Lecture Notes: Electron Arrangement in Atoms

Key Concepts

• Electron Arrangement: The distribution of electrons in an atom's shells.
• Stability of Atoms: Atoms are stable when they have a full outer shell of electrons.
• Reactivity: Atoms with incomplete outer shells tend to react with other atoms to achieve stability.

Electron Configuration

• Sodium (Na)

  • Atomic number: 11 (11 protons and 11 electrons).
  • Electron arrangement:
    • 1st shell: 2 electrons
    • 2nd shell: 8 electrons
    • 3rd shell: 1 electron
  • Stability: Unstable because the outer shell is not full; likely to react.

• Noble Gases

  • Group 0 elements with full outer shells
  • Non-reactive due to complete electron shells.
  • Example: Neon

Determining Electron Structure

• Argon (Ar)

  • Atomic number: 18
  • Electron arrangement:
    • 1st shell: 2 electrons
    • 2nd shell: 8 electrons
    • 3rd shell: 8 electrons
  • Written as: 2, 8, 8 (indicator of electron count per shell)

• Calcium (Ca)

  • Atomic number: 20
  • Electron arrangement:
    • 1st shell: 2 electrons
    • 2nd shell: 8 electrons
    • 3rd shell: 8 electrons
    • 4th shell: 2 electrons
  • Written as: 2, 8, 8, 2
  • Stability: Unstable; can lose 2 electrons to become a Ca²⁺ ion with a stable 2, 8, 8 configuration.

• Fluorine (F)

  • Atomic number: 9
  • Electron arrangement:
    • 1st shell: 2 electrons
    • 2nd shell: 7 electrons
  • Stability: Needs 1 more electron to fill the outer shell, becoming F⁻ ion (Fluoride) with 2, 8 structure.

Drawing Electron Configurations

• Methods: Electrons can be represented as either crosses or dots.
• Ions: Represent ions with square brackets and charge in the top right corner.

Exam Tips

• Be familiar with the electron arrangement of the first 20 elements.
• Practice drawing and writing electron structures.

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