Lecture on Electron Arrangement in Atoms

Importance of Electron Arrangement

• Atoms are stable with a full outer electron shell.
• Incomplete outer shells lead to reactivity.
• Atoms gain or lose electrons to achieve a stable configuration.

Sodium's Electron Arrangement

• Atomic Number: 11 (11 protons and 11 electrons)
• Electron Shells:
  • First shell: 2 electrons
  • Second shell: 8 electrons
  • Third shell: 1 electron
• Stability: Unstable due to one electron in the outer shell (third shell) needing full 8 electrons.

General Atom Reactivity

• Atoms with incomplete outer shells react to form molecules or compounds.
• Exception: Noble gases (Group 0) have full outer shells and are usually unreactive (e.g., Neon).

Electron Arrangement for Argon

• Atomic Number: 18 (18 electrons)
• Electron Shells: 2, 8, 8
• Representation: Written as 2, 8, 8 for first, second, and third shells respectively.

Drawing Electron Diagrams

• Electrons should be shown as crosses or dots.
• Notation for ions: Use square brackets and indicate charge in the top right.

Calcium's Electron Arrangement

• Atomic Number: 20 (20 electrons)
• Electron Shells: 2, 8, 8, 2
• Stability: Unstable due to two electrons in the fourth shell (needs to be removed for stability).
• Ion Formation: Becomes a calcium ion (Ca²⁺) with electron structure 2, 8, 8.

Fluorine's Electron Arrangement

• Atomic Number: 9 (9 electrons)
• Electron Shells: 2, 7
• Stability: Needs to gain one electron for full outer shell.
• Ion Formation: Becomes a fluoride ion (F⁻), written as 2, 8.

Next Steps

• Next topic will be balancing chemical equations.
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