Electronegativity and Bond Polarity: Types of Chemical Bonds

Electronegativity and Bond Polarity

Introduction to Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. Understanding electronegativity is essential for predicting bond polarity and the type of chemical bond formed between atoms.

• Electronegativity increases from left to right across a period on the periodic table.

• Electronegativity decreases going down a group.

• Nonmetals generally have higher electronegativity values, with fluorine (F) being the most electronegative element.

• Metals have lower electronegativity values.

Electronegativity and the Periodic Table

The periodic trends in electronegativity help predict how atoms will interact in chemical bonds. The following table summarizes representative electronegativity values for selected elements:

Group	1A	2A	3A	4A	5A	6A	7A	8A
H	2.1
Li	1.0	Be 1.5	B 2.0	C 2.5	N 3.0	O 3.5	F 4.0	Ne —
Na	0.9	Mg 1.2	Al 1.5	Si 1.8	P 2.1	S 2.5	Cl 3.0	Ar —
K	0.8	Ca 1.0	Ga 1.6	Ge 1.8	As 2.0	Se 2.4	Br 2.8	Kr —

Additional info: The table above is a simplified representation of periodic trends in electronegativity.

Bond Polarity and Types of Bonds

Polarity of Bonds

The difference in electronegativity between two bonded atoms determines the polarity of the bond:

• If electrons are shared equally, the bond is nonpolar covalent.

• If electrons are shared unequally, the bond is polar covalent.

• If electrons are transferred, the bond is ionic.

Example:

• In the H2 molecule, both atoms have the same electronegativity, so the bond is nonpolar.

• In HCl, chlorine is more electronegative than hydrogen, so the bond is polar.

Nonpolar Covalent Bonds

A nonpolar covalent bond occurs between nonmetals with equal or nearly equal sharing of electrons. The electronegativity difference is small (0.0 to 0.4).

Atoms	Electronegativity Difference	Type of Bond
N—N	3.0 – 3.0 = 0.0	Nonpolar covalent
Cl—Br	3.0 – 2.8 = 0.2	Nonpolar covalent
H—Si	2.1 – 1.8 = 0.3	Nonpolar covalent

Polar Covalent Bonds

A polar covalent bond occurs between nonmetal atoms with unequal sharing of electrons. The electronegativity difference is moderate (0.5 to 1.8).

Atoms	Electronegativity Difference	Type of Bond
O—Cl	3.5 – 3.0 = 0.5	Polar covalent
Cl—C	3.0 – 2.5 = 0.5	Polar covalent
O—S	3.5 – 2.5 = 1.0	Polar covalent

Dipoles and Bond Polarity

A dipole is created in a polar covalent bond due to the separation of charges as a result of unequal electron sharing. The positive and negative ends of the dipole are indicated by:

• The lowercase Greek letter delta ($\delta^+$ or $\delta^-$) to show partial charges.

• An arrow pointing from the positive to the negative end of the dipole.

Examples:

• $\ce{C^{\delta+}—O^{\delta-}}$

• $\ce{N^{\delta+}—O^{\delta-}}$

• $\ce{Cl^{\delta+}—F^{\delta-}}$

Ionic Bonds

An ionic bond forms between metal and nonmetal ions, involving the transfer of electrons. The electronegativity difference is large (greater than 1.8).

Atoms	Electronegativity Difference	Type of Bond
Cl—K	3.0 – 0.8 = 2.2	Ionic
N—Na	3.0 – 0.9 = 2.1	Ionic

Summary: Variations in Bonding

The type of chemical bond can be predicted using the difference in electronegativity ($\Delta EN$) between two atoms:

• $0.0 \leq \Delta EN \leq 0.4$: Nonpolar covalent bond (electrons shared equally)

• $0.5 \leq \Delta EN \leq 1.8$: Polar covalent bond (electrons shared unequally)

• $\Delta EN > 1.8$: Ionic bond (electrons transferred)

• $1.8 \leq \Delta EN \leq 2.0$: Bond type depends on the elements involved (metal + nonmetal = ionic; two nonmetals = covalent)

Electronegativity and Bond Types Table

Electronegativity Difference	Bond Type	Electron Bonding
0.0 to 0.4	Nonpolar covalent	Electrons shared equally
0.5 to 1.8	Polar covalent	Electrons shared unequally
1.9 to 3.3	Ionic	Electrons transferred

Learning Check: Practice Problems

Use the electronegativity difference to identify the type of bond (nonpolar covalent [NP], polar covalent [P], or ionic [I]) between the following pairs:

1. K—N

2. N—O

3. Cl—Cl

4. H—Cl

Additional info: To solve, subtract the electronegativity values and compare to the ranges above.