Discuss the general characteristics and gradation in properties of alkaline earth metals.

(i) The general electronic configuration of alkaline earth metals is [noble gas] ns².

(ii) The atomic and ionic radii of alkaline earth metals are smaller than that of alkali metals. On moving down the group their atomic and ionic radii increase due to the decrease in their effective nuclear charge.

(iii) These metals lose two electrons to acquire the nearest noble gas configuration. Therefore, their oxidation state is +2.
(iv) Due to their small size, the first ionisation enthalpies of alkaline earth metals are higher than those of the alkali metals. But their second ionisation enthalpies are found to be lower than those of the corresponding alkali metals.
v) Metallic character: They are less electropositive than alkali metals due to higher ionisation enthalpies. Metallic character increases down the group due to a decrease in ionisation enthalpy.
Chemical reactivity:
Reaction with water and air: The alkaline earth metals are less reactive than alkali metals. Be and Mg is kinetically inert of O2 and H2O due to the formation of an oxide layer on its surface. Be does not react with water or steam even at red-hot and does not get oxidised in air below 873K. Powdered be burns on ignition to form BeO and Be₃N₂
2Be +O₂ → 2BeO
3Be +N₂ → Be₃N₂
Mg is more electropositive and burns in air with dazzling light forming MgO and Mg3N2.
2Mg +O₂ →2MgO
3Mg +N₂ →Mg₃N₂

Ca, Sr and Ba readily react with oxygen to give oxides. Calcium forms oxides whereas Sr and Ba form peroxide. They react with nitrogen to form nitrides.
2Ca +O₂→ 2CaO
3Cs +N₂ → Ca₃N₂
Sr +O₂ → SrO₂
Ba +O₂→ BaO₂
Mg Reacts with hot water Ca, Ba, Sr react with cold water vigorously.
Mg +H₂O(Hot)→ MgO +H₂
Ca +2H₂O→ Ca(OH)2 +H₂
Sr +2H₂O → Sr(OH)2 +H₂
Ba +2H₂O → Ba(OH)2 +H₂

Reaction with halogens: Group 2 element react with halogens at increased temperature to from halides.
Be +Cl₂+ Heat → BeCl₂
Mg +Cl₂+ Heat →MgCl₂
Ca+Cl₂+ Heat →CaCl₂
Action with acids: The alkaline earth metals readily react with acids to form salts and liberate H2 gas.
Be +2HCl→ BeCl₂+H₂
Mg+2HCl→ MgCl₂ +H₂

Reaction with H2
All metals combine with H2 to form hydrides except Be.
Ca +H → CaH₂(Hydrolith)
Mg +H2 → MgH