Every object around us displays some unique characteristics, which sets aside that object from the rest. These properties become the reason why an element behaves a certain way and how we are able to predict its behaviour. We will learn about the characteristics of the compounds of the alkali earth metals in this topic. How are the compounds of the alkali earth metals any different from the compounds of other elements in the periodic table? Let’s find out.
What are Alkali Earth Metals?
If we consider the periodic table, the elements that would fall in the group 2 of the table are usually known as alkali earth metals. Included in these metals are beryllium(Be), magnesium(Mg), strontium(Sr), barium(Ba) and radium(Ra). Each of these elements contains two electrons in their outermost shell. Let us discuss the characteristics of the compounds of the alkali earth metals.
Physical Properties of the Compounds of Alkali Earth Metals
- They are silverish, white, and hard metals. They are soft but harder than alkali metals in comparison.
- Some of them appear whitish but beryllium and magnesium appear greyish in colour.
- Their melting and boiling points are higher compared to the alkali metals.
- These metals are strongly electropositive in nature. Alkaline earth metals give different colour with the flame test such as calcium gives brick red colour, strontium gives crimson colour and barium gives apple green colour, all of which are different for different metals.
Chemical Properties of the Compounds of Alkali Earth Metals
- All alkaline earth metals tend to form monoxide except the metal, beryllium.
- They usually have high electrical and thermal conductivities as they have a metallic bonding.
- The oxides of alkaline earth metals are basic but less basic in comparison to alkali metals.
- Hydroxides of alkaline earth metals are basic in nature except for beryllium hydroxide.
- Alkali earth metals form solid carbonates. As one moves from beryllium to barium thermal stability of carbonates usually increases.
- Alkaline earth metals are also capable to form sulphates such as BeSO4, and MgSO4. Beryllium sulphate and magnesium sulphate is soluble in water as compared to other sulphates of alkaline earth metals.
- Group 2 elements also form hydrated, crystallized nitrates. Heating of nitrates forms oxides. Barium nitrate crystallizes to form an anhydrous salt of barium oxide whereas magnesium nitrate crystallizes with six molecules of water.
2Ba(NO3)2 + heat → 2BaO + 4NO2 + O2
- Alkaline earth metals form halides only after reacting with halogens. Beryllium chloride polymerizes in the solid phase.
- All beryllium halides are essentially covalent and are soluble in organic solvents. They are hygroscopic, and fume in the air due to hydrolysis. On hydrolysis, they produce an acidic solution.
- The halides of all remaining alkaline earth metals are ionic in nature. Their ionic character increases as the size of the metal Ion increases.
- As the ionic character increases or the covalent character decreases, their tendency towards undergoing hydrolysis decreases.
- The hydrated chlorides, bromides and iodides of Ca, Ba and Sr can be dehydrated on heating but those of Be and Mg undergo hydrolysis.
- BeF2 in highly soluble in water due to the high hydration enthalpy of the small Be2+ ion. The other fluorides are almost insoluble in water.
- The chlorides, bromides and iodides of all of the elements i.e. Mg, Ca, Ba, Sr are ionic, have a lower melting point than the fluorides and are readily soluble in water. The solubility decreases somewhat with increasing atomic number.
- Except for BeCl2 and MgCl2, the other chlorides of alkaline earth metal impart characteristic colours to flame.
A Solved Question for You
Q: Discuss the solubility and thermal stability of sulphates of alkali earth metals.
Ans: The sulphates of alkaline earth metal are prepared by the action of sulphuric acid on metals, metal oxides, hydroxides and carbonates. The sulphates of alkaline earth metal are all white solids. Beryllium, magnesium and calcium sulphate crystallise in the hydrated form i.e. BeSO4·4H2O, MgSO4·7H2O, CaSO4·2H2O but sulphates of Strontium and barium crystallise without water of crystallisation.
The solubility of sulphates in water decreases down the group. The magnitude of the lattice enthalpy remains almost constant as the sulphate ion is so big that small increase in the size of cation from Be to Ba does not make any difference. The hydration enthalpy decreases from Be2+to Ba2+ as the size of the cation increases down the group.
Hence the solubility of sulphates of alkaline earth metal decreases down the group mainly due to decreasing hydration enthalpy from Be2+ to Ba2+. The high solubility of BeSO4 and MgSO4 is due to the high hydration enthalpy because of the smaller size of Be2+ and Mg2+ ions. The reason for this is because the sulphates of alkaline earth metals decompose on heating giving their corresponding oxides and SO3.