Given below are two statements: Statement I: The first ionisation enthalpy of oxygen is lower than that of nitrogen. Statement II: An electron removed from oxygen comes from a doubly occupied 2p orbital, where electron-electron repulsion is greater. Choose the correct answer.

Both Statement I and Statement II are correct

Statement I is correct but Statement II is incorrect

Both Statement I and Statement II are incorrect

Statement I is incorrect but Statement II is correct

Moving from left to right across a period, the atomic radius generally

decreases, because effective nuclear charge increases

stays constant, because shielding balances nuclear charge

increases, because the number of shells increases

decreases, because the number of shells decreases

Classification of Elements & Periodicity

11 NEET-UG practice questions on Classification of Elements & Periodicity, part of Chemistry. The first 11 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Given below are two statements: Statement I: The first ionisation enthalpy of oxygen is lower than that of nitrogen. Statement II: An electron removed from oxygen comes from a doubly occupied 2p orbital, where electron-electron repulsion is greater. Choose the correct answer.
1. ABoth Statement I and Statement II are correct
2. BStatement I is correct but Statement II is incorrect
3. CBoth Statement I and Statement II are incorrect
4. DStatement I is incorrect but Statement II is correct
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Answer: A · Both Statement I and Statement II are correct
Oxygen (2p⁴) has one 2p orbital holding two electrons, whose mutual repulsion makes one of them easier to remove. Nitrogen (2p³) has only singly occupied 2p orbitals. Both statements are therefore true.
- B: Statement II correctly describes the 2p⁴ arrangement of oxygen.
- C: Both statements match the data for N and O.
- D: Statement I correctly gives O below N.
Remember this
Oxygen's lower ionisation enthalpy comes from repulsion in its paired 2p electrons.
Question 2 · Medium
Match List-I with List-II. List-I (atomic number) A. 20 B. 26 C. 35 D. 58 List-II (block of the periodic table) I. f-block II. s-block III. p-block IV. d-block
1. AA-IV, B-II, C-III, D-I
2. BA-II, B-III, C-IV, D-I
3. CA-II, B-IV, C-III, D-I
4. DA-II, B-IV, C-I, D-III
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Answer: C · A-II, B-IV, C-III, D-I
Z = 20 is Ca, with its last electron in 4s. Z = 26 is Fe (3d), Z = 35 is Br (4p) and Z = 58 is Ce, a lanthanoid (4f).
- A: This swaps Ca and Fe, which are s-block and d-block in turn.
- B: This swaps Fe and Br, which are d-block and p-block in turn.
- D: This swaps Br and Ce, which are p-block and f-block in turn.
Remember this
The block is named after the subshell that receives the last electron by the aufbau order.
Question 3 · Hard
An element X of the third period shows these successive ionisation enthalpies (kJ/mol): 578, 1817, 2745, 11577, 14842. From this data, the formula of its oxide is
1. AX₂O
2. BX₂O₃
3. CXO₂
4. DXO
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Answer: B · X₂O₃
The large jump comes between the third and fourth values, so X has three valence electrons and the fourth comes from a noble gas core. X therefore forms X³⁺, and with O²⁻ it gives X₂O₃.
- A: X₂O needs X⁺, but the jump is not after the first value.
- C: XO₂ needs X⁴⁺, but the fourth electron comes from the inner core.
- D: XO needs X²⁺, but the jump is not after the second value.
Remember this
The largest jump in successive ionisation enthalpies marks the number of valence electrons.
Question 4 · Medium
A third-period oxide forms a salt with dilute HCl and also forms a salt with aqueous NaOH. Which oxide behaves in this way?
1. ANa₂O
2. BAl₂O₃
3. CCl₂O₇
4. DMgO
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Answer: B · Al₂O₃
Oxides change from strongly basic to strongly acidic across the period, with an amphoteric oxide between. Al₂O₃ gives AlCl₃ with HCl and an aluminate with NaOH.
- A: Na₂O is strongly basic and forms salts with acids only.
- C: Cl₂O₇ is strongly acidic and forms salts with bases only.
- D: MgO is basic and forms salts with acids only.
Remember this
Across period 3, Na₂O is strongly basic, Al₂O₃ amphoteric and Cl₂O₇ strongly acidic.
Question 5 · Easy
Moving from left to right across a period, the atomic radius generally
1. Adecreases, because effective nuclear charge increases
2. Bstays constant, because shielding balances nuclear charge
3. Cincreases, because the number of shells increases
4. Ddecreases, because the number of shells decreases
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Answer: A · decreases, because effective nuclear charge increases
Electrons enter the same shell while the nuclear charge rises, and they shield one another poorly. The outer electrons are pulled in more strongly, so atoms get smaller.
- B: Shielding within a shell is too weak to balance the rising nuclear charge.
- C: The number of shells increases down a group, not across a period.
- D: The number of shells stays the same across a period.
Remember this
Across a period the shell number stays fixed and effective nuclear charge rises, so radius falls.
Question 6 · Easy
Among B, C, N and O, the highest first ionisation enthalpy belongs to
1. AO
2. BB
3. CC
4. DN
Answer and explanation in LearnFree account
Question 7 · Medium
Arrange the following elements in increasing order of first ionisation enthalpy. A. Mg B. Al C. Si D. Na
1. AD, B, A, C
2. BB, D, A, C
3. CD, A, B, C
4. DD, B, C, A
Answer and explanation in LearnFree account
Question 8 · Hard
Which of the following statements are correct? A. The first ionisation enthalpy of Be is greater than that of B. B. The electron gain enthalpy of O is less negative than that of S. C. The first ionisation enthalpy of S is greater than that of P. D. The second ionisation enthalpy of Na is greater than that of Mg.
1. AB, C and D only
2. BA, B and D only
3. CA, C and D only
4. DA and B only
Answer and explanation in LearnFree account
Question 9 · Medium
Arrange the following isoelectronic ions in decreasing order of ionic radius. A. Al³⁺ B. Mg²⁺ C. N³⁻ D. F⁻
1. AC, D, A, B
2. BA, B, D, C
3. CD, C, B, A
4. DC, D, B, A
Answer and explanation in LearnFree account
Question 10 · Easy
Among the halogens listed, which has the most negative electron gain enthalpy?
1. AF
2. BCl
3. CBr
4. DI
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Question 11 · Medium
Assertion A: The electron gain enthalpy of fluorine is less negative than that of chlorine. Reason R: An electron added to the compact 2p subshell of fluorine suffers strong repulsion from the electrons already present. Choose the correct answer.
1. AA is correct but R is not correct
2. BBoth A and R are correct and R is the correct explanation of A
3. CBoth A and R are correct but R is not the correct explanation of A
4. DA is not correct but R is correct
Answer and explanation in LearnFree account

Classification of Elements & Periodicity

Given below are two statements: Statement I: The first ionisation enthalpy of oxygen is lower than that of nitrogen. Statement II: An electron removed from oxygen comes from a doubly occupied 2p orbital, where electron-electron repulsion is greater. Choose the correct answer.

Match List-I with List-II. List-I (atomic number) A. 20 B. 26 C. 35 D. 58 List-II (block of the periodic table) I. f-block II. s-block III. p-block IV. d-block

An element X of the third period shows these successive ionisation enthalpies (kJ/mol): 578, 1817, 2745, 11577, 14842. From this data, the formula of its oxide is

A third-period oxide forms a salt with dilute HCl and also forms a salt with aqueous NaOH. Which oxide behaves in this way?

Moving from left to right across a period, the atomic radius generally

Among B, C, N and O, the highest first ionisation enthalpy belongs to

Arrange the following elements in increasing order of first ionisation enthalpy. A. Mg B. Al C. Si D. Na

Arrange the following isoelectronic ions in decreasing order of ionic radius. A. Al³⁺ B. Mg²⁺ C. N³⁻ D. F⁻

Among the halogens listed, which has the most negative electron gain enthalpy?

Assertion A: The electron gain enthalpy of fluorine is less negative than that of chlorine. Reason R: An electron added to the compact 2p subshell of fluorine suffers strong repulsion from the electrons already present. Choose the correct answer.