🔹 Definition

Electronegativity is the tendency of an atom to attract the shared pair of electrons towards itself in a chemical bond.

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Important Points

• Applies only to bonded atoms
• It is a relative (dimensionless) value
• Most commonly used scale → Pauling scale

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Trends in Periodic Table

Across a Period (→)

Increases

• Reason: Increase in nuclear charge → stronger pull on electrons

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Down a Group (↓)

Decreases

• Reason: Increase in size → weaker attraction

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🔹 Important Order

Fluorine (F) is the most electronegative element$$\textbf{F > O > N > Cl > Br > I}$$

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Factors Affecting Electronegativity

1. Atomic Size
   • Smaller atom → higher electronegativity
2. Nuclear Charge
   • More protons → stronger attraction
3. Shielding Effect
   • More shielding → lower electronegativity
4. Hybridisation
   • sp > sp² > sp³

More s-character → electrons closer to nucleus

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Important Exceptions

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❗ Noble Gases

• Usually no electronegativity
• (Because they rarely form bonds)

Pauling Scale (Electronegativity) — JEE/NEET Concept

The Pauling scale is the most commonly used scale to measure electronegativity, proposed by Linus Pauling.

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Basic Idea

Electronegativity is calculated based on:

✔ Bond energies (bond dissociation enthalpies)

• If a bond A–B is stronger than expected, it means:
  A and B have different electronegativities

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Formula (Conceptual)

Pauling related electronegativity difference to bond energy:$$\chi_A – \chi_B \propto \sqrt{D_{AB} – \frac{D_{AA} + D_{BB}}{2}}$$

Where:

• $D_{AB}$ = bond energy of A–B
• $D_{AA}, D_{BB}$​ = bond energies of A–A and B–B

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Key Points

1. Fluorine is highest
   • Value = 4.0 (maximum on scale)
2. Values are relative
   • Not absolute, just comparison
3. Dimensionless
   • No unit

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🔹 Example Values

Element	Electronegativity
F	4.0
O	3.5
N	3.0
Cl	3.0
H	2.1

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Important Applications

1. Predict Bond Type

• ΔEN ≈ 0 → Non-polar covalent
• ΔEN small → Polar covalent
• ΔEN large → Ionic

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2. Predict Polarity

• Larger difference → more polar bond

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Mulliken Scale — Electronegativity (JEE/NEET)

The Mulliken scale was proposed by Robert S. Mulliken.

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Basic Idea

Electronegativity depends on:

✔ Ionisation Enthalpy (IE)
✔ Electron Gain Enthalpy (EGE / Electron Affinity)

So, it considers both:

• Tendency to lose electron (IE)
• Tendency to gain electron (EGE)

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🔹 Formula

$$\chi = \frac{\text{IE} + \text{EGE}}{2}$$(In some books, electron affinity is used instead of EGE)

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🔹 Meaning

• Higher IE → atom doesn’t lose electrons easily
• More negative EGE → atom gains electrons easily

✔ So, higher value = higher electronegativity

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🔹 Key Points

1. Absolute scale
   • Based on measurable energies (unlike Pauling)
2. Applies to isolated atoms
   • Not bond-based
3. Units initially in energy
   • Often converted to dimensionless values

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🔹 Comparison with Pauling Scale

Feature	Mulliken	Pauling
Based on	IE + EGE	Bond energy
Type	Absolute	Relative
Concept	Atomic property	Bond property

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Basic Relation

The two scales are related approximately by:$$\chi_{\text{Pauling}} \approx \frac{\chi_{\text{Mulliken}}}{2.8}$$