The Periodic Table

Writing the configuration

• An atom's electron arrangement can be shown in two ways:
  • As an electron shell diagram (nucleus drawn with concentric shells, electrons placed as dots)
  • As an electronic configuration (also called electronic structure): the number of electrons in each shell, comma-separated, innermost shell first
• Shell-filling rules used for the first 20 elements at IGCSE:
  • 1st shell holds up to 2 electrons
  • 2nd shell holds up to 8 electrons
  • 3rd shell holds up to 8 electrons (the IGCSE simplified model — in fact it can hold up to 18, but the next 8 are stable enough to treat it as 8 at this level)
  • 4th shell starts to fill once the 3rd reaches 8
• Once the 3rd shell has 8 electrons, the next two electrons (for potassium and calcium) go into the 4th shell first, before the 3rd shell continues filling for heavier elements

Electronic configurations of the first 20 elements

Configurations of ions

• An ion's configuration is its atom's configuration adjusted for electrons gained or lost
• Sodium atom (Na): 11 electrons, configuration 2,8,1
  • Sodium ion (Na⁺): 1 electron lost → 10 electrons → 2,8 (same as neon)
• Chlorine atom (Cl): 17 electrons, configuration 2,8,7
  • Chloride ion (Cl⁻): 1 electron gained → 18 electrons → 2,8,8 (same as argon)
• Many ions end up with the same configuration as the nearest noble gas, which is why those ions are stable

Reading position from the electronic configuration

• Period number = number of separate notations in the configuration
• Group number = the last notation (for Groups 1 to 7)
• For example, chlorine has configuration 2,8,7:
  • 3 notations → 3 occupied shells → Period 3
  • Last notation is 7 → 7 outer electrons → Group 7
• Group 0 is the exception
  • Group 0 elements have full outer shells, not a single matching outer-electron count
  • Helium is in Group 0 even though its configuration is just 2 (only one full shell)