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Classification of Elements & Periodicity

34 NEET-UG practice questions on Classification of Elements & Periodicity, part of Chemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

  1. 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
    Show answer

    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.

  2. 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
    Show answer

    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.

  3. 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
    Show answer

    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.

  4. 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
    Show answer

    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.

  5. 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
    Show answer

    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.

  7. 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

  8. 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

  9. 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

  11. 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

  12. Question 12 · Easy

    An element has the ground-state configuration 1s² 2s² 2p⁶ 3s² 3p⁴. Where is it placed in the long form of the periodic table?

    1. AGroup 16, period 4
    2. BGroup 16, period 3
    3. CGroup 6, period 3
    4. DGroup 14, period 3

    Answer and explanation in LearnFree account

  13. Question 13 · Medium

    Match List-I with List-II, considering only the four elements listed in List-II. List-I (property) A. Highest electronegativity B. Most negative electron gain enthalpy C. Highest first ionisation enthalpy D. Largest atomic radius List-II (element) I. Cs II. He III. F IV. Cl Choose the correct answer.

    1. AA-III, B-IV, C-II, D-I
    2. BA-IV, B-III, C-II, D-I
    3. CA-III, B-IV, C-I, D-II
    4. DA-IV, B-III, C-I, D-II

    Answer and explanation in LearnFree account

  14. Question 14 · Medium

    Arrange Mg, Al, P and S, all from period 3, from lowest to highest first ionisation enthalpy: A. Mg B. Al C. P D. S Choose the correct answer.

    1. AB, A, C, D
    2. BB, A, D, C
    3. CA, B, C, D
    4. DA, B, D, C

    Answer and explanation in LearnFree account

  15. Question 15 · Easy

    Electron gain enthalpies of the noble gases are

    1. Alarge and negative
    2. Blarge and positive
    3. Cexactly zero
    4. Dsmall and negative

    Answer and explanation in LearnFree account

  16. Question 16 · Hard

    Arrange in order of decreasing negative electron gain enthalpy (most negative first): A. O B. S C. F D. Cl Choose the correct answer.

    1. AC, D, A, B
    2. BD, C, A, B
    3. CC, D, B, A
    4. DD, C, B, A

    Answer and explanation in LearnFree account

  17. Question 17 · Medium

    Assertion A: The first ionisation enthalpy of beryllium is higher than that of boron. Reason R: The 2p electron of boron is shielded by the inner 2s electrons and is held less tightly than a 2s electron. Choose the correct answer.

    1. ABoth A and R are correct and R is the correct explanation of A
    2. BA is correct but R is not correct
    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

  18. Question 18 · Easy

    Lithium, the first element of group 1, resembles in many of its properties an element of the next group and next period. This diagonal partner of lithium is

    1. Asodium
    2. Bmagnesium
    3. Cberyllium
    4. Daluminium

    Answer and explanation in LearnFree account

  19. Question 19 · Medium

    Given below are two statements: Statement I: Like ionisation enthalpy, electronegativity is a directly measurable quantity with a fixed value for each element. Statement II: Across a period, the non-metallic character of elements increases as their electronegativity increases. Choose the correct answer.

    1. AStatement I is correct but Statement II is incorrect
    2. BBoth Statement I and Statement II are correct
    3. CStatement I is incorrect but Statement II is correct
    4. DBoth Statement I and Statement II are incorrect

    Answer and explanation in LearnFree account

  20. Question 20 · Hard

    Consider the following statements about periodic trends. A. The first ionisation enthalpy of oxygen is lower than that of nitrogen. B. The ionic radius of Mg²⁺ is larger than that of Na⁺. C. An anion is larger than its parent atom because the added electron increases electron–electron repulsion and lowers the effective nuclear charge per electron. D. Across a period, the electron gain enthalpy generally becomes more negative from left to right. Choose the correct answer from the options given below.

    1. AA and D only
    2. BA, C and D only
    3. CB, C and D only
    4. DA, B and C only

    Answer and explanation in LearnFree account

  21. Question 21 · Medium

    Isoelectronic species have the same number of electrons. Which set contains only species that are isoelectronic with neon?

    1. ANa⁺, Mg²⁺, F⁻ and O²⁻
    2. BLi⁺, Na⁺, Be²⁺ and Mg²⁺
    3. CMg²⁺, Al³⁺, S²⁻ and Cl⁻
    4. DNa⁺, K⁺, F⁻ and Cl⁻

    Answer and explanation in LearnFree account

  22. Question 22 · Medium

    The first ionisation enthalpy of sodium is 496 kJ mol⁻¹. How much energy is needed to convert 2.3 g of gaseous sodium atoms completely into gaseous Na⁺ ions? (Na = 23)

    1. A49.6 kJ
    2. B496 kJ
    3. C1141 kJ
    4. D4.96 kJ

    Answer and explanation in LearnFree account

  23. Question 23 · Medium

    Consider the following statements about periodic trends. A. Electronegativity generally increases from left to right across a period. B. Metallic character generally increases down a group. C. The first ionisation enthalpy of beryllium is higher than that of boron. D. The first ionisation enthalpy of oxygen is higher than that of nitrogen. Choose the correct answer from the options given below.

    1. AA, B and C only
    2. BB, C and D only
    3. CA, B, C and D
    4. DA and B only

    Answer and explanation in LearnFree account

  24. Question 24 · Easy

    Following the IUPAC rules for elements with atomic number above 100, the temporary name of element 109 is

    1. AUnunennium
    2. BUnniloctium
    3. CUnnilennium
    4. DUnnilunium

    Answer and explanation in LearnFree account

  25. Question 25 · Medium

    Gallium is slightly smaller in atomic radius than aluminium, mainly because

    1. Ait has fewer electron shells than aluminium does
    2. Bits 3d electrons shield the outer electrons poorly
    3. Cits outermost electrons occupy the 3p subshell
    4. Dits nuclear charge is smaller than that of aluminium

    Answer and explanation in LearnFree account

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