpy of atomisation is: