Single, Double, and Triple Bonds: Understanding Bonding Pairs
Imagine you're holding two pieces of string, tying them together with a single knot. This knot represents a single bond. Now, picture tying two or three knots instead—these represent double and triple bonds, respectively.
What is a Covalent Bond?
Covalent bond
A covalent bond is a chemical bond formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the bonded atoms.
In other words, it forms when two atoms share electrons to achieve a stable electron configuration, often resembling the noble gases with a "full outer shell" of electrons. Each shared pair of electrons creates one covalent bond.
But what happens when atoms share more than one pair of electrons? That’s where single, double, and triple bonds come into play.
Analogy
Think of covalent bonds as bridges between atoms.
- A single bond is like a simple wooden bridge, a double bond is a stronger, reinforced steel bridge, and a triple bond is the most robust, like a suspension bridge.
- Each additional "layer" of the bridge represents more shared electrons, making the bond stronger and shorter.
Bonding Pairs: Single, Double, and Triple Bonds
Single Bonds: Sharing One Pair of Electrons
Single bond
A single bond occurs when two atoms share one pair of electrons.
This is the simplest type of covalent bond. A single line (—) represents a single bond in a Lewis structure.
Example
The hydrogen molecule (H₂) forms when two hydrogen atoms each contribute one electron to share a pair. This shared pair creates a single bond:
Double Bonds: Sharing Two Pairs of Electrons
Double bond
A double bond forms when two atoms share two pairs of electrons.
This type of bond is stronger and shorter than a single bond. A double line (=) represents a double bond in a Lewis structure.
Example
In an oxygen molecule (O₂), each oxygen atom shares two electrons with the other, forming a double bond:
Common Mistake
- When drawing double bonds, students often forget that each atom must still satisfy the octet rule (or duet rule for hydrogen).
- For example, in O₂, each oxygen atom has four non-bonding electrons in addition to the shared pairs.
Triple Bonds: Sharing Three Pairs of Electrons
Triple bond
A triple bond occurs when two atoms share three pairs of electrons.
This bond is the strongest and shortest of the three types. A triple line (≡) represents a triple bond in a Lewis structure.
Example
In a nitrogen molecule (N₂), each nitrogen atom shares three electrons with the other, forming a triple bond:
Example
Draw the Lewis structure of acetylene (C₂H₂).
- Each carbon atom forms a triple bond with the other and a single bond with hydrogen, represented as:
Bond Length and Bond Strength
Now that you understand the types of bonds, let’s explore how they differ in terms of bond length and bond strength.
Bond Length: How Far Apart Are the Atoms?
Bond length
Bond length is the average distance between the nuclei of two bonded atoms.
Trend: As the number of shared electron pairs increases, the bond length decreases.
This is because more shared electrons create a stronger attraction between the nuclei, pulling the atoms closer together.
Single bonds > Double bonds > Triple bonds (in terms of length)
Bond Strength: How Much Energy is Needed to Break the Bond?
Bond strength
Bond strength refers to the energy required to break a bond.
Trend: As the number of shared electron pairs increases, the bond strength increases.
More shared electrons mean a stronger electrostatic attraction between the atoms.
Triple bonds > Double bonds > Single bonds (in terms of strength)
Tip
Remember that shorter bonds are stronger because the nuclei are held more tightly together by the shared electrons.
Reflection and Connections
Think about the bonds in your body and the world around you. From the oxygen you breathe (
Self review
- What is the relationship between bond length and bond strength?
- How does the number of bonding pairs affect molecular stability?
- Why are triple bonds shorter and stronger than single bonds?
Theory of Knowledge
- How does understanding bond strength and length help us explain molecular stability in different environments?
- For instance, why is nitrogen gas (
