Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 77

has atomic number 21 and mass number 45. Therefore, in a neutral atom of Sc, there are 21 protons, 21 electrons, and 24 neutrons. As with the other examples, and following the Aufbau principle, Hund's rule, and the Pauli exclusion principle, the ground state electronic configuration of scandium can be written as 1s²2s²2p⁶3s²3p⁶4s²3d¹ or [Ar]4s²3d¹. Remember, the 4s orbital is filled before the 3d because it is lower in energy.