Analysis of Periodic Trends: Ionization Energy, Electron Affinity, and Basic Character
Which of the following represents the correct trend for the mentioned property?

A. F > P > S > B    - First Ionization Energy
B. Cl > F > S > P    - Electron Affinity
C. K > Al > Mg > B    - Metallic character
D. K₂O > Na₂O > MgO > Al₂O₃    - Basic character

Choose the correct answer:
Correct Answer: B
(A, B and D only)

Detailed Analysis of Trends

A. First Ionization Energy (IE₁)

General trend: IE increases across a period and decreases down a group.
• F (Group 17, Period 2) is highest.
• P vs S: Phosphorus ([Ne] 3s² 3p³) has a half-filled p-orbital, making it more stable than Sulphur ([Ne] 3s² 3p⁴). Thus, P > S.
• B is in Period 2 but has lower Z than F and less stability than half-filled P.
Order: F > P > S > B is Correct.

B. Electron Affinity (EA)

General trend: EA increases across a period.
• Cl vs F: Chlorine has a higher EA than Fluorine because F is very small, leading to high inter-electronic repulsions that hinder the addition of an electron.
• S vs P: Sulphur (3p⁴) wants an electron more than Phosphorus (3p³ half-filled stable).
Order: Cl > F > S > P is Correct.

C. Metallic Character

General trend: Decreases across a period, increases down a group.
• Order should be K > Mg > Al > B.
• Magnesium (Group 2) is more metallic than Aluminum (Group 13).
Statement C says Al > Mg, which is Incorrect.

D. Basic Character of Oxides

Basic nature increases with the electropositivity of the metal.
• K is most electropositive, so K₂O is most basic.
• Al₂O₃ is amphoteric (least basic here).
Order: K₂O > Na₂O > MgO > Al₂O₃ is Correct.

Extensive Related Theory (2000+ Words Context)

1. Ionization Energy (IE) Fundamentals
Ionization Energy is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom. It is influenced by:

  • Nuclear Charge (Z): Higher charge means stronger pull on electrons, increasing IE.
  • Atomic Size: Larger radius means the outer electron is further from the nucleus, decreasing IE.
  • Screening Effect: Inner electrons shield the outer ones, decreasing the effective nuclear charge (\(Z_{eff}\)).
  • Electronic Configuration: Atoms with half-filled or fully-filled orbitals (like P, N, or noble gases) possess extra stability, resulting in unexpectedly high IE values.

2. Electron Affinity and Electron Gain Enthalpy
Electron Affinity is the energy released when an electron is added to a neutral gaseous atom. While it generally increases from left to right, the "Third Period Anomaly" is a critical exam topic. For groups 16 and 17, the 2nd-period elements (O, F) have lower EA than 3rd-period elements (S, Cl). This is because the 2p subshell is so compact that incoming electrons face significant repulsion from existing electrons. Chlorine, with its larger 3p subshell, accommodates the extra electron more comfortably, making it the element with the highest electron affinity in the entire periodic table.

3. Nature of Oxides and Hydroxides
The chemical nature of oxides follows a distinct pattern:

  • Metallic Oxides: Generally basic (e.g., Na₂O, K₂O). They react with water to form strong bases.
  • Non-Metallic Oxides: Generally acidic (e.g., SO₃, Cl₂O₇).
  • Metalloid/Weak Metal Oxides: Often amphoteric (e.g., Al₂O₃, ZnO, PbO), meaning they react with both acids and bases.
  • Neutral Oxides: A small subset like CO, NO, and N₂O do not show acidic or basic properties.
The basicity decreases across a period because the metallic character decreases and electronegativity increases.

4. Effective Nuclear Charge (\(Z_{eff}\))
Most periodic trends are governed by the balance between the actual nuclear charge and the repulsive forces of inner-shell electrons. According to Slater's Rules, \(Z_{eff} = Z - \sigma\). Across a period, the increase in \(Z\) outweighs the increase in the screening constant \(\sigma\), leading to a contraction in atomic size and an increase in electronegativity and ionization potential.

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Related Covered Topics

periodic table trends first ionization energy electron affinity anomaly metallic character basicity of oxides effective nuclear charge half-filled orbital stability amphoteric oxides electronegativity atomic radius slater's rules periodicity in properties

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