Chemical Bonding: Why Atoms Stick Together
3.2. Chemical Bond
Definition
Define: What is a chemical bond?
A chemical bond is a force of attraction between atoms which holds them together in the form of a molecule or a compound.
Attractive vs Repulsive Forces
When atoms approach each other, two main types of forces are at play:
Important Information:
The arrangement of electrons around the nucleus of an atom in shells and sub-shells is called electronic configuration.
Types of Bonds
We will explore three main types of chemical bonds:
- Ionic bond
- Covalent bond
- Coordinate covalent bond
3.2.1. Ionic Bond
Why do atoms form ionic bonds? What do they try to achieve?
A chemical bond is formed as a result of the tendency of atoms to lose or gain electrons to acquire the electronic configuration of the nearest noble gas, because this is a more stable electronic structure.
Formation of Sodium Chloride (NaCl)
Let’s visualize the formation of sodium chloride from sodium and chlorine.
Na: 2, 8, 1
Cl: 2, 8, 7
Na⁺ (2,8)
Cl⁻ (2,8,8)
(Electrostatic attraction)
Formation of Calcium Chloride (CaCl₂)
Calcium, an alkaline earth metal, loses two electrons to form calcium chloride (CaCl₂):
Ca: 2, 8, 8, 2
Ca²⁺
Cl: 2, 8, 7
Cl: 2, 8, 7
Definition of Ionic Bond
An ionic bond is therefore a bond which is formed by the complete transference of electron or electrons from one atom to another atom.
3.2.2 Covalent Bond
During the formation of an ionic bond, atoms lower their energy by transferring electrons. However, this is not the only way atoms can lower their energy. Some atoms decrease their energy by mutually sharing their electrons.
Forces Acting During Covalent Bond Formation
Let’s see how attractive and repulsive forces interact when two atoms form a covalent bond.
Definition of Covalent Bond
A covalent bond is a bond formed by the mutual sharing of an electron pair provided by the bonded atoms. This is called a single covalent bond.
Types of Covalent Bonds
Atoms can share one, two, or three pairs of electrons:
Single Covalent Bond
One shared electron pair
Double Covalent Bond
Two shared electron pairs
Triple Covalent Bond
Three shared electron pairs
Formation of Covalent Compounds (H₂O, CO₂)
Water Molecule (H₂O)
Structure of H₂O:
(Each H shares 1 electron with O; O shares 1 electron with each H, and has 2 lone pairs)
Carbon Dioxide (CO₂)
Structure of CO₂:
(Carbon shares 4 electrons with two oxygen atoms; each oxygen shares 2 electrons and has 2 lone pairs)
Exercise:
What type of elements form covalent bonds?
Covalent bonds are typically formed between two non-metal atoms. This is because non-metals have a high electronegativity and tend to achieve a stable noble gas configuration by sharing electrons rather than by losing or gaining them completely.
How is a covalent bond different from an ionic bond?
The primary difference lies in the mechanism of bond formation:
- Ionic bonds: Formed by the complete transfer of electrons from one atom (typically a metal) to another (typically a non-metal), resulting in the formation of oppositely charged ions that attract each other.
- Covalent bonds: Formed by the mutual sharing of electron pairs between two atoms (typically non-metals).
(For the “Draw electron dot and cross structures” exercise, an interactive drawing tool would be complex for this format. This would be a great classroom or textbook activity!)
3.2.3 Coordinate Covalent Bond
Definition and Formation
What makes a coordinate covalent bond special?
A coordinate covalent bond is a type of covalent bond in which the shared electron pair is donated by one atom only.
- The molecule donating the pair is called a donor.
- The molecule accepting it is called an acceptor.
An arrow head (→) pointing toward the acceptor represents this type of bond.
Example 1: Hydronium Ion (H₃O⁺) Formation
Acids provide protons (H⁺) when dissolved in water. Watch how water forms a hydronium ion.
Example 2: Reaction Between NH₃ and BF₃
Ammonia (NH₃) has a lone pair, and Boron Trifluoride (BF₃) has an empty orbital. Observe the donation.
Additional Example: Ammonium Chloride and Protonated Ethyl Alcohol
In ammonium chloride, a coordinate covalent bond links nitrogen of ammonia and the proton. The positive charge is spread all over the ammonium ion. All four bonds between nitrogen and hydrogen in ammonium ion behave exactly alike.
This proves that the difference between a covalent bond and a coordinate covalent bond lies in the way they are formed. Once these bonds are formed, there does not remain any difference between them.