Overview
This lecture covers the basics of atoms, elements, compounds, mixtures, atomic structure, and key terms for the Cambridge IGCSE Chemistry syllabus.
Elements, Compounds, and Mixtures
- An element is a pure substance made of only one type of atom.
- Elements are represented by chemical symbols (e.g., O for oxygen, H for hydrogen).
- Elements cannot be broken down into simpler substances.
- A compound is made when two or more different elements chemically combine.
- Compounds have properties different from the individual elements that form them.
- Chemical formulas (e.g., H₂O, CO₂) represent compounds.
- Compounds cannot be separated by physical methods; only chemical reactions can separate them.
- A mixture contains two or more substances (elements or compounds) that are not chemically bonded.
- Mixture components retain their individual properties and can be separated physically (e.g., filtration, distillation).
- Salt water is a mixture where salt dissolves in water but does not chemically bond.
Atoms, Molecules, and Compounds Explained
- An atom is the smallest indivisible unit of an element.
- A molecule is a group of atoms bonded together; can be same or different elements.
- Not all molecules are compounds; only molecules with different elements are compounds.
- Elements are substances with only one type of atom.
- Compounds contain two or more different atoms chemically bonded.
- Mixtures have elements or compounds not chemically bonded together.
Atomic Structure
- An atom consists of a nucleus (center) containing protons and neutrons, with electrons orbiting in shells.
- Protons are positively charged particles in the nucleus.
- Neutrons are neutral particles in the nucleus.
- Electrons are negatively charged and orbit the nucleus in energy levels or shells.
- Electron shells can hold specific numbers of electrons based on their energy level.
Subatomic Particles: Charges and Masses
- Protons: charge +1, relative mass ≈ 1 atomic mass unit (amu).
- Neutrons: charge 0 (neutral), relative mass ≈ 1 amu.
- Electrons: charge -1, relative mass ≈ 1/1836 amu (negligible).
Atomic Number, Mass Number, and the Periodic Table
- Atomic number (Z) is the number of protons in an atom, unique for each element.
- In a neutral atom, number of protons = number of electrons.
- Mass number (A) is the sum of protons and neutrons in the nucleus.
- Relative atomic mass is the average mass of all isotopes of an element, often rounded for exams.
- The number of neutrons can be calculated: Mass number (A) – Atomic number (Z) = Number of neutrons.
Key Terms & Definitions
- Element — Pure substance of one type of atom.
- Compound — Substance of two or more different elements chemically bonded.
- Mixture — Combination of substances not chemically bonded.
- Atom — Smallest unit of an element.
- Molecule — Group of atoms bonded together.
- Proton — Positively charged subatomic particle in the nucleus.
- Neutron — Neutral subatomic particle in the nucleus.
- Electron — Negatively charged particle orbiting the nucleus.
- Atomic Number (Z) — Number of protons in the nucleus.
- Mass Number (A) — Total number of protons and neutrons in the nucleus.
- Relative Atomic Mass — Average mass of all isotopes of an element.
Action Items / Next Steps
- Review topic notes and ensure understanding of definitions and examples.
- Practice calculating number of subatomic particles for different elements.
- Familiarize yourself with the layout of the periodic table.