Simple Molecules and Covalent Bonds
2.5 Simple Molecules and Covalent Bonds
Definition of a Covalent Bond
- A covalent bond is formed when a pair of electrons is shared between two atoms.
- The shared electrons are attracted to the nuclei of both atoms, creating a strong bond.
- Covalent bonds allow atoms to achieve noble gas electronic configurations (full outer shells).
- Occurs between non-metal atoms (which have similar electronegativities and both want to gain electrons).
- Each shared pair of electrons represents one covalent bond.
- Notation in formulas:
- Single covalent bond: represented by one line (e.g., H–H)
- Double covalent bond: represented by two lines (e.g., O=O)
- Triple covalent bond: represented by three lines (e.g., N≡N)
General Characteristics of Covalent Bonding
- Involves electron sharing, not electron transfer.
- Bonds are strong due to the electrostatic attraction between the shared electrons and both nuclei.
- Molecules formed are discrete particles (simple molecules) rather than a continuous lattice.
- Can occur between:
- Atoms of the same element (e.g., Clâ‚‚, Oâ‚‚, Nâ‚‚)
- Atoms of different elements (e.g., Hâ‚‚O, CHâ‚„, COâ‚‚)
Dot-and-Cross Diagrams
- Represent valence (outer shell) electrons of atoms.
- Dots represent electrons from one atom, crosses represent electrons from another atom.
- Shared pairs are placed between the atoms to show covalent bonding.
- Remaining unshared pairs are called lone pairs.
- All atoms aim to follow the octet rule (8 electrons in outer shell), except hydrogen (duet rule: 2 electrons).
Formation of Covalent Bonds in Specific Simple Molecules
Hydrogen molecule (Hâ‚‚)
- Each hydrogen atom has 1 electron.
- To achieve a stable configuration (2 electrons like helium), each atom shares 1 electron with the other.
- Dot-and-cross diagram:
- H(•) + H(×) → H–H
- Single covalent bond formed.
Chlorine molecule (Clâ‚‚)
- Each chlorine atom has 7 valence electrons.
- Each needs 1 more electron to complete its octet.
- They share 1 electron each.
- Dot-and-cross diagram:
- Cl(••••••×) + Cl(•••••••) → Cl–Cl
- Single covalent bond formed.
Water molecule (Hâ‚‚O)
- Oxygen has 6 valence electrons; hydrogen has 1 valence electron.
- Oxygen needs 2 more electrons → shares one with each hydrogen.
- Each hydrogen achieves duet, oxygen achieves octet.
- Dot-and-cross diagram:
- O atom in the centre, 2 H atoms bonded, 2 lone pairs on O.
- 2 single covalent bonds.
Methane (CHâ‚„)
- Carbon has 4 valence electrons, hydrogen has 1 each.
- Carbon needs 4 more electrons → bonds with 4 hydrogen atoms.
- Each H gets duet; carbon gets octet.
- Structure: Tetrahedral shape, all single bonds.
Ammonia (NH₃)
- Nitrogen has 5 valence electrons.
- Needs 3 more electrons → bonds with 3 hydrogen atoms.
- Nitrogen has one lone pair after bonding.
- Pyramidal shape.
Hydrogen chloride (HCl)
- Hydrogen has 1 electron, chlorine has 7 electrons in the valence shell.
- Share one pair → hydrogen achieves duet, chlorine achieves octet.
- One single bond.
Methanol (CH₃OH)
- Carbon bonds with 3 hydrogens and 1 oxygen.
- Oxygen bonds with carbon and 1 hydrogen; has 2 lone pairs.
- Structure shows both C–H and O–H covalent bonds.
Ethene (Câ‚‚Hâ‚„)
- Each carbon has 4 valence electrons.
- Each carbon bonds with 2 hydrogens and double bonds with the other carbon.
- Double covalent bond = 2 shared pairs of electrons.
Oxygen molecule (Oâ‚‚)
- Each oxygen has 6 valence electrons.
- Needs 2 more → shares 2 pairs with the other oxygen atom.
- Forms a double bond.
Carbon dioxide (COâ‚‚)
- Carbon has 4 valence electrons; oxygen has 6.
- Each oxygen forms a double bond with carbon → linear molecule.
Nitrogen molecule (Nâ‚‚)
- Each nitrogen has 5 valence electrons.
- Needs 3 more → share 3 pairs of electrons (triple bond).
- Very strong and short bond.
Properties of Simple Molecular Compounds
Low melting points and boiling points
- Molecules held together by weak intermolecular forces (van der Waals forces).
- Only small amounts of energy needed to separate molecules.
- Covalent bonds within molecules are strong, but forces between molecules are weak.
Poor electrical conductivity
- Molecules have no free ions or delocalised electrons.
- Cannot conduct electricity in any state (solid, liquid, gas).
Solubility
- Many are soluble in non-polar solvents.
- Some polar covalent compounds dissolve in water (e.g., sugar), but most non-polar ones do not.
Comparison: Ionic vs Covalent
| Property | Ionic compounds | Simple molecular compounds |
|---|---|---|
| Bonding | Transfer of electrons | Sharing of electrons |
| Structure | Giant lattice | Discrete molecules |
| Melting/boiling pt | High | Low |
| Conductivity | Conduct when molten/aqueous | Never conduct |
| Strength of bonding | Strong ionic bonds | Strong covalent bonds within molecules, weak forces between |