One line or two?
Single and double bonds.
Counting the pairs
Two hydrogen atoms form H₂. Two oxygen atoms form O₂. Hydrogen needs one more electron and oxygen needs two more.
How many shared pairs of electrons do you think hold the two atoms of an oxygen molecule together?
What this lesson covers
The idea
Two atoms sharing one pair of electrons are joined by a single bond, drawn as one line between their symbols; two shared pairs form a double bond, drawn as two lines.
Counting the pairs
Two hydrogen atoms form H₂. Two oxygen atoms form O₂. Hydrogen needs one more electron and oxygen needs two more.
How many shared pairs of electrons do you think hold the two atoms of an oxygen molecule together?
- Two
- One
- Three
Pick a molecule
Each line between the symbols is one shared pair of electrons.
Single and double bonds
If two atoms share one pair of electrons they are joined by a single bond, drawn as one line. If they share two pairs they are joined by a double bond, drawn as two lines.
Two atoms sharing one pair of electrons are joined by a single bond, drawn as one line between their symbols. Two shared pairs form a double bond, drawn as two lines.
Notes
One shared pair is a single bond (one line). Two shared pairs are a double bond (two lines), as in O=O.
Check yourself
How many pairs of electrons are shared in an oxygen molecule, O₂?
Answer: 2
The two oxygen atoms are joined by two pairs of shared electrons, which is a double bond, drawn O=O.
How is the chlorine molecule, Cl₂, drawn?
What is a double bond?
An oxygen atom has six valence electrons. How many more does it need to complete its octet, and what bond forms in O₂?
- Cl=Cl, a double bond. Each chlorine atom needs only one electron, so one pair is shared. That is a single bond.
- Cl-Cl, a single bond — correct. Yes!
- H-Cl, a single bond with hydrogen. Cl₂ is a molecule of two chlorine atoms. H-Cl is the compound hydrogen chloride.
- One shared pair of electrons, drawn as two lines. One shared pair is a single bond, drawn as one line.
- Two shared pairs of electrons, drawn as one line. Each shared pair is drawn as one line, so two pairs are drawn as two lines.
- Two shared pairs of electrons, drawn as two lines — correct. Yes!
- Two more, giving a double bond — correct. Yes!
- One more, giving a single bond. Oxygen needs 8 − 6 = 2 more electrons, so two pairs are shared in O₂.
- Two more, giving a single bond. Two shared pairs make a double bond, not a single bond.