Demarcation of periodic table
Into s and p block
Answers
Answer:
Elements in the long form of periodic table have been divided into four blocks i.e. s ,p ,d and f. This division is based upon the name of the orbitals which receives the last electron.
S block elements
1)Elements in which the last electron enters the s orbital of their respective outermost shells are called s block elements.
2)s sub shell has only 1 orbital which can accommodate only 2 electrons, therefore, there are only two groups of s block elements.
3)Elements of and group 1(hydrogen and alkali metals) and group 2 (alkaline earth metals) and helium comprises s block element.
4)Hydrogen has only one electron and this enters 1s orbital. The rest of the elements i.e. Li, Na, K, Rb, Cs and Fr receive the last electron in 2s, 3s, 4s, 5s,6s and 7s orbitals.
5)Helium and the elements of group 2 i.e. Be, Mg, Ca, Sr, Ba and Ra.The last electron in these elements also enter the s orbital i.e. 1s, 2s, 3s, 4s, 5s, 6s and 7s orbital.These elements also belong to s block and have ns2 as their general outer electronic configuration.
6)There are 14 s-block elements in the periodic table.
General electronic configuration of s-block elements: ns1-2 where n=2-7
Helium belongs to s – block but its positioning with in the p block along with other group 18 elements is justified because it has a completely filled valence shell and thus exhibits properties characteristic of other noble gases.
Because of its unique behaviour, we shall place hydrogen separately at the top of the periodic table it because:
1) It has a single s-electron and hence can be placed along with alkali metal group.
2)It can also gain one electron to acquire a noble gas configuration and hence can be placed along with halogens.
Characteristics of s-block elements
1)They are soft metals with low melting and boiling points.
2)They have low ionisation enthalpies and are highly electropositive.
3)They lose the valence electrons readily to form +1 and +2 ions.
4)They are very reactive metals .