The Big Idea
When two non-metal atoms come together, they share pairs of electrons to form covalent bonds. This shared pair makes both atoms feel like they have a full outer shell. The molecules that form this way have very strong bonds holding the atoms together, but weak forces between molecules—and that's why they usually melt and boil at low temperatures and can't conduct electricity.
Why this matters: Understanding covalent bonding explains everything about why water boils at 100°C, why sugar is a solid at room temperature, and why plastic coating on electrical wires doesn't conduct electricity.
What is a Covalent Bond?
Definition
A covalent bond is a bond formed when two atoms share a pair of electrons. This happens between non-metal atoms (atoms from the right side of the periodic table—things like oxygen, nitrogen, carbon, chlorine, hydrogen).
The key idea: each atom has electrons in its outer shell, and they want a full outer shell (noble gas configuration). Instead of losing or gaining electrons (like in ionic bonding), they share electrons with each other. Each shared electron counts towards *both* atoms' outer shells.
Why Do Atoms Form Covalent Bonds?
Think of it like two people wearing the same coat. Normally, you'd want your own coat to stay warm. But if you share a coat with someone, you both stay warm without anyone giving up their coat. Similarly:
- Hydrogen atom: Needs 1 more electron to fill its first shell (which holds max 2).
- Chlorine atom: Needs 1 more electron to fill its third shell (which holds max 8).
- Solution: They share one pair of electrons → both atoms now feel like they have full outer shells → H—Cl bond forms.
Key Rule: In a covalent bond, a pair of electrons is shared between two atoms. Both atoms count the same pair towards their own electron count.
Dot-and-Cross Diagrams
A dot-and-cross diagram is a way to show which electrons came from which atom. It's a visual way to prove the atoms are sharing:
- Electrons from atom A = shown as dots (•)
- Electrons from atom B = shown as crosses (×)
- Overlapping circles = the shells overlap, showing they share electrons
- The shared electrons sit in the overlap region of the two circles
Cl atom Cl atom
× •
×× × • × • After bonding:
× 17P × • 17P •
×× × • × • Shared pair
× • of electrons
BEFORE Cl — Cl
(chlorine molecule)
Dot-and-cross diagram: Two chlorine atoms sharing one pair of electrons
Common Mistake: Students often confuse dot-and-cross diagrams with ionic diagrams. Remember:
- Covalent = overlapping circles (atoms share electrons, stay together)
- Ionic = separated brackets (atoms have given/gained electrons, ions are apart)
Practice Question 1
1 Draw a dot-and-cross diagram for hydrogen chloride (HCl).
Hint: Hydrogen has 1 electron, chlorine has 7 electrons in its outer shell. They share 1 pair.
Types of Covalent Bonds
Single Covalent Bonds
A single bond is when two atoms share one pair of electrons (written as a single line: —).
Examples:
Double Covalent Bonds
Example: