Lecture on Electron Arrangement in Atoms

Importance of Electron Arrangement

• Atoms have incomplete outer electron shells, e.g., fluorine.
• Stability is achieved with a full outer shell.
• Incomplete shells lead to reactions to gain or lose electrons.

Electron Arrangement Example: Sodium

• Atomic Number: 11 ➜ 11 protons and 11 electrons.
• Electron Shells:
  • First shell can hold 2 electrons (lowest energy level).
  • Second shell can hold up to 8 electrons.
  • Third shell holds the remaining electron.
• Stability:
  • Sodium has 1 electron in the outer shell ➜ unstable.
  • Wants to react to fill the outer shell.

General Behavior of Atoms

• Most single atoms have incomplete outer shells.
• Atoms react to form molecules/compounds to achieve full shells.
• Exceptions: Noble gases (Group 0) have full outer shells ➜ non-reactive.

Example Exercise: Argon

• Atomic Number: 18 ➜ 18 electrons.
• Electron Configuration:
  • First shell: 2 electrons.
  • Second shell: 8 electrons.
  • Third shell: 8 electrons.
• Symbolic notation: 2, 8, 8.

Example Exercise: Calcium

• Atomic Number: 20 ➜ 20 electrons.
• Electron Configuration:
  • First shell: 2 electrons.
  • Second shell: 8 electrons.
  • Third shell: 8 electrons.
  • Fourth shell: 2 electrons.
• Stability:
  • Fourth shell is not full (2/8 electrons).
  • Removing 2 outer electrons gives full third shell ➜ stable.
  • Resulting ion: Calcium becomes Ca²⁺.
  • New configuration: 2, 8, 8.

Example Exercise: Fluorine

• Atomic Number: 9 ➜ 9 electrons.
• Electron Configuration:
  • First shell: 2 electrons.
  • Second shell: 7 electrons.
• Stability:
  • Needs 1 more electron to fill shell.
  • Gains an electron to become a fluoride ion (F⁻).
  • Resulting configuration: 2, 8.

Conclusion

• Understanding electron arrangement is key for predicting atom stability and reactions.
• Practice with exercises by finding configurations for different elements.
• Next topic: Balancing Chemical Equations.