What is a Covalent Bond?
When two non-metal atoms come close together, they can share electrons with each other. This sharing creates a covalent bond — one of the most important types of chemical bonds in chemistry.
A chemical bond formed by the sharing of a pair of electrons between two non-metal atoms, so that each atom achieves a full outer electron shell (usually 8 electrons, following the octet rule).
Think about why atoms bond in the first place: atoms are most stable when their outer electron shell is full. Non-metals, which are on the right-hand side of the periodic table, can reach this stable arrangement not by transferring electrons (as happens in ionic bonding, where a metal transfers electrons to a non-metal), but by sharing them.
Imagine two students who each need two textbooks to complete a set. Instead of one student buying both books (like ionic bonding — complete transfer), they decide to share the books — each gets to use both. Both students benefit, and both reach their goal. This is covalent bonding!
The shared pair of electrons is attracted to both nuclei at the same time, and this mutual attraction holds the two atoms together. The bond is the region between the atoms where the shared electrons spend most of their time.
Drawing Covalent Bonds — Dot-and-Cross Diagrams
Chemists use dot-and-cross diagrams (also called Lewis structures) to show how electrons are arranged in covalent molecules. Each atom's electrons are drawn as dots or crosses, and the shared pairs sit in the overlapping region between atoms.
Rules for drawing dot-and-cross diagrams:
- Draw only the outer shell electrons of each atom.
- Use dots (•) for electrons from one atom and crosses (×) for electrons from the other.
- Shared pairs sit between the two atoms, in the overlapping region.
- Lone pairs (non-bonding pairs) sit on the outside of atoms.
- Check that each atom has a full outer shell (usually 8 electrons; hydrogen only needs 2).
Example: Hydrogen molecule (H₂)
Hydrogen has 1 electron in its outer shell. It needs 1 more to reach a full shell of 2.
Two hydrogen atoms each contribute 1 electron → they share 1 pair of electrons.
In the dot-and-cross diagram, one dot (from one H atom) and one cross (from the other H atom) are drawn between the two hydrogen atoms, showing the shared pair. Each hydrogen atom now counts 2 electrons in its shell. ✓
This shared pair is called a single covalent bond.
Example: Water (H₂O)
Oxygen has 6 outer electrons — it needs 2 more to complete its shell.
Each hydrogen needs 1 more electron.
Oxygen forms 2 single bonds, one with each hydrogen.
Oxygen ends up with: 2 shared pairs + 2 lone pairs = 8 electrons ✓
Each hydrogen ends up with: 1 shared pair = 2 electrons ✓
Note that the 2 lone pairs on oxygen are drawn on the outside of the oxygen atom, not between atoms.
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