First member of each group of representative elements (i.e., s and p-block elements) shows anomalous behaviour. Illustrate with two examples.

Explanation

The first member of each group of the representative elements (i.e., s- and p- block elements ) show anomalous behaviour from rest of members of the same group because of the following reasons.
(i) small size (ii) high ionization enthalpy (iii) high electronegativity and (iv) absence of d- orbitals s- Block Elements. Some example of anomalous behaviour of Li from rest of the alkali metals and Be from rest of teh alkaline earth metals are discussed below .
(i) Lithium unlike other alkali metals and beryllium unlike other alkaline earth metals form compound which have significant covalent character while other members of each of the these groups form compound which are predominantly ionic.
(ii) Lithium combines with nitrogen to form lithium nitride while all other alkali metals do not form nitrides .Similarly , lithium carbonate decomposes to give `CO_(2)`1 while all other alkali metals carbonates do not.
`6 Li + N_(2) overset(Heat)(to) underset("Lithium nitride ")(2Li_(3)N)`
` Li_(2) CO_(3) overset(Heat)(to) Li_(2)O + CO_(2) , Na_(2) CO_(3) overset(Heat)(to) No` action
Similarly Be differs from all other alkaline earth metals as shown below ,
(i) Beryllium carbide reacts with water to produce methane gas while carbides of other alkaline earth metals give acetylene gas.
`Be_(2) C+ 4 H_(2) O to 2 Be (OH)_(2) + CH_(4) , CaC_(2) + 2H_(2) O to Ca (OH)_(2) + HC -= CH`
(ii) Beryllium shows a coordination number of 4 due to the presence of one s- and three p-orbitals in valence shell but other alkaline earth metals such as `Mg^(2+) ` and `Ca^(2+)` show a coordination number of making use of d- orbitals in addition to s- and p- orbitals .For example
`BeF_(2) + F tp ]BeF_(4)]^(2-) -` four coordinate complex
`Ca^(2+) + Na_(2) [H_(2) EDTA]^(2-) to [Ca (EDTA)]^(2-) +2 Na^(+) +2 H^(+) -Six` coordinate complex

Disodium salt of ethylenediamine tetraacetic and
The EDTA (ethylenediamine tetraacetic acid ) Complexes of `Mg^(2+)` and `Ca^(2+)` are used to determine the hardness water by titration method.
p- block element . Some examples of anomalous behaviour of first element of each group of p- block element from the rest of the member of the same group are discussed below :
maximum covalency of four. the first member of each group of the p- block elements has four orbitals (one 2s and three 2p- orbitals ) in the valence shell for bonding and hence can accommodate 4 pairs or 8 electrons . In other words , these elements show a maximum covalency of four. In contrast , other member of the same group or different froup of p - block element have vacent d- orbitals and hence can accommodate more electrons and thus can expand their covalency beyond four . for example,
(a) boron forms `[BF_(4)]^(-)` or `[BH_(4)]^(-)` ion while A1 gives `[A1F_(6)]^(3-)` ion.
( b) Carbon forms only tetrahalites `(CX_(4), X =F, C1 , Br, I)` while other member form hexahalides , i.e., `[SiF_(6)]^(2-) , [GeC1_(6)]^(2-), [SiC1_(6)]^(2-) , [PbC1_(6)]^(2-),` etc
( c) Nitrogen forms only trihalides such as `NF_(3)` (having an octer of electrons in the valence shell ) while P and As form both trihalites `(PF_(3), "As" C1_(3)` etc ) and pentahalides `(PF_(3), "As" C1_(5))` having 10 electrons in valence shell.
(d ) Fluorine does not form `FC1_(3)` (having 10 valence electrons ) while chlorine forms `C1F_(3)` (having 10 valence electrons )