What is a Covalent Bond?
Electron sharing in H₂
Sharing Electrons: The Hydrogen Molecule
A covalent bond forms when atoms share electrons to fill their outer shell.
In H₂, two hydrogen atoms share one electron each, creating a single shared pair.
Key Points:
- Covalent bond = electron sharing, not transfer.
- Hydrogen molecule has 1 shared pair (2 electrons).
- Sharing lets each H feel its shell is full.
Single Bond
One Pair Shared = Single Covalent Bond
A single covalent bond forms when two atoms share one pair of electrons.
In a hydrogen molecule (H₂), each hydrogen donates one electron to the shared pair.
Key Points:
- Exactly 2 electrons in the bond pair.
- Represented by a single dash: H–H.
Double Bond
Lewis structure of O₂ showing a double bond
What is a Double Covalent Bond?
A double covalent bond forms when two atoms share two pairs of electrons.
The oxygen molecule \(O_2\) is held together by one double bond.
Key Points:
- 2 shared electron pairs = 4 electrons.
- Represented by two lines: O = O.
- Oxygen \(O_2\) is a common example.
Triple Bond
Lewis structure of N₂
Three shared pairs
A triple bond forms when two atoms share three electron pairs.
Nitrogen molecule (N₂) is held together by this strong triple bond.
Key Points:
- 3 shared pairs = 6 bonding electrons.
- Notation: N≡N.
- Triple bonds are very strong, making N₂ inert.
Diamond – A Carbon Allotrope
3-D tetrahedral lattice of diamond
Why is diamond so hard and non-conductive?
Diamond is an allotrope of carbon where every atom forms four strong covalent bonds in a tetrahedral arrangement.
This rigid, endless 3-D lattice leaves no free electrons, making diamond extremely hard and a poor conductor.
Key Points:
- Allotrope: carbon atoms in a continuous tetrahedral network.
- Each carbon forms 4 sp³ covalent bonds.
- No mobile electrons, so electricity cannot pass.
- Strong bonds in all directions give extreme hardness.
Methane Molecule
Electron dot diagram of methane
Electron Dot Structure of Methane
Methane, formula CH₄, is the simplest alkane.
Carbon forms four single covalent bonds, one with each hydrogen.
Each bond shares a pair of electrons, filling carbon’s octet and hydrogen’s duet.
After this slide, you can draw methane’s electron dot structure yourself.
Key Points:
- Write element symbols: C in center, four H around.
- Place one dot from carbon and one from each hydrogen to form each bond.
- Check: carbon has 8 shared electrons, every hydrogen has 2.
Saturated: Ethane
Ball-and-stick model of ethane
Ethane is a saturated hydrocarbon
Ethane \(C_2H_6\) is an alkane with two carbon atoms and six hydrogens.
All its C–C and C–H bonds are single, leaving no space for extra atoms—so it is termed “saturated.”
Key Points:
- Alkanes contain only single covalent bonds.
- Single bonds make the molecule saturated.
- Ethane is the simplest saturated hydrocarbon after methane.
Unsaturated: Ethene
Structural formula of ethene showing the C=C bond.
Ethene – An Unsaturated Hydrocarbon
Ethene \( \mathrm{C_2H_4} \) is the simplest alkene and an unsaturated hydrocarbon.
Its carbon–carbon double bond is easy to spot and makes the molecule highly reactive.
Key Points:
- Has one C=C double bond.
- Unsaturated: fewer hydrogens than an alkane.
- Double bond can open for addition of atoms.
- Hence more reactive than ethane (single bonds only).
Isomers of Butane
n-butane (straight) vs iso-butane (branched)
Same formula, two different shapes
Butane has molecular formula \( \mathrm{C_4H_{10}} \).
It can join its four carbon atoms in a straight or branched chain, creating two distinct molecules.
Key Points:
- Different structures with the same formula are called isomers.
- Straight-chain isomer: n-butane.
- Branched isomer: iso-butane (2-methylpropane).
- Isomerism shows that one formula can yield different structures and properties.