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 title: 1
 description: 
 published: true
-date: 2026-02-11T15:31:01.086Z
+date: 2026-02-11T15:34:58.131Z
 tags: 
 editor: ckeditor
 dateCreated: 2026-02-11T14:58:54.870Z
@@ -371,3 +371,16 @@ dateCreated: 2026-02-11T14:58:54.870Z
   </table>
 </figure>
 <p>&nbsp;</p>
+<h3><span style="font-family:Arial, Helvetica, sans-serif;">Pauli's Exclusion principle</span></h3>
+<p><span style="font-family:Arial, Helvetica, sans-serif;">Each orbital can be occupied by no more than 2 electrons.</span></p>
+<p><span style="font-family:Arial, Helvetica, sans-serif;">If 2 electrons occupy the same orbital, they must have opposite spins.</span></p>
+<p>&nbsp;</p>
+<h3><span style="font-family:Arial, Helvetica, sans-serif;">Hund's rule</span></h3>
+<p><span style="font-family:Arial, Helvetica, sans-serif;">Electron must occupy each orbital singly before pairing.</span></p>
+<p>&nbsp;</p>
+<h3><span style="font-family:Arial, Helvetica, sans-serif;">Exception: Half and full filled orbital</span></h3>
+<p><span style="font-family:Arial, Helvetica, sans-serif;">The electronic configuration of Cr (Chromium) and Cu (Copper) do not follow the order. The outer subshell become half or fully filled to gain extra stability.</span></p>
+<p><span style="font-family:Arial, Helvetica, sans-serif;">It is also because that 4s and 3d energy difference is small.</span></p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>