Classification of Elements & Periodicity — Key Reactions — Summary

Pair	Reaction	Significance
Li–Mg (normal oxide formation)	$4\text{Li} + \text{O}_2 \rightarrow 2\text{Li}_2\text{O}$	Li forms normal oxide (not peroxide like Na)
Li–Mg (normal oxide formation)	$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$	Mg also forms normal oxide
Li–Mg (carbonate decomposition)	$\text{Li}_2\text{CO}_3 \xrightarrow{\Delta} \text{Li}_2\text{O} + \text{CO}_2$	Both $Li_{2}CO_{3}$ and $MgCO_{3}$ decompose on heating
Be–Al (dissolve in NaOH)	$\text{Be} + 2\text{NaOH} \rightarrow \text{Na}_2\text{BeO}_2 + \text{H}_2\uparrow$	Amphoteric Be reacts with base
Be–Al (dissolve in NaOH)	$2\text{Al} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow 2\text{NaAlO}_2 + 3\text{H}_2\uparrow$	Amphoteric Al reacts with base
B–Si (chloride hydrolysis)	$\text{BCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{B(OH)}_3 + 3\text{HCl}$	Covalent chloride of B hydrolyzed
B–Si (chloride hydrolysis)	$\text{SiCl}_4 + 4\text{H}_2\text{O} \rightarrow \text{Si(OH)}_4 + 4\text{HCl}$	Covalent chloride of Si hydrolyzed

Process	Equation	$\Delta H$ Sign
First ionization	$\text{M}(g) \rightarrow \text{M}^+(g) + e^-$	+ve (endothermic)
Electron gain	$\text{X}(g) + e^- \rightarrow \text{X}^-(g)$	−ve for non-metals (exothermic)
EGE of Cl	$\text{Cl}(g) + e^- \rightarrow \text{Cl}^-(g),\ \Delta H = -349\ \text{kJ/mol}$	Most negative among halogens
EGE of F	$\text{F}(g) + e^- \rightarrow \text{F}^-(g),\ \Delta H = -328\ \text{kJ/mol}$	Less negative than Cl (anomaly)

$\boxed{Z_{eff} = Z - \sigma}$

where $\sigma$ is the shielding constant (Slater's rules). All periodic trends flow from this single equation.