Chemistry (Combined Science)
Key chemistry topics: atomic structure, bonding, quantitative chemistry, and chemical changes.
In this topic
Atomic Structure
Atoms have a nucleus (protons + neutrons) surrounded by electrons in shells. - Protons: +1 charge, mass 1 - Neutrons: 0 charge, mass 1 - Electrons: -1 charge, negligible mass
Electronic structure determines chemical properties. Elements in the same group have similar properties because they have the same number of outer electrons.
Isotopes: same element, same protons, different neutrons.
Key Points
- Atomic number = protons = electrons (in neutral atom)
- Mass number = protons + neutrons
- Group number = outer electrons
- Period number = number of shells
Example Questions
2Potassium has atomic number 19 and mass number 39. How many protons, neutrons, and electrons?
19 protons, 20 neutrons (39-19), 19 electrons
[2 marks]
Bonding and Structure
Three types of bonding: 1. Ionic (metal + non-metal): electrons transferred, giant lattice, high MP, conducts when molten/dissolved 2. Covalent (non-metal + non-metal): electrons shared, simple molecular or giant covalent 3. Metallic (metal + metal): sea of delocalised electrons, conducts, malleable
Simple molecular: low MP (weak intermolecular forces), don't conduct Giant covalent (diamond, graphite): very high MP, diamond doesn't conduct, graphite does
Key Points
- Ionic: transfer electrons, form ions, strong electrostatic attraction
- Covalent: share electrons, strong bonds within molecules
- Metallic: delocalised electrons explain conductivity
- Properties depend on structure, not just bonding
Example Questions
4Explain why sodium chloride has a high melting point but methane has a low melting point.
NaCl has a giant ionic lattice with strong electrostatic forces between ions — lots of energy needed. Methane is a simple molecular substance with weak intermolecular forces between molecules — little energy needed to overcome them.
[4 marks]