Answer to Question #91952 in Inorganic Chemistry for Gaurav patwal

Question #91952
Explain the variation in ionization energy across the elements of period 2 in the periodic table.
Expert's answer

As we move across the second period from left to right, in general, the first ionisation energy increases. But we can see two drops: between Be and B and between N and O. The explanation lies in the electronic structures of atoms.

Li [He] 2s1

Be [He] 2s2

B [He] 2s2 2p1

We can see in atom of B one more electron is added but it is added to the p orbital. P orbital is higher in energy than s orbital and electron on this orbital is further from the nucleus. The first ionization energy decreases 1) due to the screening effect of 2S2 electrons that reduces the pull from the nucleus and 2) due to the increased distance between

2p electron and nuclei that reduces attraction between them.

C [He] 2s2 2p2

N [He] 2s2 2p3

O [He] 2s2 2p4

We can see that three p orbitals in N atom are half- filled (px1py1pz1) whereas in O atom one p orbital is fully filled (px2py1pz1). The screening effect is the same for all p electrons, we remove an electron from the same p orbital (px) but for the pair of electrons repulsion exists therefore it is easier to remove electron from the pair. For the next atoms (F and Ne)addition of one more proton has more influence on p electrons than repulsion between electrons of a pair.

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