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THE S- BLOCK ELEMENTS

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INTRODUCTION :

The s-block elements of the Periodic Table are those in which the last electron enters the outermost s-orbital. As the s-orbital can accommodate only two electrons, two groups (1 & 2) belong to the s-block of the Periodic Table.
Group 1 of the Periodic Table consists of the elements: lithium, sodium, potassium, rubidium, calcium and francium. They are collectively known as the alkali metals. These are so called because they form hydroxides on reaction with water which are strongly alkaline in nature. The elements of Group 2 include beryllium, magnesium, calcium, strontium, barium and radium. These elements
with the exception of beryllium are commonly known as the alkaline earth metals. These are so called because their oxides and hydroxides are alkaline in nature and these metal oxides are found in the earth’s crust.

Among the alkali metals sodium and potassium are abundant and lithium, rubidium and calcium have much lower abundances. Francium is highly
radioactive; its longest-lived isotope 223Fr has a half-life of only 21 minutes. Of the alkaline earth metals calcium and magnesium rank fifth and sixth in abundance respectively in the earth’s crust. Strontium and barium have much
lower abundances. Beryllium is rare and radium is the rarest of all comprising only 10–10 per cent of igneous rocks.

the general electronic configuration of s-block elements is (noble gas) ns2 fpr alkaline earth metals.

Lithium and beryllium, the first elements of Group 1 and Group 2 respectively exhibit some properties which are different from those of the other members of the respective group. In these anomalous properties they resemble the second element of the following group. Thus, lithium shows similarities to magnesium and beryllium to aluminium in many of their properties. This type of diagonal similarity is commonly referred to as diagonal relationship in the periodic table. The diagonal relationship is due to the similarity in ionic sizes and /or charge/radius ratio of the elements. Monovalent sodium and potassium ions and
divalent magnesium and calcium ions are found in large proportions in biological fluids. These ions perform important biological functions such as maintenance of ion balance and nerve impulse conduction.

GROUP 1 ELEMENTS: ALKALI METALS :

The alkali metals show regular trends in their physical and chemical properties with the increasing atomic number. The atomic, physical and chemical properties of alkali metals are discussed below.

Electronic Configuration :

All the alkali metals have one valence electron, ns1 outside the noble gas core.
The loosely held s-electron in the outermost valence shell of these elements makes them the most electropositive metals. They readily lose
electron to give monovalent M+ ions. Hence they are never found in free state in nature.
increase in atomic number, the atom becomes larger. The monovalent ions (M+) are smaller than the parent atom. The atomic and ionic radii of alkali metals increase on moving down the group i.e., they increase in size while going
from Li to Cs.

Ionization Enthalpy :

The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs. This is because the effect of increasing size outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge