Explain ionization enthalpy in group and period

1. In a group from top to bottom Ionization Enthalpy decreases due to higher atomic size. 
2. And in period from left to right Ionization Enthalpy increases. So it varies as 16<17<18. 
3. Group 15 elements have higher Ionization Enthalpy than that of group 16 due to half filled orbital which gives extra stability to atom.