Physical Science Matric Revision: Atomic structure

Revision Notes for CAPS Physical Science Grade 12: Matter and Materials – Atomic Structure

Introduction to Atomic Structure

Understanding atomic structure is fundamental in Physical Science as it provides insight into how elements combine, interact and form various materials. This knowledge lays the foundation for comprehending more complex topics like chemical bonding, reactions, and material properties.

Learning Objectives:

  • Identify and describe subatomic particles.
  • Understand the arrangement of electrons in atoms.
  • Relate atomic structure to the periodic table.
  • Comprehend atomic theories and models.

Key Points

Subatomic Particles

  1. Protons: Positively charged particles located in the nucleus.
  2. Neutrons: Neutral particles, also in the nucleus.
  3. Electrons: Negatively charged particles orbiting the nucleus in electron shells.

Atomic Number and Mass Number

  • Atomic Number (Z): Number of protons in the nucleus. Defines the element.
  • Mass Number (A): Total number of protons and neutrons in the nucleus.
  • Number of Neutrons (N): N = A – Z.


  • Atoms of the same element with different numbers of neutrons, hence different mass numbers.

Electron Arrangement

  • Electrons are arranged in energy levels or shells around the nucleus.
  • Shells are filled in a sequence (2, 8, 18, 32…) in steps following the order of increasing energy.

Atomic Models

  1. Bohr Model: Electrons orbit the nucleus in fixed paths or shells.
  2. Quantum Mechanical Model: Electrons have probabilities of being in certain regions around the nucleus called orbitals.

Real-World Applications

Practical Example: Carbon Isotopes

Carbon has two stable isotopes: Carbon-12 (12C) and Carbon-14 (14C). While 12C is used to study organic compounds, 14C is used in radiocarbon dating to determine the age of archaeological finds.

Example Problem:
Calculate the number of protons, neutrons, and electrons in 14C.

  • Protons = Atomic number of Carbon = 6
  • Neutrons = Mass number – Atomic number = 14 – 6 = 8
  • Electrons = Number of protons (for neutral atoms) = 6

Step-by-Step Problem Solving

If you are given an element with Atomic Number ( Z ) and Mass Number ( A ):

  1. Identify Protons: This is the Atomic Number.
  2. Calculate Neutrons: Subtract the Atomic Number from the Mass Number ( N = A – Z ).
  3. Electrons: Equal to the number of protons for a neutral atom ( e^- = Z ).

Common Misconceptions and Errors

  1. Confusing Atomic Number and Mass Number: Remember, Atomic Number is the number of protons; Mass Number is the total number of protons and neutrons.
  2. Wrong Shell Filling Order: Electrons fill from lower to higher energy levels, often mistakenly filled without following the sequence.
  3. Isotopic Mass: Misunderstanding that isotopic mass is not average atomic mass found on the periodic table but specific to the isotope.

Strategy to Avoid Errors:

  • Double-check the sequence of shell filling using the ( 2n^2 ) rule.
  • Always confirm if the atom is neutral or ionized (number of electrons can differ in ions).

Practice and Review

Practice Questions

  1. Identify the number of protons, neutrons, and electrons in (_{11}^{23}\mathrm{Na}).
  2. Explain how the Bohr model differs from the Quantum Mechanical Model.
  3. Draw the electron configuration for Silicon (Si), Atomic Number = 14.

Examination Tips

  • Keywords: Proton, Neutron, Electron, Atomic Number (Z), Mass Number (A), Isotope.
  • Time Management: Allocate time according to marks; typically, 1 minute per mark.
  • Diagrams: Practice drawing atomic models and electronic configurations.

Connections and Extensions

  • Chemical Bonding: Knowledge of valence electrons applies to covalent, ionic bonds.
  • Periodic Table Trends: Atomic structure explains periodicity in element properties.
  • Nuclear Chemistry: Isotopes play key roles in nuclear reactions and stability.

Summary and Quick Review

  1. Atomic Number: Number of protons.
  2. Mass Number: Sum of protons and neutrons.
  3. Isotopes: Same protons, different neutrons.
  4. Electron Configuration: Distribution of electrons in shells/orbitals.

Quick Reference:

  • Protons = Atomic Number
  • Neutrons = Mass Number – Atomic Number
  • Electrons = Number of protons (if neutral)
  • First Shell Capacity = 2, Second = 8…

Additional Resources

For more practice and in-depth reading:
Khan Academy: Videos and articles on atomic structure.
Chemguide: Detailed chemistry tutorials.
PhET Interactive Simulations: Visualize atomic models.

Addressing any specific doubts with your teacher or through additional reading or videos can significantly enhance understanding. Happy studying!

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