Atomic Structure and Isotopes

This lesson covers the structure of the atom, the sub-atomic particles, atomic number, mass number, nuclear notation and isotopes. Understanding atomic structure is essential for the Radioactivity section of the Edexcel GCSE Combined Science specification (1SC0).

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The Structure of the Atom

An atom consists of a small, dense, positively charged nucleus surrounded by negatively charged electrons that orbit in shells (energy levels).

Part of the atom	Location	Detail
Nucleus	Centre	Contains protons and neutrons; very small compared with the atom; contains almost all the mass
Electron shells	Around the nucleus	Electrons orbit in shells at different energy levels

graph TD
    subgraph "Atom"
    N["Nucleus<br/>(protons + neutrons)"] --> E1["1st electron shell"]
    E1 --> E2["2nd electron shell"]
    E2 --> E3["3rd electron shell"]
    end

Size of the Atom

• The radius of an atom is about $1 \times 10^{-10}$1×10−10 m (0.1 nm).
• The radius of the nucleus is about $1 \times 10^{-14}$1×10−14 m — roughly 10 000 times smaller than the atom.
• Most of the atom is empty space.