Structure of Elements & Compounds
Bonding: The attraction(s) that hold particles of a substance together (ionic, covalent, or metallic).
Structure: The arrangement of a substance's particles and the type of bonding present determine its structure.
- The bonding/structure determines properties such as melting/boiling point, hardness, solubility in water, conductivity, etc.
- Structure → giant structures/simple structures
- Giant structures have large numbers of atoms/ions arranged regularly in a fixed ratio. There is no fixed total number of particles. Strong bonds hold the particles together. There are giant ionic, covalent, and metallic structures, depending on the bonding present.
- Simple molecular structures have individual molecules, each made up of a fixed number of atoms joined by strong covalent bonds. Between molecules are weaker intermolecular forces of attraction. The number of atoms per molecule is typically small, although it can be larger in some cases, such as DNA.
Types of Structures
Giant Metallic:
- All metals and alloys have giant metallic structures. Metallic bonds hold the metal atoms in place.
Giant Ionic:
- All solid ionic compounds have giant ionic structures. Ionic compounds are compounds containing ions.
Simple Molecular:
- Most non-metal elements and compounds have a simple molecular structure, exist as molecules. Covalent bonds bind atoms together to form molecules.
Exceptions:
- Diamond (carbon), graphite (carbon), and silicon dioxide are nonmetals that have a giant covalent structure. Covalent bonds hold the atoms together.
- The noble gases don't form bonds → can't exist as molecules → have simple atomic structure.
Non-metals Only
Bonding: Covalent → the electrostatic attraction between 2 or more positive nuclei and the shared pair of electrons.
Structure: Simple molecular or giant covalent.
1) Simple molecular properties:
The material is non-conductive.
- Covalent bonding: No ions/delocalised electrons → molecules are neutral.
- ∴ nothing to carry charge through structure.
2) Low m.p./b.p.:
- Weak intermolecular forces that don't require much energy to overcome.
- CH4 → C2H6 → C3H8 increasing m.p./b.p.
→ Molecules have larger relative molecular mass (Mr), ∴ and there are stronger, weaker intermolecular forces that require more energy to overcome.
3) Insoluble in water:
- Generally, molecules are neutral and aren't attracted to water.
Giant Covalent Structures
- Diamond, Graphite + SiO₂ (silicon dioxide)
- Graphite and diamond are allotropes of carbon.
Diamond (Carbon/Diamond);
- Each carbon atom is bonded to 4 others.
- Very hard, as strong covalent bonds throughout the structure require lots of force to break.
→ These items serve as jewellery and cutting/drilling tools.
- It never conducts electricity because it lacks ions or delocalised electrons, which prevents current flow.
Graphite (Carbon/Graphite):
- In hexagonal rings (3 carbons bonded to each other).
- Graphene = One layer of graphite that has uses in technology.
- Conducts electricity when solid (giant covalent structure) as 1 out of 4 electrons are delocalised ∴ those delocalised electrons can move throughout thestructure.
- The layers are soft because the weak forces between them allow them to slide over each other.
→ used as lubricants and in pencils.
- Both graphite and diamonds have a high b.p. (≈3000°C) as lots of energy is needed to break the many strong covalent bonds.
- C60: Buckminster fullerene is an allotrope of carbon with a simple molecular structure (i.e., it doesn't have that many carbons).
Structure and Bonding
Metals and Nonmetals:
- Ionic bonding: The electrostatic attraction between positive and negative ions.
- Structure: Giant ionic lattice
→ E.g., NaCl (1:1 ratio of ions in lattice)
(1) Properties:
High m.p./b.p.:
- Strong electrostatic attractions between positive and negative ions require lots of energy to break.
E.g., NaF → MgO
→ The m.p./b.p. increases due to a higher charge on the anion and cation.
→ Stronger EA between cations and anions means more energy is needed to separate ions.
(2) Conducts electricity when molten/aqueous.
- It has ionic bonding, which permits ions in liquids or aqueous solutions to flow freely, in contrast to solids where ions are immobile.
(3) Brittle
- Ions pack together in a regular lattice, producing crystals.
- If you force ions with the same charge next to each other, they will fall apart.
(4) Often, water is soluble.
- Water is a polar solvent, and there are significant attractions between polar water molecules and the ions in the lattice.
Metals Only:
- Metallic bonding: Electrostatic attraction between positive metal ions and delocalised electrons (sea).
- Structure: Giant metallic lattice
Properties:
(1) Malleable and ductile
Layers of cations can slide over each other, as there are no weak forces.
(2) Conducts electricity (when solid)
- Has metallic bonding and is a metal.
- The structure has a sea of delocalised electrons that are free to move throughout the structure.
(3) High m.p./b.p.:
- Strong electrostatic attraction between the positive metal ions and the sea of delocalised electrons requires more energy to break.
- Na → Mg → Al increasing m.p./b.p.
→ A higher charge on the metal ion results in more delocalised electrons.
- A stronger electrostatic attraction (EA) between cations and a sea of delocalised electrons requires more energy to overcome.
Alloy: A mixture of metals