Chemistry Inorganic - How to Solve: Periodic Table Trends (IIT JEE)

How to Solve: Periodic Table Trends (IIT JEE)

Introduction

"Mastering periodic trends doesn’t just get you 5–10 marks in IIT JEE—it’s the key to predicting chemical reactivity, bond strength, and even why your phone battery works. One question on ionisation energy or electronegativity can decide whether you clear JEE Main or miss the cut-off by 1 mark."

WHAT YOU NEED TO KNOW FIRST

Before diving into trends, ensure you understand:
1. Electronic configuration (Aufbau principle, Pauli exclusion, Hund’s rule).
2. Effective nuclear charge (Z_eff) – How inner electrons shield outer electrons from the nucleus.
3. Periodic table structure – Groups (columns) and periods (rows), s/p/d/f blocks.

KEY TERMS & FORMULAS

1. Ionisation Energy (IE)

Definition: Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms in the ground state. Formula: IE = E_final (ion) – E_initial (atom) - MEMORISE THIS: IE increases across a period (left → right) and decreases down a group (top → bottom). - Exceptions: Half-filled and fully-filled subshells (e.g., Be > B, N > O).

2. Electronegativity (EN)

Definition: Ability of an atom to attract shared electrons in a covalent bond. Scale: Pauling scale (F = 4.0, highest). - MEMORISE THIS: EN increases across a period and decreases down a group. - No formula – given on exam sheet as a table.

3. Atomic Radius (AR)

Definition: Half the distance between two identical bonded atoms. - Covalent radius (for non-metals), metallic radius (for metals), van der Waals radius (for noble gases). - MEMORISE THIS: AR decreases across a period (due to increasing Z_eff) and increases down a group (due to new shells).

4. Electron Gain Enthalpy (Δ_egH)

Definition: Energy released when 1 mole of electrons is added to 1 mole of gaseous atoms. Formula: X(g) + e⁻ → X⁻(g) + Δ_egH - MEMORISE THIS: - More negative = higher affinity for electrons. - Trend: Becomes more negative across a period (except noble gases, which have positive Δ_egH). - Exceptions: Group 2 (Be, Mg) and Group 15 (N, P) have less negative Δ_egH due to stable configurations.

STEP-BY-STEP METHOD

Step 1: Identify the Trend Direction

• Across a period (left → right):
• IE ↑, EN ↑, AR ↓, Δ_egH becomes more negative (except noble gases).
• Down a group (top → bottom):
• IE ↓, EN ↓, AR ↑, Δ_egH becomes less negative.

Step 2: Check for Exceptions

• IE Exceptions:
• Group 13 (B, Al): Lower IE than Group 2 (Be, Mg) due to p-electron shielding.
• Group 16 (O, S): Lower IE than Group 15 (N, P) due to electron repulsion in p⁴.
• Δ_egH Exceptions:
• Noble gases: Positive Δ_egH (no affinity for electrons).
• Group 2 (Be, Mg): Less negative Δ_egH (stable s² configuration).
• Group 15 (N, P): Less negative Δ_egH (half-filled p³ stability).

Step 3: Compare Two Elements

• If in the same period: Compare Z_eff (higher Z_eff = smaller AR, higher IE/EN).
• If in the same group: Compare number of shells (more shells = larger AR, lower IE/EN).
• If diagonally placed: Use Z_eff vs. shielding trade-off (e.g., Li vs. Mg).

Step 4: Apply to Bonding & Reactivity

• Higher EN = More polar bonds (e.g., H-F > H-Cl).
• Lower IE = More metallic character (e.g., Na > Mg).
• More negative Δ_egH = More likely to form anions (e.g., Cl > S).

WORKED EXAMPLES

Example 1 – Basic: Compare IE of Na and Mg

Question: Which has higher ionisation energy, Na or Mg? Solution:
1. Same period (Period 3).
2. Mg has higher Z_eff (12 vs. 11).
3. IE increases across a period.
4. Mg has higher IE than Na. What we did and why: Compared Z_eff in the same period—higher Z_eff means stronger nuclear pull, so higher IE.

Example 2 – Medium: Compare AR of O²⁻ and F⁻

Question: Which has a larger atomic radius, O²⁻ or F⁻? Solution:
1. Both are isoelectronic (10 electrons).
2. O²⁻ has 8 protons, F⁻ has 9 protons.
3. Higher nuclear charge (F⁻) pulls electrons closer.
4. O²⁻ has a larger radius than F⁻. What we did and why: Compared nuclear charge in isoelectronic species—more protons = smaller radius.

Example 3 – Exam-Style: Which has the highest EN?

Question: Arrange in increasing order of electronegativity: C, N, O, F. Solution:
1. All in Period 2.
2. EN increases left → right.
3. Order: C < N < O < F.
4. F has the highest EN (4.0 on Pauling scale). What we did and why: Applied the period trend—EN increases with Z_eff.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is your 60-second cheat sheet for periodic trends:
1. Across a period (left → right): - IE ↑, EN ↑, AR ↓, Δ_egH becomes more negative (except noble gases).
2. Down a group (top → bottom): - IE ↓, EN ↓, AR ↑, Δ_egH becomes less negative.
3. Exceptions to remember: - IE: Be > B, N > O. - Δ_egH: Noble gases (positive), Group 2 (less negative), Group 15 (less negative).
4. Isoelectronic species? More protons = smaller radius.
5. Bonding? Higher EN = more polar bonds. Lower IE = more metallic. Now go crush that exam!