Answering difference in ionization enthalpy of Element

1. Increase in number of proton --> effective nuclear charge: increase
   
2. Screening effect of inner shell electron --> : decrease
   
3. Extra stability(write out ground state)
   
4. Electron to electron repulsion --> : decrease
   

進階問題答法(基本問題只涉及以上一項)
同 + 異 + 比較-->outweighing effect

Example: O and N
Oxygen and nitrogen has the same number of electron shell(同), although oxygen has 1 more number of proton than nitrogen(異), the electronic configurations of N and O are [He]2s2 2p3 and [He]2s2 2p4 respectively, where first electron of N being remove is in completely filled 2p3 orbital which give extra-stability . (異)

On the other hand, the first electron of O being removed is in a paired orbital, the electron to electron repulsion will lower its ionization enthalpy.(異)

The extra-stability and electron to electron repulsion together outweigh the effect in increase in proton number.(比較--> outweighing effect) Therefore the ionizationentalpy of N is higher than O