Electron Configurations for the First Period

Elements in the First Period

• Hydrogen
• Helium

Hydrogen (Atomic Number: 1)

• Atomic Structure: 1 proton, 1 electron (in a neutral atom)
• Electron Configuration
  • Uses the Aufbau principle ("building up")
  • Goal: place electron as close to the nucleus as possible
  • Lowest energy level: n = 1
  • Allowed quantum numbers
    • l = 0 (s orbital)
    • ml = 0 (only one orientation for s orbital)
  • Orbital Shape: s orbital is spherical
  • Notation:
    • Electron Configuration: 1s¹
    • Orbital Notation: Line labeled as 1s orbital with one electron (up spin)
    • Spin: ms = +1/2 (up spin)

Helium (Atomic Number: 2)

• Atomic Structure: 2 protons, 2 electrons
• Electron Configuration
  • Still in the first shell (n = 1)
  • Same quantum numbers as hydrogen
    • l = 0, ml = 0
  • Orbital Notation
    • First electron: spin up
    • Second electron: spin down
    • Pauli Exclusion Principle: No two electrons can have the same four quantum numbers
      • Differentiate by ms (+1/2 and -1/2)
  • Complete Electron Configuration
    • 1s²: Two electrons in the s orbital of the first energy level
    • First shell is closed (maximum of two electrons per s orbital)

Key Principles

• Aufbau Principle: Building up from lowest energy levels
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers
• Orbital Capacity: Maximum of two electrons in an s orbital within the same shell

Conclusion

• The first energy level (n=1) is filled with helium, closing the shell.
• Moving beyond helium requires electrons to be placed into the next shell, transitioning to the second period of the periodic table.