Periodic property:

The property in which there shall be a regular gradation is called periodic property.

I. a) Atomic radius : 

Period : 

Atomic radius of elements decreases across a period from left to right because the nuclear charge increases due to increase in atomic number. 

Group : 

Atomic radius increases from top to bottom in a group due to addition of new shell. 

b) Ionization energy: 

Period : 

When we move from left to right it does not follow a regular trend but generally increases due to increase in atomic number.

Group : 

In a group from top to bottom, the ionization energy decreases due to increase in atomic size. – 

c) Electron affinity: 

Period : 

Electron affinity values increase from left to right in a period. 

Group : 

Electron affinity values decrease from top to bottom in a group. 

d) Electronegativity : 

Period : 

Electronegativity increases from left to right in a period. 

Group : 

Electronegativity decreases from top to bottom in a group.

II. Ionization energy order : 

a) Na, Al, Cl :  In a period ionisation energy increases so the order is Na < kl < Cl. 

b) Li, Be, B : Beryllium has stable configuration 1s² 2s². So it has more ionisation energy. So the order is Li < B < Be.  

c) C, N, O : Nitrogen has half-filled p-orbitals. So it has greater ionisation energy. So the order is C < O < N. 

d) F, Ne, Na : Ne is inert gas right to F. Whereas Na is a metal ion in third period. So, the order is Na < F< Ne. 

e) Be, Mg, Ca : In a group ionisation energy decreases. So the order is Be > Mg > Ca.