Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 73

has atomic number 16 and mass number 32. Therefore, a neutral atom of S has 16 protons, 16 electrons, and 16 neutrons. When putting the electrons into the orbitals, it is important to always start with the lowest‐energy orbital.

Electrons are placed in the energy levels starting at the lowest energy level (1s) in order of increasing energy (Aufbau principle). When filling the third shell, electrons are firstly placed in the 3s orbital, as this has the lowest energy in the third shell, followed by four electrons in the 3p orbitals. However, when putting the remaining four electrons in the 3p orbitals, each p orbital is filled with one electron first (Hund's rule). When each p orbital is half full, there is one electron left over, and it is at this point that the electrons are paired. When they are paired, they have opposing spins, which in this case is represented by drawing each half‐arrow in opposite directions, fulfilling the requirements of the Pauli exclusion principle.

The ground state electronic configuration of sulfur can be written as 1s²2s²2p⁶3s²3p⁴.

The reason we say ‘ground state’ electron configuration is because ‘excited states’ can also exist. The discussion of excited states is beyond the scope of this book but is discussed at degree level.