Chapter 3
Bonds and Forms of Molecules
Lesson One: Lewis Dot Symbols and Ionic Bonds
Lewis Dot Symbols
Lewis dot symbols represent the valence electrons of an atom as dots placed around the element symbol.
The number of dots equals the number of valence electrons (group number for main-group elements).
Dots are placed one at a time on each of the four sides of the symbol before pairing begins.
Na· (1 dot — Group 1A)
·Mg· (2 dots — Group 2A)
·Ċl: (7 dots — Group 7A)
Paired dots = lone pairs; unpaired dots = bonding electrons
Types of Chemical Bonds
Chemical bonds form because bonded atoms have lower energy than isolated atoms.
There are three main types of strong (primary) bonds: ionic, covalent, and metallic.
| Bond Type | Between | Transfer/Share | Example |
|---|---|---|---|
| Ionic | Metal + Non-metal | Transfer of electrons | NaCl |
| Covalent | Non-metal + Non-metal | Sharing of electrons | H₂O |
| Metallic | Metal + Metal | Sea of delocalised electrons | Cu, Fe |
Ionic Bond
An ionic bond forms by the complete transfer of one or more electrons from a metal atom to a non-metal atom.
The metal loses electrons → forms a positive cation.
The non-metal gains electrons → forms a negative anion.
The electrostatic attraction between cation and anion is the ionic bond.
Ionic compounds form giant lattice structures — not discrete molecules.
High melting and boiling points due to strong electrostatic forces.
Conduct electricity when molten or dissolved in water (ions are free to move).
Do NOT conduct electricity in solid state (ions are fixed in lattice).
G.R. — Ionic compounds conduct electricity when molten or dissolved: because the ions are free to move and carry charge. In the solid state, ions are held in fixed positions in the lattice and cannot move.
Electronegativity and Bond Type
Electronegativity (EN) is the ability of an atom in a molecule to attract the shared pair of electrons towards itself.
The difference in electronegativity (ΔEN) between two bonded atoms determines the bond type.
| ΔEN | Bond Type | Example |
|---|---|---|
| 0 | Pure covalent (nonpolar) | H–H, Cl–Cl, O=O |
| 0.1 – 1.7 | Polar covalent | H–Cl, H–O, H–N |
| > 1.7 | Ionic | Na–Cl, Mg–O |
G.R. — HF is more polar than HCl: because fluorine has a higher electronegativity than chlorine, so the ΔEN in H–F is greater, making the bond more polar.