S-Block Elements of periodic table 

Introduction 

The s-block elements are the metals in Group IA and Group IIA of the periodic table. They are called the s-block elements because s-orbitals are being filled, in their outermost shells. The elements of group IA except hydrogen are called “Alkali metals” while those of IIA Are named “Alkaline-earth metals”.

The name alkali came from Arabic, which means ‘The Ashes’. The Arabs used this term For these metals because they found that the ashes of plants were composed chiefly of sodium and Potassium. Alkali metals include the elements, lithium, sodium, potassium, rubidium, cesium And francium. These are very reactive metals, produce strong alkaline solutions with water. The Alkaline-earth metals are beryllium, magnesium, calcium, strontium, barium and radium. They Are called alkaline-earth because they produce alkalies in water and are widely distributed in Earth’s crust. The alkali and alkaline earth metals include the most reactive electropositive elements And a study of their electronic configurations will help in understanding their properties.

Electronic Configurations of s-Block Elements.

Alkali Metal

Alkali metals have only one electron in ‘s’ orbital of their valence shell. All alkali metals Lose their one electron of the valence shell to form monopositive ions M¹ because their ionization Energy values are very low. They form ionic compounds and show +1 oxidation state. The electronic configurations and some physical constants of alkali metals are given in Configurations and Physical Constants of Alkali Metals

Alkaline Earth Metals

Alkaline earth metals have two electrons in ‘s’ orbital of their valence shell. All alkaline earth Metals lose their two electrons to form dipositive ions M², because their ionization energy values Are low. They form ionic compounds and show + 2 oxidation state. The electronic configurations And some physical constants of alkaline earth metals are given in Table 2.2. 

In going down a group the number of shells increases by one at each step and equal to the Number of the period to which the element belongs.

Occurrence of Alkali Metals

Due to high reactivity, the alkali metals Occur in nature in the combined state. None of The alkali metals is found free in nature. Sodium And potassium are abundant alkali metals and Each constitutes about 2.4 percent of earth’s Crust.

Most of the earth’s crust is composed of Insoluble alumino-silicates of alkali metals. Lithium deposits, usually in the form Of complex minerals, are widely scattered over The earth. An important commercial source of Lithium is the mineral spodumene, LiAl(SiO₂),. Small amounts of rubidium and Caesium are found in potassium salts deposits. Francium has not been found in nature. It has Been prepared artificially in the laboratory and Is very unstable, thus a very little is known About this metal. 

Occurrence of Alkaline Earth Metals

Being very reactive, alkaline earth metals Also do not occur in free state. The compounds of These metals occur widely in nature. Magnesium and calcium are very abundant In earth’s crust. The outer portion of the earth was Originally in the form of silicates and alumino- Silicates of alkaline earth metals. Magnesium and Calcium, with sodium and potassium are present in The rocks as cations.

Magnesium halides are found in sea Water. Magnesium is an essential constituent of Chlorophyll. Calcium phosphate, Ca,(PO,), and calcium Fluoride, CaF, are also found as minerals. Calcium Is an essential constituent of many living Organisms. It occurs as skeletal material in bones, Teeth, sea-shells and egg shells.

Peculiar Behaviour of Lithium

The deviation shown by lithium can be explained on the basis of its small radius and high Charge density. The nuclear charge of Li ion is screened only by a shell of two electrons. The so Called ‘anomalous’ properties of lithium are due to the fact that lithium is unexpectedly far less Electropositive than sodium. Some of the more important differences of lithium from other alkali metals are listed In many of its properties, lithium Quite different from the other alkali metals. This Behavior is not unusual, because the first member of each main group of the periodic table shows Marked deviation from the regular trends of the group as a whole.

Below:

1. Radium is a rare element. It is of great interest because of its radioactive nature.

2. Lithium is much harder and lighter than the other alkali metals.

3. The lithium salts of anions with high charge density are generally less soluble in water Than those of the other alkali metals, e.g. LiOH, LiF, Li,PO4, Li₂CO3.

4. Lithium forms stable complex compounds, although complex formation generally is not A property of alkali metals.One of the stable complexes formed by lithium is [Li(NH3)4] Lithium reacts very slowly with water, while other alkali metals react violently.

5. Lithium salts of large polarizable anions are less stable than those of other alkali metals.

6. Unlike other alkali metals lithium does not form bicarbonate, tri-iodide or hydrogen Sulphide at room temperature.

7. When burnt in air lithium forms only normal oxide, whereas the others form peroxides or Obrzo siduloent Superoxides.

8. Lithium hydride is more stable than the hydrides of other alkali metals.

9. Lithium compounds are more covalent, that is why its halides are more soluble in Organic solvents and the alkyls and aryls of lithium are more stable than those of other Alkali metals.

10. Lithium is the least reactive metal of all the alkali metals.

11. When acetylene is passed over strongly heated lithium, it does not produce lithium Acetylide, but other alkali metals form the corresponding metallic acetylides.

12. Lithium has low electropositive character, thus its carbonate and nitrate are not so stable And therefore decompose giving lithium oxide. Carbonates of other alkali metals do not Decompose. 

13. Decomposition of lithium nitrate gives different products than the nitrates Of other alkali metals.

14. Lithium reacts with nitrogen to form nitride, while the other members of the group do not Give this reaction.

15. Lithium carbide is the only alkali metal carbide formed readily by the direct reaction.

Peculiar Behaviour of Beryllium

Beryllium is the lightest member of the series and differs from the other group IIA Elements in many ways. This is due to its small atomic size and comparatively high Electronegativity value.

Lithium hydroxide when strongly heated, forms lithium oxide but the other alkali metal Hydroxides do not show this behaviour. Lithium chloride has an exothermic heat of solution, whereas chlorides of sodium andPotassium have endothermic heats of solution. The main points of difference are:

1. Beryllium metal is almost as hard as iron and hard enough to scratch glass.

2. The other Alkaline earth metals are much softer than beryllium but still harder than the alkali Metals.

3. The melting and boiling points of beryllium are higher than other alkaline earth metals.

4. As reducing agents, the group IIA metals are all powerful enough to reduce water, at least In principle. However, with water, beryllium forms insoluble oxide coating that It from further attack.

5. Protects Beryllium in particular is quite resistant towards complete oxidation, even by acids, Because of its BeO coating.

6. Beryllium is the only member of its group which reacts with alkalies to give hydrogen. 

7. The other members do not react with alkalies.