Properties and Chemical Reactivity — Core Principles

Core Principles

Alkaline earth metals are Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra) with an $ns^2$ outer electronic configuration. They are highly electropositive and readily lose two electrons to form stable $M^{2+}$ ions, acting as strong reducing agents.

Key trends down the group include increasing atomic/ionic radii, decreasing ionization enthalpy, increasing metallic character and reactivity. Their oxides are basic (except amphoteric BeO), and their hydroxides are increasingly soluble and basic down the group.

They react with oxygen to form oxides (and sometimes peroxides/nitrides), with water to form hydroxides and hydrogen (reactivity increases down the group), and with halogens to form halides. Beryllium is anomalous due to its small size and high charge density, forming covalent compounds and exhibiting a diagonal relationship with Aluminium.

Flame coloration is observed for Ca, Sr, and Ba due to electron excitation. Solubility trends for hydroxides increase down the group, while for sulfates, they decrease.

Important Differences

vs Alkali Metals (Group 1)

Aspect	This Topic	Alkali Metals (Group 1)
Valence Electrons	1 ($ns^1$)	2 ($ns^2$)
Ion Formation	Monopositive ($M^+$)	Dipositive ($M^{2+}$)
Metallic Character	More metallic, softer	Less metallic, harder
Ionization Enthalpy	Lower	Higher (for both $IE_1$ and $IE_2$ compared to Group 1 $IE_1$)
Reducing Power	Stronger reducing agents	Strong reducing agents, but weaker than alkali metals
Hydration Enthalpy of Ions	Lower (for $M^+$)	Higher (for $M^{2+}$ due to higher charge density)
Nature of Oxides/Hydroxides	Strongly basic	Basic (except amphoteric BeO/Be(OH)$_2$)
Flame Coloration	All impart characteristic colors	Be and Mg do not impart color

Alkaline earth metals differ significantly from alkali metals despite both being s-block elements. Group 2 elements have two valence electrons, forming $M^{2+}$ ions, leading to higher ionization enthalpies and stronger metallic bonding, making them harder with higher melting points. Their $M^{2+}$ ions have higher charge densities, resulting in greater hydration enthalpies and influencing solubility trends differently. While both are strong reducing agents, alkali metals are generally stronger. The anomalous behavior of Beryllium and the amphoteric nature of its compounds further distinguish Group 2 from Group 1.