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Electronegativity

Understanding the ability of atoms to attract electrons in chemical bonds

What is Electronegativity?

Electronegativity is a measure of an atom's ability to attract the shared electrons in a chemical bond. It's not about gaining electrons (that's electron affinity) or the energy to remove them (ionization energy)—it's about how strongly an atom PULLS on shared electrons when bonded to another atom. Most commonly measured on the Pauling scale (0-4.0), where higher values = stronger pull on electrons.

📊 Factors Affecting Electronegativity

Four main factors determine electronegativity:

1. Atomic Size

Smaller atoms = Higher electronegativity

Smaller atoms have the nucleus closer to the valence electrons, exerting stronger pull on shared electrons in a bond.

Example: F (smallest) = 3.98, I (largest) = 2.66. F pulls much harder on bonded electrons.

2. Effective Nuclear Charge

More protons = Higher electronegativity

More protons in the nucleus = stronger positive charge pulling on bonded electrons.

Example: C = 2.55, N = 3.04, O = 3.44. As you go across, more protons increase pull.

3. Electron Configuration

Valence electrons matter most

Atoms with more valence electrons (especially p-block) are more electronegative. They "want" more electrons to fill their shell.

Example: Halogens are highly electronegative because they need just one more electron for a full octet

4. Energy Availability

Can the atom stabilize extra electrons?

Electronegativity relates to both IE and EA. An atom that strongly repels removal but strongly attracts addition is highly electronegative.

Example: Nonmetals are highly electronegative, metals are not. Nonmetals want shared electrons.

📈 Periodic Trends in Electronegativity

Trend 1: Across a Period (Left to Right) → Electronegativity Increases
Moving left to right across a period, electronegativity steadily increases. Atoms get smaller and have more protons, pulling harder on shared electrons.
Observation: Period 2 shows clear increase from Li (0.98) to F (3.98). Alkali metals very low, halogens very high.
Trend 2: Down a Group (Top to Bottom) → Electronegativity Decreases
Moving down a group, electronegativity decreases. Atoms get larger, and valence electrons are farther from the nucleus, weakening their pull on bonded electrons.
Halogen Period Atomic Size Electronegativity (Pauling)
Fluorine (F) 2 Smallest 3.98
Chlorine (Cl) 3 Small 3.16
Bromine (Br) 4 Large 2.96
Iodine (I) 5 Very Large 2.66
Pattern: F is the MOST electronegative element (3.98). Even Cl, larger and more shielded, is significantly less electronegative.
Trend 3: Metals vs Nonmetals
Metals have LOW electronegativity (they give up electrons easily). Nonmetals have HIGH electronegativity (they attract electrons). This explains metallic vs nonmetallic character!
Category Electronegativity Range Examples Behavior
Alkali Metals 0.7 - 1.3 Li (0.98), Na (0.93) VERY willing to give up electrons in bonds
Alkaline Earth 1.0 - 1.6 Mg (1.31), Ca (1.00) Willing to give up electrons
Nonmetals 2.5 - 3.5 C (2.55), O (3.44) Want to share/pull electrons
Halogens 2.66 - 3.98 F (3.98), Cl (3.16) VERY hungry for shared electrons
Key Insight: Electronegativity difference drives bond character. High difference = ionic bond. Low difference = covalent bond.
The Electronegativity Scale (Pauling)
Understanding the Numbers:
0 - 1.5 Metallic: Prefers to lose electrons. Examples: Li, Na, K
1.5 - 2.0 Mixed: Weakly nonmetallic. Examples: B, Si, P
2.0 - 2.5 Covalent: Form covalent bonds. Examples: C, H, I
2.5 - 3.0 Covalent (polar): Form polar covalent bonds. Examples: N, Cl, S
3.0 - 4.0 Nonmetallic: Very hungry for electrons. Examples: F, O, N

🔗 Electronegativity & Bond Type

Electronegativity Difference Determines Bond Type:
EN Difference Bond Type Examples Electron Behavior
< 0.5 Nonpolar Covalent H-H, C-C, Cl-Cl Electrons shared equally
0.5 - 1.7 Polar Covalent H-F, C-N, H-O Electrons pulled toward more electronegative atom
> 1.7 Ionic Na-Cl, Mg-O, Ca-F Electrons transferred (not really shared)
Practical Use: If you know the electronegativity of two atoms, you can predict if their bond will be ionic, polar, or nonpolar!

🔬 Why This Matters