Question

a) the first ionization energies (IE) of hydrogen and helium are about 1300 kJ/ mol and 2300 kJ/ mol respectively. Yet the electron removed from both of these originated in a 1s orbital. Explain in 1-3 sentences the large IE difference observed between these elements .

b) Another particulary interesting exception to the trend in 1st ionization energy is found by comparing N and O. The 1st IE is higher for N than O, even though O has 1 more proton in its nucleus. Offer an explanation. Hint: it may be helpful to look at the valence electron diagrams for N and O.

Answer 1

(A) The ionisation energy depends upon NUCLEAR CHARGE and the ORBITAL FROM WHICH ELECTRON IS REMOVED among other things. In He and H the orbital from which is removed is same i.e; 1s. However the NUCLEAR CHARGE OF HE IS GREATER THAN H. That is why it has greater IE

(B) N has half filled valence shell

2S²2P³

Oxygen 2s²2p⁴

Complete and exactly half filled shells represent more stable staes. Hence more energy is required to remove an electron from N Than O