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GCSE Physics 09 — Atomic Structure and Radioactivity

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Topics include Atomic Structure and Subatomic Particles, Development of the Atomic Model, Electron Energy Levels and Ionisation, Isotopes, Types of Nuclear Radiation, Ionising Power and Penetration, Alpha Decay Equations, and Beta and Gamma Decay Equations.

Physics EN
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Atomic Structure and Subatomic Particles

An atom consists of a tiny, dense, positively charged nucleus surrounded by negatively charged electrons in shells.

Key points

  • Protons: Mass 1, Charge +1, located in the nucleus.
  • Neutrons: Mass 1, Charge 0, located in the nucleus.
  • Electrons: Mass $\approx \frac{1}{1840}$ (negligible), Charge -1, located in energy levels (shells).
  • Atomic Number ($Z$): Number of protons (defines the element).
  • Mass Number ($A$): Total number of protons + neutrons.

Worked example

Question

An atom of Sodium is represented as $^{23}_{11}\text{Na}$. Determine the number of protons, neutrons, and electrons in a neutral atom of Sodium.

Solution

1. Protons: Look at the atomic number (bottom number), $Z = 11$. There are 11 protons.
2. Electrons: Since the atom is neutral, electrons = protons = 11.
3. Neutrons: Subtract atomic number from mass number ($A - Z$). $23 - 11 = 12$. There are 12 neutrons.

Common pitfalls

  • Confusing the mass number (top) with the atomic number (bottom).
  • Thinking electrons have zero mass (they have negligible mass, not zero).
  • Forgetting that the nucleus is positively charged due to protons.

Prerequisites

  • Basic charge interactions (positive repels positive)
  • Standard form notation
Further resources