The alkali metals are a group of elements in the periodic table. They are all in the first column of the periodic table. The only element in the column that is not usually considered an alkali metal is hydrogen. […]

Alkali metals share many similar properties including :

• they are very reactive ;
• they all have one valence electron in the outermost shell which they seek to lose in order to have a full outer shell. This is what makes them so reactive ;
• they are soft enough to be cut with a knife ;
• when exposed to air, they tarnish due to oxidation ;
• they are not found in nature as a free element, but generally as salts ;
• they react when coming into contact with water. Some of them will even explode when they come into contact with water ;
• they are malleable, ductile, and good conductors of electricity and heat.

• _{24}\text{Cr} : ... 3d⁵4s¹ ;
• _{29}\text{Cu} : ... 3d¹⁰4s¹ ;
• _{41}\text{Nb} : ... 4d⁴5s¹ ;
• _{42}\text{Mo} : ... 4d⁵5s¹.

Il existe des exceptions à la règle de Klechkowski, certains éléments ne respectent pas cet ordre. On observe alors un transfert entre leur sous-couche s au profit de leur sous-couche d ce qui permet à cette dernière d'être remplie (dix électrons) ou demi-remplie (cinq électrons). L'énergie globale du cortège électronique est ainsi plus faible.