Why Lithium and Magnesium shows similar chemical reactions?

Are in groups 1 and 2 of the periodic table meaning that they have 1 and 2 electrons in the outer shell meaning they are reactive metals.

Lithium and magnesium....

There is an interesting diagonal relationship between elements in the second and third periods where the elements exhibit similar physical properties and chemical properties. These include Li/Mg, Be/Al and B/Si.

Clearly, lithium and magnesium don't have the same number of valence electrons and so combine with different numbers of other elements (LiCl vs MgCl2), but the compounds which are formed also have similar characteristics. Also, lithium and magnesium have similar atomic radii and ionic radii and similar electronegativities.

With lithium and magnesium, it is interesting to note that both are used to treat neurological conditions. Both play a role in the treatment of bipolar disorder and schizophrenia, as well as depression.

So does sodium and potassium, that's why they're in the same 'group'. Like the halogens are in a group and the noble gases are another group.