First member of each group of representative elements ( i.e. s and p-block elements ) shows anomalous behaviour due to
(i) small size
(ii) high ionisation enthalpy
(iii) high electronegativity and
(iv) absence of d- orbitals.
For example in s-block elements, lithium shows anoimalous behaviour from rest of the alkali metals.
(i) Compounds of lithium have significant covalent character. While compounds of other alkali metals are predominantly ionic.
(ii) Lithium reacts with nitrogen to form lithium nitride while other alkali metals do not form nitrides.
In p-block elements, first member of each group has four orbitals, one 2s- and three 2p-orbitals in their valence shell. So, these elements show a maximum covalency of four ehile other members of the same group or different group show a maximum covalency beyond four due to availability of vacant d-orbitals.

As we move from left to right in period, the number of valence electrons increases by one at each succeeding element but the number of shells remains same. Due to this, effective nuclear charge ivcreases.
More is the effective nuclear charge, more is the attraction between nuclei and electron.
Hence, the tendency of the element to lose electrons decreases, this results in decrease in metallic character.
Furthermore, the tendency of an element to gain electrons increases with increase in effective nuclear charge, so non-metallic character increases on moving from left to right in a period.

ionization\(({ _{ 7 }N }={ 1s }^{ 2 },{ 2s }^{ 2 },{ 2p }^{ 3 }),\) enthalpy of nitrogen is greater than oxygen \(({ _{ 8 }O }={ 1s }^{ 2 },{ 2s }^{ 2 },{ 2p }^{ 4 }),\)due to extra stable half-filled 2p-orbitals. Similarly, ionization enthalpy of phosphorus is \(({ { _{ 15 }{ P } } }={ 1s }^{ 2 },{ 2s }^{ 2 },{ 2 }p^{ 6 },{ 3s }^{ 2 },{ 3p }^{ 3 })\) greater than sulfur on\(({ { _{ 16 }{ S } } }={ 1s }^{ 2 },{ 2s }^{ 2 },{ 2 }p^{ 6 },{ 3s }^{ 2 },{ 3p }^{ 4 })\) moving down the group, ionization enthalpy decreases with increasing atomic size. so, the increasing order of first ionization enthalpy is S.

Non-metallic character across a period (left to right) increase but on moving down the group it decreases. So, the increasing order of non0metalic character is P.

Electron gain enthalpy generally becomes more negative across a period as we move from left to right. Within a group, electron gain enthalpy becomes less negative down a group. However, adding an electron to the 2p-orbital leads to greater repulsion than adding an electron to the larger 3p-orbital. Hence the element with most negative electron gain enthalpy is chlorine; the one with the least negative electron gain enthalpy is phosphorus.