s Block Elements for IIT JEE

The s-Block Elements - Characteristics of s-Block Elements for JEE

s-block elements

In the s-block elements, the last electron enters the ns energy shell. The maximum capacity of ns energy shell is of two electrons, thus, these elements have valency shell configuration of either ns1 or ns2.

The members of this block lie on the extreme left of the Periodic Table. The elements having ns1 configuration are called alkali metals (Group IA elements) while those having ns2 configuration are called alkaline earth metals (Group IIA elements). The elements of s-block are also called as reactive metals.

General characteristics of s-block elements:

The general characteristics of alkali metals (Li, Na, K, Rb, Cs and FrY and alkaline earth metals (Be, Mg, Ca, Sr, Ba and Ra), i.e. s-block elements are as mentioned below:

  • 1. They are soft metals, possess low melting and boiling points, have the largest atomic radii in their corresporiding periods and are good conductors of heat and electricity.

  • 2. They have low values of ionisation energies and are hence highly electropositive.

  • 3. They are very reactive and readily form ionic compounds. They show a fixed valency which depends on the number of electrons present in the outermost shell. The alkali metals show monovalency while alkaline earth metals show divalency. They are never found in free state in nature due to their reactive nature.

  • 4. On account of low ionisation energies and highly negative electrode potentials, they act as strong reducing agents. The alkali and alkaline earth metals cannot be prepared by doing electrolysis of aqueous solutions of their salts.

  • 5. Except Be and Mg, they impart a characteristic colour to the flame.

  • 6. The compounds of s-block elements are predominantly ionic and colourless. However, lithium and beryllium compounds are covalent in nature.

  • 7. They have great affinity for oxygen and non-metals. The oxides are basic. The hydroxides are strong alkali.

  • 8. They displace hydrogen from acids and form corresponding salts.

  • 9. With the exception of Be and Mg, they decompose water and readily evolve hydrogen.

  • Alkali metals in the Periodic Table:

    The members of Group IA or I, except hydrogen are called the alkali metals,because they react with water to form alkaline solutions. All the group IA elements are soft silvery-grey metals. Their valence electron configurations are nsl where n is the period number, and their physical and chemical properties are dominated by the ease with which the single valence electron can be removed. These elements are collectively called as alkali metals and group IA is known as alkali group as the hydroxides of these metals are soluble in water and these solutions are highly alkaline in nature.

    Properties of Alkali metals

  • 1. All the alkali elements are silvery white solids.

  • 2. Soft and can be possibly cut with the help of knife (except lithium).

  • 3. Atomic and ionic radii: Group IA atoms are the largest in their horizontal periods in the Periodic Table.

  • 4. Density: All are light metals. The densities are low. Lithium, sodium and potassium are lighter than water and for this very reason, they float on water. Density increases while moving from Li to Cs. Potassium is, however, lighter than sodium.

  • 5. Melting and boiling points: The energy binding the atoms in the crystal lattices of these metals is relatively low on account of a single electron in the valency shell.

  • 6. Ionisation energies and electropositive character: Due to their large size, the outermost electron is far from the nucleus and can easily be removed. Their ionisation energies or ionisation enthalpies are relatively low. Thus, the metals have a great tendency to lose the ns1 electron to change into M+ ions. These metals are highly electropositive in nature.

  • 7. Hydration of ions, hydrated radii and hydration energy: The salts of alkali metals are ionic and soluble in water. The solubility is due to the fact that cations get hydrated by water molecules.

  • 8. Electronegativity: The tendency to attract electrons is low as the alkali metals are electropositive. The electronegativity, thus, decreases from Li to Cs as the electropositive character increases.

  • 9. Conductivity: The alkali metals are good conductors of heat and electricity. This is due to the presence of loosely held valency electrons which are free to move throughout the metal structure.

  • 10. Specific heats: The specific heat values decrease from Li to Cs.

  • 11. Heat of atomisation: Heat of atomisation decreases—from Li to Cs.

  • 12. Reducing nature: An element, which acts as a reducing agent, must have low ionisation energy. Alkali metals act as strong reducing agents as their ionisation energy values are low. Since ionisation energy decreases on moving down from Li to Cs, the reducing property increases in the sameorder. Thus, Li is the weakest reducing agent while Cs is the strongest reducing agent amongst the alkali metals in free-state.

  • Alkali earth metals in periodic table:

    The group ITA of the Periodic Table consists of six elements--beryllium, magnesium, calcium, strontium, barium and radium. These elements are collectively called as alkaline earth metals because their earths are basic (alkaline) and group IIA is known as alkaline earth group. The oxides of the three principal members namely calcium, strontium and barium were known to humans much earlier than the metals themselves. These oxides were alkaline in nature and existed in the earth and were named alkaline earths. The metals when discovered were also called alkaline earths. This term is now applied to all the six elements of group IIA.

    The first member beryllium is less active than other members and shows some abnormal properties like lithium in IA group. However, it shows resemblance with aluminium (a member of IIIrd group), i.e. diagonal relationship. The last member, radium is radioactive in nature. Each member of this group occupies a place just after the members of IA group in various periods of the periodic table except for the first period.

    Properties of Alkali earth metals

    (a) Physical state: All the group IIA elements are metals and are too reactive, so that they cannot occur in the uncombined state in nature. They are all silvery white metals.
    (b) Atomic and ionic radii: The size of the atom increases gradually from Be to Ra, on account of the presence of an extra energy shell at each step.
    (c) Density: These metals are denser than alkali metals in the same period because these can be packed more tightly due to their greater nuclear charge and -smaller size. The density decreases slightly upto calcium and then increases considerably upto radium. Irregular trend is due to the difference in the crystal structure of these elements.
    (d) Melting and boiling points: The melting and boiling points of these elements are higher than the corresponding alkali metals. This is due to the presence of two electrons in the valency shell and thus, they are strongly bonded in the solid state. However, melting and boiling points do not show any regular trends because atoms adopt different crystal structures.
    (e) Ionisation energies and electropositive character: The first and second ionisation energies of these metals decrease from Be to Ba. The second ionisation energy in each case is higher than the first, nearly double the first ionisation energy.

    Difference between alkaline earth metals and alkali metals

    Both alkaline earth metals and alkali metals are s-block elements as the last differentiating electron enters the ns-orbital. They resemble each other in many respects but still there are certain dissimilarities in their properties on account of the different number of electrons in the valency shell, smaller atomic radii, high ionisation potential, higher electronegativity, etc. The main points of difference between alkaline earth metals and alkali metals are given below:

    PropertiesAlkaline earth metalsAlkali metals
    (i) Electronic configurationTwo electrons are present in the valency shell. The configurations is ns2One electron is present in the valency shell. The configuration is ns1
    (ii) ValencyBivalent.Monovalent.
    (iii) Electropositive natureLess electropositive.More electropositive.
    (iv) HydroxidesWeak bases, less soluble and decompose on heating. Strong bases, highly soluble and stable towards heat.
    (v) BicarbonatesThese are not known to exist in free-state. Exist only in solution. These are known to exist in solid state.
    (vi) CarbonatesInsoluble in water. Decompose on heating.Soluble in water. Do not decompose on heating (Li2CO3 is an exception).
    (vii) Action of nitrogenDirectly combine with nitrogen and form nitrides. Do not directly combine with nitrogen.
    (viii) Action of carbonDirectly combine with carbon and form carbides. Do not directly combine with carbon.
    (ix) NitratesDecompose on heating evolving a mixture of NO2 and oxygen. Decompose on heating evolving only oxygen.
    (x) Solubility of saltsSulphates, phosphates, fluorides chromates, oxalates, etc, are insoluble in water. Sulphates, phosphates, fluorides, chromates, oxalates, etc., are soluble in water.
    (xi) Physical propertiesAre less reactive and comparatively harder metals. High melting points. Diamagnetic. Soft, low melting points. Paramagnetic.
    (xii) Hydration of compoundsThe compounds are extensively hydrated. MgCl2.6H2O, CaO2.6H2O and BaCl2.2H2O are hydrated chlorides. The compounds am less hydrated. NaCI, KCl and RbC1 form non-hydrated chlorides.
    (xiii) Reducing powerWeaker, as ionisation potential values are high and oxidation potential values are low.Stronger, as ionisation potential values are low and oxidation. Potential values arc high.