Question

Explain trend or variation in periodic table ( for s-block and p-block elements).
i.Atomic radius. ii.Ionisation enthalpy. iii.Electrons gain enthalpy.

Answer 1

To observe the variation in s-block and p-block elements, draw the electronic configuration of each element of the group.
The s-block elements include group 1 ( Li, Na, K, Rb, Cs, Fr) and group 2 (Be, Mg, Ca, Sr, Ba, Ra) elements.
The p-block elements include elements from group 13 to group 18.

Complete answer:
For s-block:
Atomic radius: The alkali metals and alkaline earth metals have the largest sizes in a particular period. However alkaline earth metals are slightly smaller than alkali metals. On moving down the group from Li to Cs (in group 1) and Be to Ba(in group 2) , the size of the atom increases because of the addition of an extra shell in respective periods The increase in the size of the atom implies an increase in the atomic radius too.
-Ionisation Enthalpy: It actually refers to the ease with which an element can lose an electron. The amount of energy required to remove the most loosely bound electron in its ground state is called ionisation enthalpy. The ionisation enthalpy of s-block elements is less as compared to other elements of the periodic table. They have one valence electron , they can readily lose electrons to attain noble gas configuration . On moving down the group, their ionisation enthalpy further decreases because the effect of increasing size outweighs the effect of increasing charge, and the outermost electron is screened from the effect of the increased charge.
-Electron Gain Enthalpy: The electron gain enthalpy of s-block elements is very less negative , as they have a very low tendency of accepting an electron . They prefer to form bonds by losing their electrons. Further on moving down , it becomes less negative because the increase in size keeps the outer electron very well repulsed from the nucleus .
For p-block:
-Atomic radius: Every time on moving down the group , an extra shell is added ,this is expected to increase in the size of the atom. In group 13 , on moving down the group , the atomic radius increases except for Ga , which has slightly less radius than Al . This happens because of the addition of 3-d electrons in Ga. Atomic radius increases down the group except for Ga which shows some exception here.
The covalent radius increases from C to Si and from Si to Pb , a decrease is observed because of the presence of d and f- electrons.
-Ionisation Enthalpy: The trend of ionisation enthalpy is irregular in group 13 because of the presence of 3-d electrons. The ionisation enthalpy decreases from B to Al ( because of increase in size).
The ionisation enthalpy of group 14 elements is higher than group 13 because of an extra electron in their valence shell. In general , the enthalpy decreases down the group . However , small decreases from Si to Ge to Sn and then increases from Sn to Pb( because of intervening d and f –electrons).
-Electron Gain Enthalpy: The electron gain enthalpy of p-block elements is more negative than s-block elements. It refers to the ease of acceptance of an electron . As we move from group 13 to group 18 across the period , the electron gain enthalpy becomes more and more negative.

Note:
Any trend in chemical or physical property is due to the electronic configuration of the element. The presence of s, p, d, f shells affect the properties of the elements . The elements having similar valence shell electronic configuration resemble each other.