What is a Covalent Bond?

 

What is a Covalent Bond?

A covalent bond is a type of chemical bond formed by the mutual sharing of electrons between two atoms to achieve stable electronic configuration . (usually octet).

* Covalent bonding generally occurs between non-metallic elements.

* The shared electrons are counted in the valence shell of both atoms.

Properties of Covalent Compounds

Physical State         -   Usually gases or liquids, some solids

Melting/Boiling Point     - Low (except giant covalent structures like diamond)

Electrical Conductivity    - Poor conductors (no free ions/electrons)

Solubility     -  Soluble in non-polar solvents (e.g., benzene)

Important Terms

Bond Pair:  Electron pair involved in bonding.

Lone Pair: Electron pair not involved in bonding.

Electronegativity: Tendency of an atom to attract shared electrons.

Formation of Covalent Bond

Example: Hydrogen Molecule (H₂)

Each hydrogen atom has 1 electron and needs 1 more to complete its duplet.

Two hydrogen atoms share 1 pair of electrons:

H • + • H → H:H or H–H

Example: Oxygen Molecule (O₂)

Each oxygen atom has 6 valence electrons, needs 2 more.

Share 2 pairs of electrons (double bond):

O = O

 

Lewis Structures

* Lewis symbols represent valence electrons as dots.

* A Lewis structure shows:

                   Bond pairs (shared electrons)

                   Lone pairs (unshared electrons)

Example: Water (H₂O)

 

Coordinate (Dative) Covalent Bond

A coordinate covalent bond is a type of covalent bond in which both electrons in the shared pair are donated by only one atom.

* Represented by an arrow (→)

* Still behaves like a covalent bond

Example: Ammonium Ion (NH₄⁺)

NH₃ + H⁺ → NH₄⁺

 

Non-Polar Covalent Bonds

Definition:

A non-polar covalent bond is formed when the electrons are equally shared between two atoms.

* Equal sharing of electrons

* Usually between identical atoms or atoms with very small difference in electronegativity (∆EN ≈ 0)

* No partial charges on atoms

* Molecule is electrically neutral and symmetrical

Examples:

Molecule     Structure     Electronegativity Difference (∆EN)

H₂                      H–H                          0

Cl₂                    Cl–Cl                          0

O₂                    O=O                          0

CH₄                   C–H                       0.4 (considered nearly non-polar)

 

 Polar Covalent Bonds

 Definition:

A polar covalent bond is formed when the electrons are unequally shared between two atoms with a significant difference in electronegativity.

Key Features:

* usually ∆EN between 0.4 and 1.7.

* Unequal sharing of electrons .The atom with higher electronegativity attracts the shared electrons more strongly

* Leads to partial charges:

δ⁻ (partial negative charge) on more electronegative atom

δ⁺ (partial positive charge) on less electronegative atom

* Molecule may have a net dipole moment

Examples:

Molecule     Structure     ∆EN (Difference)      Nature

HCl                    H–Cl                 0.9                        Polar

H₂O                   H–O–H             1.4                       Polar

NH₃                     N–H                  0.9                       Polar

HF                       H–F                  1.9               Highly polar

 

Definition of Electronegativity

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

* Depends on          -  Atomic size, nuclear charge, and shielding

* Units  -                   No units (relative scale)

* Highest value           Fluorine (F) – 4.0

* Lowest                   Cesium (Cs) – 0.7

Factors Affecting Electronegativity

1.  Atomic Size-   Smaller atom → higher EN

Electrons are closer to the nucleus → stronger attraction.

2.  Nuclear Charge (Z)

More protons = stronger pull on electrons = higher EN

3. Electron Shielding / Screening Effect

More inner electrons = more shielding → lower EN

4. Hybridization

Greater s-character → higher EN

EN of sp >  EN of sp² >  EN of sp³

Electronegativity difference (∆EN) to predict bond type:

∆EN Range                  Bond Type

0 – 0.4                           Non-Polar Covalent

0.4 – 1.7                         Polar Covalent

> 1.7                                 Ionic Bond

 

Electronegativity of Common Elements (Pauling Scale)

Element                 Symbol          Electronegativity

Hydrogen                   H                     2.1

Carbon                        C                      2.5

Nitrogen                     N                     3.0

Oxygen                        O                     3.5

Fluorine                       F                4.0 (highest)

Chlorine                      Cl                    3.0

Bromine                      Br                    2.8

Iodine                          I                       2.5

Sulfur                          S                      2.5

Phosphorus                P                     2.1

Sodium                      Na                    0.9

Magnesium              Mg                   1.2

Aluminum                 Al                    1.5

Silicon                         Si                     1.8

Calcium                     Ca                    1.0

Potassium                  K                      0.8

Lithium                       Li                     1.0

Boron                                    B                       2.0

Quick Notes:

* Fluorine (F) has the highest electronegativity: 4.0

* Metals (like Na, K, Ca) generally have low electronegativity

* Non-metals (like O, N, Cl) have high electronegativity

* Noble gases usually don’t have electronegativity values (as they don’t form bonds easily)

Electronegativity Order (for non-metals)

F > O > N ≈ Cl > Br > C ≈ S > H > P