Chemistry (5070)
Topic 2 of 5Cambridge O Levels

Atomic Structure

What atoms are made of and how elements are organised

Atoms are the building blocks of all matter. They are incredibly tiny — about 0.1 nanometres across!


Structure of an Atom:

  • Nucleus (centre): Contains protons (+) and neutrons (0)
  • Electron shells (orbiting): Contain electrons (-)
  • Protons and neutrons have mass of ~1, electrons have almost no mass

    • Key Numbers:

  • Atomic number (Z): Number of protons (defines the element)
  • Mass number (A): Protons + Neutrons
  • Number of neutrons: A - Z
  • In a neutral atom: protons = electrons

    • Electron Configuration:

    Electrons fill shells: 2, 8, 8, 18...

  • 1st shell: max 2 electrons
  • 2nd shell: max 8 electrons
  • 3rd shell: max 8 electrons (at GCSE level)
  • Example: Sodium (Na, Z=11): 2, 8, 1

    • Isotopes: Same element (same protons) but different number of neutrons.

  • Carbon-12 (6p, 6n) and Carbon-14 (6p, 8n) are isotopes
  • Same chemical properties, different physical properties

    • The Periodic Table:

  • Organised by atomic number
  • Groups (columns): Same number of outer electrons = similar properties
  • Periods (rows): Same number of electron shells

    • Key Points to Remember

    • 1Protons (+), Neutrons (0) in nucleus; Electrons (-) in shells
    • 2Atomic number = protons, Mass number = protons + neutrons
    • 3Electron shells fill: 2, 8, 8...
    • 4Isotopes: same protons, different neutrons

    Pakistan Example

    Pakistan's Nuclear Story

    Pakistan became a nuclear state in 1998 thanks to understanding atomic structure. Nuclear energy comes from the nucleus of uranium atoms. Uranium-235 (92 protons, 143 neutrons) can undergo fission — its nucleus splits into smaller atoms, releasing enormous energy. Dr. Abdul Qadeer Khan's work on isotope separation involved distinguishing Uranium-235 from Uranium-238 (same element, same 92 protons, but 238 has 3 extra neutrons). Isotopes have the same chemistry but different nuclear properties — this difference powers nuclear reactors at Karachi Nuclear Power Plant (KANUPP).

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