Electronegativity
The "measuring stick" for predicting bond types between atoms
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It's the critical link between the Periodic Table and Chemical Bonding.
1. Definition
Electronegativity is defined as a measure of the tendency of an atom to attract a bonding pair of electrons.
The Pauling Scale
Electronegativity is quantified using the Pauling scale, ranging from approximately 0.7 to 4.0.
Fluorine (4.0) is the most electronegative element. Francium (0.7) is the least electronegative.
2. Periodic Table Trends
Across a Period
Electronegativity INCREASES
Atomic radius decreases, stronger nuclear pull
Li (1.0) → F (4.0)
Down a Group
Electronegativity DECREASES
Electrons further from nucleus, more shielding
F (4.0) → At (2.2)
Electronegativity Trend on the Periodic Table
2.2 He
1.0 Be
1.5 B
2.0 C
2.5 N
3.0 O
3.5 F
4.0 Ne
← EN increases →
3. Predicting Bond Types
Calculate the difference in electronegativity (ΔEN) between two bonding atoms:
Bond Type Predictor
Select two elements to calculate ΔEN:
Element 1:
Element 2:
Non-polar Covalent
ΔEN = 0.0
Equal sharing of electrons
Example: H₂, Cl₂
Non-polar Covalent
ΔEN < 0.4
Equal sharing of electrons
H₂, Cl₂, N₂
Polar Covalent
ΔEN 0.4 - 1.7
Unequal sharing
H₂O, HCl
Ionic Bond
ΔEN > 1.7
Electron transfer
NaCl, MgO
4. Polarity of Bonds
In a polar covalent bond, the more electronegative atom pulls electrons closer, becoming slightly negative (δ-), while the other becomes slightly positive (δ+).
Water (H₂O) - Polar Covalent Bond
Oxygen (EN = 3.5) pulls electrons away from Hydrogen (EN = 2.1)
ΔEN = 1.4 → Polar Covalent
Hydrogen Chloride (HCl)
Chlorine (EN = 3.0) pulls electrons away from Hydrogen (EN = 2.1)
ΔEN = 0.9 → Polar Covalent
Sodium Chloride (NaCl) - Ionic
Sodium (EN = 0.9) transfers electron to Chlorine (EN = 3.0)
ΔEN = 2.1 → Ionic Bond (complete transfer)
Electronegativity Summary
| Bond Type | ΔEN Range | Electron Behavior | Example | ΔEN |
|---|---|---|---|---|
| Non-polar Covalent | < 0.4 | Equal sharing | H₂ | 2.1 - 2.1 = 0.0 |
| Non-polar Covalent | < 0.4 | Equal sharing | Cl₂ | 3.0 - 3.0 = 0.0 |
| Polar Covalent | 0.4 - 1.7 | Unequal sharing | HCl | 3.0 - 2.1 = 0.9 |
| Polar Covalent | 0.4 - 1.7 | Unequal sharing | H₂O | 3.5 - 2.1 = 1.4 |
| Ionic | > 1.7 | Electron transfer | NaCl | 3.0 - 0.9 = 2.1 |
| Ionic | > 1.7 | Electron transfer | MgO | 3.5 - 1.3 = 2.2 |
Test Your Understanding
1. Which element has the highest electronegativity?
2. What happens to electronegativity as you go down a group?
3. If ΔEN = 1.2 between two atoms, what type of bond will form?
4. In a polar covalent bond, the more electronegative atom becomes:
Key Terms
Key Takeaways
- Electronegativity measures an atom's attraction for bonding electrons (Pauling scale 0.7-4.0)
- Fluorine (4.0) is most electronegative; Francium (0.7) is least
- Trends: Increases across a period, decreases down a group
- ΔEN < 0.4 = Non-polar covalent (equal sharing, e.g., H₂)
- ΔEN 0.4-1.7 = Polar covalent (unequal sharing, e.g., H₂O)
- ΔEN > 1.7 = Ionic (electron transfer, e.g., NaCl)
- In polar bonds, more electronegative atom becomes δ-, less becomes δ+
