Diagonal Relationships: Li-Mg and Be-Al

What is a Diagonal Relationship? Elements in adjacent groups but consecutive periods (e.g., Li in Period 2/Group 1 vs. Mg in Period 3/Group 2) that have similar ionic radius-to-charge ratios (charge density), leading to similar chemical properties.

Li-Mg Diagonal Relationship

Shared Property	Li	Mg
Oxide type	$Li_{2}O$ (normal)	MgO (normal)
Carbonate	$Li_{2}CO_{3}$ decomposes on heating	$MgCO_{3}$ decomposes on heating
Nitrate decomposition	$LiNO_{3}$ → $Li_{2}O$ + $NO_{2}$ + $O_{2}$	Mg( $NO_{3}$ )_{2} → MgO + $NO_{2}$ + $O_{2}$
Bicarbonate	No solid $LiHCO_{3}$	No solid Mg( $HCO_{3}$ )_{2}
Chloride character	LiCl: covalent, soluble in EtOH/ether	$MgCl_{2}$ : partial covalent character
Reaction with $N_{2}$	Li + $N_{2}$ → $Li_{3}N$	Mg + $N_{2}$ → $Mg_{3}$$N_{2}$
High polarizing power	High (small $Li^{+}$ )	High (small, +2 $Mg^{2+}$ )

Be-Al Diagonal Relationship

Shared Property	Be	Al
Oxide	BeO: amphoteric	$Al_{2}O_{3}$ : amphoteric
Chloride	$BeCl_{2}$ : covalent, polymeric	$AlCl_{3}$ : covalent, forms $Al_{2}Cl_{6}$ dimer
Max covalency	4 ( $sp^{3}$ )	4 ( $sp^{3}$ , though can use d)
With NaOH	BeO/Be dissolve → $BeO_{2}^{2-}$	$Al_{2}O_{3}$ /Al dissolve → $AlO_{2}^{-}$
With acids	BeO + HCl → $BeCl_{2}$	$Al_{2}O_{3}$ + HCl → $AlCl_{3}$
Electronegativity	1.57	1.61