Electron Arrangement in Atoms

Key Concepts

• Atoms need a full outer shell of electrons to be stable.
• Atoms with incomplete outer shells tend to react to gain or lose electrons to achieve stability.

Sodium (Na) Electron Arrangement

• Atomic Number: 11 (11 protons and 11 electrons)
• Electron Shells:
  • 1st shell: 2 electrons (full)
  • 2nd shell: 8 electrons (full)
  • 3rd shell: 1 electron
• Sodium is unstable due to its incomplete outer shell (1 electron rather than 8).
• Tends to react to achieve a full outer shell.

Noble Gases

• Group 0 of the periodic table.
• Have completely full outer shells (e.g., Neon), thus do not react easily.

Argon (Ar) Electron Arrangement

• Atomic Number: 18 (18 electrons)
• Electron Shells:
  • 1st shell: 2 electrons
  • 2nd shell: 8 electrons
  • 3rd shell: 8 electrons
• Notation: 2, 8, 8

Calcium (Ca) Electron Arrangement

• Atomic Number: 20 (20 electrons)
• Electron Shells:
  • 1st shell: 2 electrons
  • 2nd shell: 8 electrons
  • 3rd shell: 8 electrons
  • 4th shell: 2 electrons
• Notation: 2, 8, 8, 2
• To become stable, calcium prefers to lose 2 outer electrons, forming a Calcium ion (Ca²⁺).
• Notation for ion: 2, 8, 8

Fluorine (F) Electron Arrangement

• Atomic Number: 9 (9 electrons)
• Electron Shells:
  • 1st shell: 2 electrons
  • 2nd shell: 7 electrons
• Notation: 2, 7
• Fluorine gains an electron to fill its outer shell, becoming a fluoride ion (F⁻).
• Ion Notation: 2, 8

Exam Tips

• Be prepared to determine electron arrangements for the first 20 elements.
• Practice drawing electron shells using crosses or dots.
• Remember to adjust electron count and notation when forming ions, and denote charge with square brackets and charge notation.

Next Steps

• Future lesson will cover balancing chemical equations.