Li-Mg parallels: (1) Both form nitrides directly with N2 (Li3N, Mg3N2). (2) Both form normal oxides only $\frac{not peroxides}{superoxides}$. (3) Carbonates decompose on heating (Li2CO3 → Li2O + CO2; MgCO3 → MgO + CO2). (4) Chlorides are hygroscopic and soluble in organic solvents (covalent character). (5) Both have relatively high IE compared to their group members. (6) LiF and MgF2 are insoluble. (7) Neither forms stable bicarbonates in solid state. Be-Al parallels: (1) Both form amphoteric oxides and hydroxides. (2) Both form covalent chlorides with bridging (BeCl2 polymer, AlCl3 dimer). (3) Both form [M(OH)4]^n- in excess NaOH. (4) Both carbides give CH4 with water. (5) Both form electron-deficient compounds. (6) Both have max covalence of 4 (Be) and practical max coordination 4-6 (Al). Cause of diagonal similarity: similar charge density $\frac{charge}{radius ratio}$ and electronegativity.