JEE Main 2025 : Periodic Table & Periodicity – Class 11 Chemistry (Ionisation Enthalpy Trend)
JEE Main 2025 Question :
The atomic number of the element from the following with lowest 1st ionisation enthalpy is:
(A) 32
(B) 35
(C) 19
(D) 87
Answer : (D)
Step 1: Understand Ionisation Enthalpy Trends
First ionisation enthalpy is the energy required to remove the outermost electron from a gaseous atom.
- It generally decreases down a group.
- It generally increases across a period in the periodic table.
Thus, the lowest ionisation enthalpy is typically found in elements that:
- are large in size,
- have their valence electrons furthest from the nucleus,
- experience less effective nuclear charge.
These are usually alkali metals in lower periods.
Step 2: Identify the Elements from the Given Atomic Numbers
(A) $Z = 32$: Germanium (Ge) → Group 14, Period 4 (metalloid)
(B) $Z = 35$: Bromine (Br) → Group 17, Period 4 (halogen)
(C) $Z = 19$: Potassium (K) → Group 1, Period 4 (alkali metal)
(D) $Z = 87$: Francium (Fr) → Group 1, Period 7 (alkali metal)
Step 3: Compare Ionisation Enthalpies
Francium (Fr, $Z = 87$):
Group 1, Period 7.
Largest atom among the given options.
At the bottom of Group 1 → very low ionisation enthalpy.
Potassium (K, $Z = 19$):
Group 1, Period 4.
Alkali metal but much smaller than Francium.
Germanium (Ge, $Z = 32$) and Bromine (Br, $Z = 35$):
Period 4 elements but not alkali metals.
Higher effective nuclear charge.
Smaller atomic radius → higher ionisation enthalpy.
Since ionisation enthalpy decreases down a group,
$$ \text{Fr} < \text{K} < \text{Ge}, \text{Br} $$
Therefore, Francium ($Z = 87$) has the lowest first ionisation enthalpy.
Final Answer
The atomic number of the element with the lowest 1st ionisation enthalpy is:
$$ \boxed{87 \quad \text{(D)}} $$
Correct Option: (D) 87
Concept Takeaway
Exam Tip:
Ionisation enthalpy trends in the periodic table follow a predictable pattern: it increases across a period due to higher nuclear charge and decreases down a group because of increasing atomic size and shielding effect.
The lowest first ionisation enthalpy is found in heavy alkali metals like Francium.
This concept is crucial for mastering the chapter Periodic Table and Periodicity (Class 11 Chemistry).
It is often asked in JEE PYQs chapterwise, and students should prepare with study material, preparation notes, and download notes for effective revision.
Learning such periodic trends with clarity from Anand Classes helps in systematic preparation of chapterwise chemistry notes for competitive exams.
Best way to practice is using class 11 chemistry preparation notes, JEE PYQs chapterwise, and download notes study material for the chapter Periodic Table and Periodicity.
JEE Main 2025 : Periodic Table & Periodicity – Class 11 Chemistry (Bond Dissociation Enthalpy Trend)
JEE Main 2025 Question :
Given below are two statements:
Statement I: $H_2Se$ is more acidic than $H_2Te$
Statement II: $H_2Se$ has higher bond enthalpy for dissociation than $H_2Te$
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) Statement I is true but Statement II is false
(C) Both Statement I and Statement II are false
(D) Statement I is false but Statement II is true
Answer : Correct Option: (D) Statement I is false but Statement II is true
Step 1: Acidic Strength
The acidity of hydrides of Group 16 elements increases down the group in the periodic table.
Reason: The bond strength between hydrogen and the central atom decreases as the size of the atom increases, making it easier to release $H^+$ ions.
Thus,
$$ H_2Se < H_2Te $$
So, Statement I is false because $H_2Te$ is more acidic than $H_2Se$.
Step 2: Bond Enthalpy
Bond enthalpy refers to the energy required to dissociate a bond.
It decreases down the group due to weaker bonds forming as the atomic size increases.
Therefore,
$$ \Delta H: H_2Se > H_2Te $$
Numerical values:
- $H_2Se = 276 \, \text{kJ/mol}$
- $H_2Te = 238 \, \text{kJ/mol}$
Thus, Statement II is true because $H_2Se$ has higher bond enthalpy compared to $H_2Te$.
Step 3: Final Evaluation of Statements
- Statement I → False
- Statement II → True
Final Answer
$$ \boxed{\text{(D) Statement I is false but Statement II is true}} $$
Concept Takeaway
- Acidity of hydrides increases down a group because the bond with hydrogen becomes weaker.
- Bond enthalpy decreases down a group due to larger atomic size and weaker overlap.
Hence, $H_2Te$ is more acidic than $H_2Se$, but $H_2Se$ has higher bond enthalpy.
Exam Tip: In JEE Main, whenever you compare acidity of hydrides (like $H_2O, H_2S, H_2Se, H_2Te$), remember: Acidity increases down the group, but bond enthalpy decreases down the group.
This is an important concept in Periodic Table and Periodicity (Class 11 Chemistry).
This topic is frequently asked in JEE PYQs, and students can strengthen their preparation using well-organized study material, preparation notes, and downloadable class 11 chemistry notes from Anand Classes.
JEE Main 2025 : Periodic Table & Periodicity – Class 11 Chemistry (Ionic Radii Trend)
JEE Main 2025 Question :
Choose the incorrect trend in the atomic radii ($r$) of the elements:
(A) $r_{Rb} < r_{Cs}$
(B) $r_{At} < r_{Cs}$
(C) $r_{Br} < r_{K}$
(D) $r_{Mg} < r_{Al}$
Answer : Correct Option: (D) $r_{Mg} < r_{Al}$
Step 1: Analyze option A: $r_{Rb} < r_{Cs}$
Rubidium (Rb) and Cesium (Cs) are in the same group (Group 1, Alkali Metals). Cesium is below Rubidium in the periodic table.
Therefore, the atomic radius of Cesium is larger than Rubidium.
The trend $r_{Rb} < r_{Cs}$ is correct.
Step 2: Analyze option B: $r_{At} < r_{Cs}$
Astatine (At) is in Group 17 (Halogens) and Cesium (Cs) is in Group 1 (Alkali Metals). Both are in period 6.
Atomic radius generally decreases across a period, and alkali metals have the largest atomic radii in their period, while halogens have smaller atomic radii.
Thus, Cesium’s atomic radius is significantly larger than Astatine.
The trend $r_{At} < r_{Cs}$ is correct.
Step 3: Analyze option C: $r_{Br} < r_{K}$
Bromine (Br) is in Group 17 (Halogens) and Potassium (K) is in Group 1 (Alkali Metals). Both are in period 4.
Atomic radius generally decreases across a period.
Alkali metals have larger atomic radii than halogens in the same period.
The trend $r_{Br} < r_{K}$ is correct.
Step 4: Analyze option D: $r_{Mg} < r_{Al}$
Magnesium (Mg) and Aluminum (Al) are in the same period (Period 3). Mg is in Group 2, and Al is in Group 13.
Atomic radius generally decreases across a period due to increasing effective nuclear charge.
Thus, Magnesium has a larger atomic radius than Aluminum, so the trend $r_{Mg} < r_{Al}$ is incorrect.
Final Answer
$$ \boxed{\text{(D) } r_{Mg} < r_{Al}} $$
Concept Takeaway
- Atomic radius increases down a group and decreases across a period because of the effective nuclear charge.
- Alkali metals have the largest radii in their periods, while halogens have the smallest radii.
- Comparing Mg and Al in the same period illustrates how effective nuclear charge reduces atomic size across a period.
This concept is important in Periodic Table and Periodicity (Class 11 Chemistry).
Students can strengthen their preparation using download notes, study material, and preparation notes from Anand Classes, which are helpful for solving JEE PYQs chapterwise.
Exam Tip: Remember that atomic radius increases down a group and decreases across a period. A quick memory rule: “Down the group → bigger atom, across the period → smaller atom”. Always practice this trend using class 11 chemistry study material and preparation notes from Anand Classes to master JEE Main questions on Periodic Table and Periodicity.
JEE Main 2025 : Periodic Table & Periodicity – Class 11 Chemistry (Enthalpy of Atomisation Trend)
JEE Main 2025 Question :
The number of valence electrons present in the metal among Cr, Co, Fe, and Ni which has the lowest enthalpy of atomisation is:
(A) 10
(B) 6
(C) 9
(D) 8
Answer : Correct Option: (A) 10
Step 1: Understand enthalpy of atomisation and its relation to metallic bonding
Enthalpy of atomisation is the energy required to break one mole of bonds in a substance to form individual gaseous atoms. In metals, this relates to the strength of metallic bonding. Stronger metallic bonding arises from a larger number of delocalized electrons and effective overlap of atomic orbitals, leading to higher enthalpy of atomisation.
Step 2: Analyze the electronic configurations of Cr, Co, Fe, and Ni
- Cr (Chromium): $[Ar]3d^5 4s^1$, 6 valence electrons
- Co (Cobalt): $[Ar]3d^7 4s^2$, 9 valence electrons
- Fe (Iron): $[Ar]3d^6 4s^2$, 8 valence electrons
- Ni (Nickel): $[Ar]3d^8 4s^2$, 10 valence electrons
Step 3: Determine which metal has the lowest enthalpy of atomisation
Within a period, enthalpy of atomisation increases with the number of unpaired d-electrons, reaching a maximum around the middle of the transition series due to stronger metallic bonding. As the number of paired electrons increases, metallic bonding weakens, lowering enthalpy of atomisation.
- Cr has 6 valence electrons with 6 unpaired electrons → strong metallic bonding → high enthalpy of atomisation
- Fe has 8 valence electrons with 4 unpaired electrons → slightly weaker metallic bonding
- Co has 9 valence electrons with 3 unpaired electrons → weaker metallic bonding
- Ni has 10 valence electrons with 2 unpaired electrons → weakest metallic bonding among these metals
Step 4: Identify the number of valence electrons for the metal with the lowest enthalpy of atomisation
Nickel (Ni) has the fewest unpaired d-electrons, resulting in comparatively weaker metallic bonding and the lowest enthalpy of atomisation. Ni has 10 valence electrons (8 from 3d and 2 from 4s).
Final Answer
$$ \boxed{10} $$
Concept Takeaway
- Enthalpy of atomisation in transition metals depends on the number of unpaired d-electrons: more unpaired electrons → stronger metallic bonding → higher enthalpy of atomisation.
- Metals like Ni with mostly paired d-electrons have weaker metallic bonding and lower enthalpy of atomisation.
This concept is important in Periodic Table and Periodicity (Class 11 Chemistry).
Students can enhance their preparation using download notes, study material, and preparation notes from Anand Classes, which are helpful for solving JEE PYQs chapterwise.
Exam Tip: For transition metals, always check the number of unpaired d-electrons to predict enthalpy of atomisation. A simple memory rule: “More unpaired electrons → stronger bonding → higher enthalpy of atomisation; more paired electrons → weaker bonding → lower enthalpy of atomisation”. Practice with class 11 chemistry study material and preparation notes from Anand Classes to excel in Periodic Table and Periodicity questions.
JEE Main 2025 : Periodic Table & Periodicity – Class 11 Chemistry (Ionisation Enthalpy Trend of group 14 Elements)
JEE Main 2025 Question :
The group 14 elements A and B have the first ionisation enthalpy values of 708 and 715 kJ mol⁻¹ respectively. The above values are lowest among their group members. The nature of their ions A²⁺ and B⁴⁺ respectively is:
(A) both reducing
(B) oxidising and reducing
(C) both oxidising
(D) reducing and oxidising
Answer : Correct Option: (D) reducing and oxidising
Step 1: Understand the elements and periodic trend
- Elements A and B are in Group 14, which contains C, Si, Ge, Sn, Pb.
- The first ionisation enthalpy values are 708 and 715 kJ/mol, which are lowest in the group, indicating these elements are heavier (Sn or Pb).
- Trend: Ionisation enthalpy decreases down the group because the valence electrons are farther from the nucleus and experience less effective nuclear charge.
- These heavier elements are more likely to show the inert pair effect, which stabilizes the +2 oxidation state relative to +4.
Step 2: Oxidation states and inert pair effect
- Group 14 elements generally show +4 oxidation state (loss of all 4 valence electrons: 2 from ns and 2 from np orbitals) and +2 oxidation state (loss of only 2 np electrons; the ns² “inert pair” remains).
- The inert pair effect is more pronounced in heavier elements like Sn and Pb. This means the +2 state becomes more stable, and the +4 state becomes less stable.
Step 3: Behavior of A²⁺
- A²⁺ has lost 2 electrons (likely from the np orbital) and retains the ns² inert pair.
- Since A²⁺ can further lose 2 electrons to form A⁴⁺, it can act as a reducing agent, donating electrons in a redox reaction.
- Example: Pb²⁺ can be oxidized to Pb⁴⁺:
$$ \text{Pb}^{2+} \rightarrow \text{Pb}^{4+} + 2e^- $$
Step 4: Behavior of B⁴⁺
- B⁴⁺ has lost all 4 valence electrons, reaching the maximum oxidation state for Group 14 elements.
- It cannot lose more electrons, but it can accept electrons to reduce to the +2 state.
- Therefore, B⁴⁺ acts as an oxidising agent, taking electrons from other species:
$$ \text{Pb}^{4+} + 2e^- \rightarrow \text{Pb}^{2+} $$
Step 5: Combine the behaviors
- A²⁺ → reducing agent (can be oxidized to +4)
- B⁴⁺ → oxidising agent (can be reduced to +2)
Final Answer
$$ \boxed{\text{(D) reducing and oxidising}} $$
Concept Takeaway
- The inert pair effect explains why heavier Group 14 elements favor +2 oxidation state over +4.
- A²⁺ is reducing because it can lose electrons to reach +4.
- B⁴⁺ is oxidising because it can gain electrons to reach +2.
- Understanding oxidation states, periodic trends, and the inert pair effect is essential for Periodic Table and Periodicity questions in JEE.
This concept is important in Periodic Table and Periodicity (Class 11 Chemistry).
Students can strengthen their preparation using download notes, study material, and preparation notes from Anand Classes, which are helpful for solving JEE PYQs chapterwise.
Exam Tip: For heavier Group 14 elements like Sn and Pb, always check for the inert pair effect. Remember the memory rule: “+2 ions → reducing; +4 ions → oxidising”. Practicing such examples with class 11 chemistry study material and preparation notes from Anand Classes helps in mastering Periodic Table and Periodicity questions for JEE Main.
JEE Main 2025 : Periodic Table & Periodicity – Class 11 Chemistry (Ionisation Enthalpy Trend of Transition Metal Ions)
JEE Main 2025 Question :
The incorrect relationship in the following pairs in relation to ionisation enthalpies is:
(A) Mn²⁺ < Fe²⁺
(B) Mn⁺ < Mn²⁺
(C) Mn⁺ < Cr⁺
(D) Fe²⁺ < Fe³⁺
Answer : Correct Option: (A) Mn²⁺ < Fe²⁺
Step 1: Understand ionisation enthalpy and influencing factors
- Ionisation enthalpy (IE) is the energy required to remove an electron from a gaseous atom or ion.
- Factors affecting IE include:
- Nuclear charge (Z) – higher Z increases IE.
- Shielding effect – more inner electrons reduce IE.
- Atomic size – larger atoms have lower IE.
- Stability of electronic configuration – half-filled or fully-filled orbitals are extra stable.
Step 2: Analyze each option
- Option A: Mn²⁺ < Fe²⁺
- Mn²⁺: $[\text{Ar}]3d^5$ → half-filled 3d⁵, stable configuration.
- Fe²⁺: $[\text{Ar}]3d^6$ → removing an electron from Mn²⁺ (stable half-filled) requires more energy than removing from Fe²⁺.
- Therefore, Mn²⁺ > Fe²⁺, making the given statement incorrect. ❌
- Option B: Mn⁺ < Mn²⁺
- Successive ionisation enthalpies increase with increasing positive charge.
- IE of Mn²⁺ > IE of Mn⁺. ✅ Correct statement.
- Option C: Mn⁺ < Cr⁺
- Mn⁺: $[\text{Ar}]3d^5 4s^1$
- Cr⁺: $[\text{Ar}]3d^5$
- Removing an electron from Cr⁺ (half-filled 3d⁵) requires more energy than from Mn⁺. ✅ Correct statement.
- Option D: Fe²⁺ < Fe³⁺
- IE increases with increasing positive charge.
- Removing an electron from Fe³⁺ requires more energy than from Fe²⁺. ✅ Correct statement.
Step 3: Identify the incorrect relationship
- The incorrect relationship is:
$$ \boxed{\text{(A) Mn²⁺ < Fe²⁺}} $$
Concept Takeaway
- Half-filled or fully-filled d-subshells are unusually stable, increasing ionisation enthalpy.
- Successive ionisation enthalpy always increases with higher positive charge.
- Always compare electronic configurations carefully when analyzing IE trends in transition metals.
This concept is important in Periodic Table and Periodicity (Class 11 Chemistry).
Students can strengthen their preparation using download notes, study material, and preparation notes from Anand Classes, which are helpful for solving JEE PYQs chapterwise.
Exam Tip: When analyzing ionisation enthalpies of ions, first check for half-filled or fully-filled d-subshell stability. Then consider successive ionisation energies. Practicing such comparisons with class 11 chemistry preparation notes from Anand Classes ensures faster accuracy in JEE Main Periodic Table questions.
