Why Don’t We Put 18 Electrons in the Third Shell Despite Its Capacity?

Learn why the third electron shell doesn't always hold 18 electrons due to energy levels and the Aufbau principle in electron configuration.

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We don't put 18 electrons in the third shell initially because electron configuration follows the Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level to higher energy levels. The third shell begins to fill after the first and second shells are full, but before it reaches 18 electrons, 4s orbital (of the fourth shell) gets filled due to its lower energy. So, it's the energy levels and not just capacity that determine electron arrangement.

FAQs & Answers

  1. Why does the 4s orbital fill before the 3d orbital? The 4s orbital fills before the 3d because it has a lower energy level than the 3d orbital, in accordance with the Aufbau principle.
  2. What determines the order in which electron shells and orbitals fill? Electron shells and orbitals fill based on their energy levels, starting from the lowest energy orbital to higher ones, rather than their maximum capacity.
  3. Can the third shell ever hold 18 electrons? Yes, the third shell can hold up to 18 electrons, but in electron configuration, the 4s orbital fills before the 3d orbitals, which affects the electron arrangement.

Watch More

For a deeper understanding of atomic structure and electron arrangements, explore our related videos on the Aufbau principle, Hund’s rule, and electron orbital filling sequences to see how they shape the periodic table.

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