What are S- Block Elements?
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 other than hydrogen are called “Alkali metals” while those of IIA is called “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 made up chiefly of salt and potassium. Alkali metals consist of the elements, lithium, sodium, potassium, rubidium, caesium 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 configuration will assist in understanding their properties.
Electronic Configurations of s-Block Components
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 mono positive ions M+ because their ionization energy values are really low. They form ionic compounds and show +1 oxidation state.
Alkaline earth metals have 2 electrons in’s’ orbital of their valence shell. All alkaline earth metals lose their two electrons to form dipositive ions M2+, due to the fact that their ionization energy values are low. They form ionic substances and show + 2 oxidation state.
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 elements belong.
The occurrence of Alkali Metals
Due to high reactivity, the alkali metals are present in nature in the combined state. None of the alkali metals is discovered complimentary in nature. Sodium and potassium are abundant alkali metals and each makes up about 2.4 percent of earth’s crust. The majority of the earth’s crust is made up of insoluble alumino-silicates of alkali metals.
Lithium deposits, typically in the form of complicated minerals, are commonly spread over the earth. An important commercial source of lithium is the mineral spodumene, LiAl (SiO3)2. Percentages of rubidium and caesium are discovered in potassium salts deposits. Francium has actually not been discovered in nature. It has actually been prepared artificially in the laboratory and is very unsteady so that very little is known about this metal.
The occurrence of Alkaline-Earth Metals
Being really reactive, alkaline earth metals also do not occur in a free state. The substances of these metals occur commonly in nature. Magnesium and calcium are really abundant in earth’s crust. The outer portion of the earth was originally in the form of silicates and aluminosilicates of alkaline-earth metals.
Magnesium and calcium, with salt and potassium, exist in the rocks as cations. Magnesium halides are found in seawater. Magnesium is an essential constituent of chlorophyll.
Calcium phosphate, Ca3(PO4) 2 and calcium fluoride, CaF2 are also found as minerals. Calcium is a necessary constituent of numerous living organisms. It takes place as a skeletal product in bones, teeth, sea-shells and eggshells. Radium is a rare element. It is of great interest because of its radioactive nature.
Peculiar Behaviour of Lithium
In a number of its properties, lithium is rather different from the other alkali metals. This behaviour is not unusual, due to the fact that the very first member of each main group of the periodic table reveals marked deviation from the regular patterns of the group as a whole.
The deviation revealed 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 2 electrons. The so-called ‘anomalous’ properties of lithium are due to the fact that lithium is unexpectedly far less electropositive than sodium. A few of the more crucial distinctions of lithium from other alkali metals are listed below:
- Lithium is much harder and lighter than the other alkali metals.
- The lithium salts of anions with high charge density are normally less soluble in water than those of the other alkali metals, e.g. LiOH, LiF, Li3PO4, Li2CO3.
- Lithium forms steady complex substances, although complex development normally is not a property of alkali metals. One of the stable complexes formed by lithium is [Li (NH3) 4] +.
- Lithium reacts extremely slowly with water, while other alkali metals respond strongly.
- Lithium salts of big polarizable anions are less stable than those of other alkali metals. Unlike other alkali metals, lithium does not form bicarbonate, tri-iodide or hydrogen sulphide at room temperature.
- When burnt in air lithium forms just typical oxide, whereas the others form peroxides or superoxides.
- Lithium hydride is steadier than the hydrides of other alkali metals.
- Lithium substances are more covalent, that is why its halides are more soluble in natural solvents and the alkyls and aryls of lithium are more stable than those of other alkali metals.
- Lithium is the least reactive metal of all the alkali metals.
- When acetylene is passed over highly heated up lithium, it does not produce lithium acetylide, but other alkali metals form the matching metallic acetylides.
- Lithium has low electropositive character; thus, its carbonate and nitrate are not so stable and for that reason decomposition giving lithium oxide. Carbonates of other alkali metals do not decompose. Decomposition of lithium nitrate offers different products than the nitrates of other alkali metals.
- Lithium chloride has an exothermic heat of solution, whereas chlorides of salt and potassium have endothermic heats up of the solution.
- Lithium carbide is the only alkali metal carbide formed easily by the direct reaction.
- Lithium hydroxide when highly heated up produces lithium oxide however the other alkali metal hydroxides do not show this behaviour.
- Lithium reacts with nitrogen to form nitride, while the other members of the group do not give this reaction.
Peculiar Behaviour of Beryllium
Beryllium is the lightest member of the series and differs from the other group IIA elements in numerous ways. This is due to its small atomic size and relatively high electronegativity value. The bottom lines of difference are:
- Beryllium metal is almost as hard as iron and hard adequate to scratch glass. The other alkaline earth metals are much softer than beryllium but still harder than the alkali metals.
- The melting and boiling points of beryllium are higher than other alkaline earth metals.
- As reducing agents, the group IIA metals are all-powerful adequate to minimize water, at least in principle. Nevertheless, with water, beryllium forms insoluble oxide coating that secures it from additional attack.
- Beryllium, in particular, is quite resistant towards complete oxidation, even by acids, because of its BeO coating.
- Beryllium is the only member of its group which reacts with alkalies to form hydrogen. The other members do not react with alkalies.