Points of Similarities between Lithium and Magnesium

The lithium–magnesium resemblance (the "diagonal relationship") arises because the two ions happen to be almost the same size — atomic radii of 152 pm (Li) and 160 pm (Mg), ionic radii of 76 pm ($Li^+$) and 72 pm ($Mg^{2+}$) — so their charge-to-size ratios, and hence their chemistries, converge even though they sit in different groups. The main parallels are:

• Both metals are noticeably harder and lighter than the other members of their respective groups.
• Both react only slowly with water; their oxides and hydroxides are sparingly soluble, and their hydroxides decompose on heating. Both combine directly with nitrogen to give a nitride — $Li_3N$ and $Mg_3N_2$ respectively.
• Neither oxide, $Li_2O$ nor $MgO$, takes up further oxygen to form a superoxide, unlike the oxides of the heavier alkali/alkaline-earth metals.
• The carbonates of both metals decompose readily on heating to the oxide and $CO_2$, and neither forms an isolable solid hydrogencarbonate. …