This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
On moving across a period, the size of atoms decreases and on moving down a group the size of atoms increases.  The effects seem to cancel out. Similar to size, covalence, basic nature, electronegativity also cancel out differences. So these diagonally next elements in 2nd and 3rd periods have similar chemistry.
Lithium and magnisium is a pair.  B barium and Si silicon are similar.

Li & Mg :  charge density on ions is nearly same
               they form only normal oxides
               Chlorides of both absorb moister and soluble in alcohol

1 4 1